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5/23/2017 Dr. N. RAMACHANDRAN, NITC 1 METAL JOINING • Even the simplest object is an assembly of components • Complex ones - greater number of partssubassemblies joined to perform the function • METHODSWELDING, BRAZING, SOLDERING, ADHESIVE BONDING, MECHANICAL JOINING 5/23/2017 Dr. N. RAMACHANDRAN, NITC 2NITC WHY JOINING? • IMPOSSIBLE TO MAKE AS ONE PIECE • EASINESS AND ECONOMY IN MANUFACTURE • EASY IN REPAIRS AND MAINTENANCE • FUNCTIONAL PROPERTIES DIFFERe.g.: Carbide tips of tools,corrosion resistant parts, tungsten carbide tip of pens, brake shoes to metal backing etc… • TRANSPORTING SITE/ CUSTOMER 5/23/2017 Dr. N. RAMACHANDRAN, NITC 3NITC CLASSIFICATION • According to the STATE of the materials being joined • Extent of external heating- PRESSURE • Use of FILLER materials 5/23/2017 Dr. N. RAMACHANDRAN, NITC 4NITC NITC Joining Processes LIQUID SOLID CHEMICAL CUTTING ARC CONSUMABLE Oxy-fuel Thermit 5/23/2017 SMAW SAW GMAW FCAW EGW ESW RESISTANCE NON CONSUMABLE Forge Cold Spot GTAW Ultrasonic Seam PAW Friction Projection Explosion EBW Flash LBW Diffusion Stud Dr. N. RAMACHANDRAN, NITC percussion MECH. JOINING LIQUIDSOLID Brazing Soldering Adhesive Bonding Fastening Crimping Seaming Stitching 5 History of welding And American Welding Society 5/23/2017 Dr. N. RAMACHANDRAN, NITC 6 Vulcan – The Roman Fire God 5/23/2017 Dr. N. RAMACHANDRAN, NITC 7 5/23/2017 Dr. N. RAMACHANDRAN, NITC 8 5/23/2017 Dr. N. RAMACHANDRAN, NITC 9 • Welding Heat Exchanger Dr. N. RAMACHANDRAN, NITC 5/23/2017 10 • Thermite Welding Patent 729573 Dr. N. RAMACHANDRAN, NITC 5/23/2017 11 5/23/2017 Dr. N. RAMACHANDRAN, NITC 12 • 1948 The Ohio State University Board of Trustees established the Department of Welding Engineering on January 1 as the first of its kind for a Welding Engineering cirriculum at a University. OSU pioneered the Welding Engineering through an emphasis in the Industrial Engineering Department the previous nine years. The advantages of this engineering degree is 1) Enable satisfactory administration of problems relating to education and research in the welding field. 2) Recognition is given to the Welding Engineer as an entity among applied sciences. 3) A degree is authorized which is descriptive of a particular discipline imposed in training for professional work in the field. Air Reduction Company develops the Inert-Gas MetalArc (MIG) process. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 13 SIGMA Welding (Shielded Inert Gas Metal Arc) was developed to weld plate greater than1/8 inch instead of the "Heli-Arc" welding process. The arc is maintained in a shield of argon gas between the filler metal electrode and the workpiece. No flux is used. Licensed by Linde Air Products Co. •1948-1949 Curtiss-Wright Corporation looks at brazing as a strong, lightweight process for durable assemblies. •1949 American Westinghouse introduces and markets welding machines using Selenium Rectifiers. US Navy uses inert-gas metal arc welding for aluminum hulls of 100 feet in length. •1950 The Kurpflaz Bridge in Germany was built as the first welded orthotropic deck. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 14 •1950s Electron Beam (EB) welding process developed in France by J. A. Stohr of the French Atomic Energy Commission. First Public disclosure was 1957. Wave soldering is introduced to keep up with the demand of Printed Wiring Boards used in the electronics age. Research on testing of brazed joint begins as serious endeavor for the next ten years. •1950 Electroslag Welding (ESW) is developed at the E. O. Paton Welding Institute, Ukraine USSR. Third Edition of the Welding Handbook is printed by AWS. Flash Butt Welding is the standard for welding rail line construction. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 15 •1951 Russia use Electroslag Welding (ESW) process in production. The Philip Roden Co. of Milwaukee Wisconsin announces the DryRod electrode oven. This oven is intended to provide a controlled moisture environment of 0.2% moisture standard set forth by the government. This oven provides adjustable temperature control of 200-550 F, vented and holding 350 pounds of electrodes. •1953 Modifying the Gas Metal Arc Welding (GMAW) process, Lyubavskii and Novoshilov used CO2 with consumable electrodes. Resulted in hotter arc, uses higher current, and larger diameter electrodes. The Ohio State University established a Welding Engineering College curriculum out of the Industrial Engineering Department. • 5/23/2017 Dr. N. RAMACHANDRAN, NITC 16 •1957 Flux Cored-Arc Welding (FCAW) patented and reintroduced by National Cylinder Gas Co. Plasma Arc Welding (PAW) Process developed by Robert M. Gage Russia, Britain, and USA independently develop a short-circuiting transfer for low-current low-voltage welding in a carbon dioxide atmosphere. Braze repair process for cracks in jet engine combustion chambers and transition ducts. 1958 The Soviet Union introduced the Electroslag Welding (ESW) Process at the Brussels World Fair in Belgium. This welding process had been used since 1951 in the USSR which was based on the concept and work of an American, R. K. Hopkins. Perfected at the Paton Institute Laboratory in Kiev, Ukraine, USSR and the Welding Research Laboratory in Braitislava, Czechoslovakia. AWS Committee on Brazing and Soldering is formed to develop a test for evaluating strength of brazed joints. Robert Peaslee proposes a test in the Welding Journal. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 17 • 1959 Electroslag welding process was first used at the Electromotive Division of General Motors in Chicago and was called the "Electro-Molding Process". Development of Inside-Outside Electrode which did not require an external gas shielding - Innershield from Lincoln Electric Co. • 1958-1959 Short Arc (Micro-wire Short Arc) developed from refined power supplies and smaller diameter wires. • 1960s Pulsed Arc Welding...(more to follow) Space Program is underway...(more to follow) Difficult to stabilize GTAW at below 15 amps, Microplasma is developed to overcome the limitation. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 18 1960 Development of a cold wall vacuum furnace. First laser beam produced using a ruby crystal for the Light Amplification Stimulated Emission Radiation (LASER). Explosive welding is developed in USA. Hughes Aircraft Company (Mainar) develops the first ruby laser (springtime). Bell Telephone Laboratories (Ali Javan) developed and presented the first gas laser using neon and helium (fall time) 1962 The Mercury Space Capsule is formed using inner and outer titanium shell, seam welded together using a three-phase resistance welder by Sciaky. 1963 U.S.S. Thresher sinks off the coast of New Hampshire and by December, the U.S. Navy charters the Submarine Safety Program (SUBSAFE) to control the fabrication, inspection and quality control of submarine construction. The presumed failure was with a silver-brazed piping joint, but after the investigation, the whole welding and brazing program was suspect. Included was the material properties of the welding and brazing filler metals. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 19 • 1965-1967 CO2 lasers are developed for cutting and welding. • 1967 H. J. Clarke makes the following Predictions during the AWS Plummer Lecture in Houston as he ties the current state of technology of welding to the future of progress: World's Population would be greater than 5 Billion. Large scale farming of the ocean and fabrication of synthetic protein. Controlled thermonuclear power as a source of energy. General immunization against bacteria and virile infections, perfected and available. Primitive forms of life will created in the lab. Automation will have advance for performance of menial chores and complicated functions. Housewives would be ordering groceries and everyday items from central stores linked to the home electronically. (!!!) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 20 • Children will be receiving education at home - "either by television or with personal teaching machines and programmed instructions" Moon - mining and manufacture of propellant and on Mars, permanent unmanned research stations. Weather manipulation by the military. Effective anti-ballistic missile defense in the form of air-launched missiles and directed energy beams. Libraries will be "computer-run" Gravity welding is introduced in Britain after its initial discovery by Japan. • 1969 The Russian Welding Program in Space began by producing Electron Beam welds on SOYUZ-6. Welding an AMG6 and DM-20 aluminum alloys with the Vulkan process. Sponsored by the E. O. Paton Welding Institute Academy of Science. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 21 • 1970 As miniaturization developed from the pressure to increase component densities, Surface Mount Technology is developed. This required new ways to make soldered joints, including the development of vapor phase, infrared, hot gas and other re-flow technologies. First AWS International Brazing Conference including 24 papers presented created much interest in the brazing process. BP discovers oil off the coast of Scotland. • 1971 British Welding Institute (Houldcroft) adds oxidizing gas jet around laser beam to develop laser cutting. • 1973 The American Astronauts used Electron Beam welding process in June 1973 welding Aluminum Alloy 2219-T87, Stainless 304 and Pure Tantalum. Welding equipment manufacturers concentrate on equipment refinement instead of new processes. Two Supertankers, Globtik Tokyo and Globtik London (476025 DWT) were 5/23/2017 Dr. N. RAMACHANDRAN, 22 built for carrying 153 million gallons (3 millionNITC barrels) of crude oil. • 1976 First automotive production application of lasers weld begins with General Motors Corporation, Dayton Ohio using two 1.25 kW CO2 lasers. for welding valve assemblies for emission control systems. • 1977 The US Federal Highway Administration issues a moratorium of Electroslag Welding (ESW) when cracks are discovered during an inspection of a bridge in Pittsburgh, Pennsylvania on an interstate highway. Failure analysis was conducted by Lehigh University on Interstate 79. • 1980 The Fort McHenry tunnel contract, for 750 Million Dollars, is awarded to begin construction, completing Intestate 95 through Baltimore, Maryland. This is the largest tunnel of its kind, 180 feet at the bottom with two separate four lane immersed tunnels removing 3.5 million cubic yards of dredge. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 23 • 1983 Homopolar pulse welding variation of the upset welding process research begins at the University of Texas at Austin at the Center for Electromechanics. • 1987 Laser research begins a unique method for depositing complex metal alloys (Laser Powder Fusion). • 1991 TWI of Cambridge England develops the Friction Stir Weld (FSW) process in its laboratory. This process differs from conventional rotary technology whereby a hard, non consumable, cylindrical tool causes friction, plasticizing two metals into a Solid-State Bond. No shielding gas or filler metal is required. Metals joined successfully include, the 2XXX, 6XXX and 7XXX series aluminum. NASA is the first US venture which welded the massive fuel tank for the Space Shuttle. Brazing Handbook (Fourth Edition) shows the data of the filler metal/base metal failure transitions between 1T and 2T overlap and is the key for the design data (factor of safety). 5/23/2017 Dr. N. RAMACHANDRAN, NITC 24 1996 Over 7,00,000 brazements are produced for the aircraft industry in the US and Canada. Over 132,010,00 units of brazed automotive parts are produce. 1999 The Edison Welding Institute develops a solution to obtaining deeper penetration of a GTA weld by introducing FLUX onto the surface of the weld. This FLUX helps drive the welding arc heat deeper into the weld joint and permits 300 percent more penetration. 2000 Magnetic Pulse Welding (MPW) is introduced by Pulsar Ltd. of Israel using capacitive power as a solid state welding process. Discharging 2 Million amps in less than 100 microseconds this process can create a metallurgical, a nonmetallurgical or a mechanical lock, depending on the substrate involved. No heat affected zone (HAZ) is created since only a rise of 30oC occurs. Tailored welded blanks of aluminum are used where spot welding was once performed. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 25 2000 Researchers from Argonne National Laboratory use the energy of the x-ray to weld metal-matrix composite (Ti or Al / Al2O3 or SiC) materials. Diode laser welding, once limited to compact disks, laser printers, and laser pointers, are now making their way to the manufacturing floor. Welding Type 304 Stainless steel (0.024 inch), Titanium foil (0.005 inch thick) and laser brazing with a silicon-bronze brazing wire. Conductive heat resistance seam welding (CHRSEW) is developed. The process uses steel cover sheets placed on top of aluminum butted together. Using conventional seam welding, the heat generated from the steel forms a molten interface on the aluminum and fusion is made at the butt joint. The steel covers are then removed. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 26 • 2001 AWS D17.1, "Specification for Fusion Welding for Aerospace Applications" is published in March. The efforts of approximately 50 individuals from a cross-section of the Aviation Industry and government produces the first commercial aviation welding specification. Flame brazing 5XXX aluminum alloys using non-corrosive flux. Sulzar Elbar introduces laser powder welding technology. Permits rebuilding of substrate material (High Creep Resistance) and reproduction of the single crystal structure. • 2002 From Linde Gas in Germany, a Diode laser using process gases and "active-gas components" is investigated to enhance the "key-holing" effects for laser welding. The process gas, Argon-CO2, increases the welding speed and in the case of a diode laser, will support the transition of heat conductivity welding to a deep welding, i.e., 'key-holing'. Adding active gas changes the direction of the metal flow within a weld pool and produces narrower, high-quality weld. CO2 Lasers are used to weld polymers. The Edison Welding Institute is using through-transmission lasers in the 230-980 nm range to readily form welded joints. Using silicon carbides embedded in the surfaces of the polymer, the laser is capable of melting the material leaving a near invisible joint line. 2003 2004 2005 Future developments. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 27 ABOUT AWS The American Welding Society (AWS) was founded in 1919 as a multifaceted, nonprofit organization with a goal to advance the science, technology and application of welding and related joining disciplines • The Engineering Societies Building (left) in New York City was the home of AWS until 1961 when the Society moved to the United Engineering Center, also in New York City. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 28 From factory floor to high-rise construction, from military weaponry to home products, AWS continues to lead the way in supporting welding education and technology development to ensure a strong, competitive and exciting way of life for all Americans. • The Society moved its headquarters to Miami in 1971 (left). 5/23/2017 Dr. N. RAMACHANDRAN, NITC 29 • The American Welding Society, in conjunction with the Department of Energy, has put together a vision that will carry the welding industry through 2020. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 30 • Technical Publications • AWS offers over 300 books, charts, videos, replicas, proceedings, and software. 160 AWSdeveloped codes, recommended practices, and guides are produced under strict American National Standards Institute (ANSI) procedures, including one of the most consulted codes in the world, D1.1 Structural Welding Code - Steel. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 31 Foundation • Founded in 1989, to support research and education in welding and related technologies. It is committed to annually awarding fellowships to deserving graduate students for important research in areas important to the requirements of industry. Accordingly, each year the AWS Foundation administers six $20,000 grants matched in kind by the participating universities. The award of scholarships to vocational and undergraduate college students is also a high priority and a student loan program has also been developed to prepare students for welding relatedDr. careers. 5/23/2017 N. RAMACHANDRAN, NITC 32 • The Professional Program The AWS Professional Program offers a broad spectrum of Technical Papers describing the latest findings in welding research, processes and applications. Special sessions and gatherings exploring the boundaries of industry issues are also significant features of the convention. Subjects cover an entire range of industry concerns from the joining of space age materials to production management techniques, testing, quality assurance and more. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 33 Which welding process(es) will see an increase in use and which will see a decrease in use during the next decade? 5/23/2017 • There was much speculation, but almost unanimously the process chosen for decline was shielded metal arc welding (SMAW). A very few speculated a decline in the use of gas metal arc (GMAW) and gas tungsten arc welding (GTAW). A significant group felt the continuous wire processes (FCAW, GMAW) would experience the most use. The GTAW process was the next most mentioned. One of the reasons stated for its increase was "the need for high-quality work on thin materials." NITC Dr. N. RAMACHANDRAN, 34 Welding Forges into the Future Where do you see the use of welding automation heading in your industry? 5/23/2017 Dr. N. RAMACHANDRAN, NITC 35 • In what areas of welding do we need more knowledge? • Safety and Health. The industry needs more knowledge and awareness regarding the hazards of welding, according to the respondents. Welding of the newer grades of high-strength steels, high- alloy steels and heat treatable steels. • We need to "keep up the 'how to weld' information with the increase in 'new' alloys, which are becoming more difficult to weld." Automation. A variety of topics relating to automation. These included training in computerization and automation; information on short-run automation; and the need to create standard platforms for welding equipment, robot controllers, sensing devices and other automation peripherals. The basics While universities and institutions are doing basic research, they cannot tell you the best process and fastest speed for a 1Ž4-in. fillet weld." 5/23/2017 Dr. N. RAMACHANDRAN, NITC 36 • What are the strengths of the welding industry? What are its weaknesses? • What business improvements during the next ten years would be in your company's best interests? • What has to be done in the future to keep the welding industry healthy? More than 50% of the respondents believe improving the image of welding so top students will be drawn to the industry and bettering training methods for welders and welding engineers are the keys to welding's future. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 37 • Are you optimistic or pessimistic about the future of your particular industry? 92% of respondents indicated they are at least optimistic about the future. One respondent summed up his reasons this way: Metallics will be around for a long time and they will need to be joined. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 38 • Since time machines still exist only in the stories of H. G. Wells and other works of science fiction, no one can tell us exactly how welding will fare in the 21st century. However, the people who responded to the Welding Journal survey represent a cross section of fabricators of welded products and producers of welding equipment and related products. Together they offer a wide range of experience and knowledge. Answering the questions separately, in their respective cities, they still formed a consensus. They agree the future looks promising for welding. It remains and will continue to be a productive, cost-effective manufacturing method. However, steps must be taken to bring more skilled personnel into the industry, or changes must be made to accommodate for the lack of skilled personnel (e.g., welding automation). They also indicated the welding industry must embrace all of the modern-day technological tools to keep pace with the rest of the 5/23/2017 Dr. N. RAMACHANDRAN, NITC 39 world. . LIQUID STATE PROCESSES • Partial melting and fusion of joint • Physical and mechanical changes taking place • Can be with application of pressure or by addition of filler material • Prior to joining, PREPARATION TO BE DONE STANDARDS- AWS; ASTMTYPES OF GROOVES, JOINTS 5/23/2017 Dr. N. RAMACHANDRAN, NITC 40NITC Types of welds and symbols • • • • • • • • FILLET, SQUARE BUTT, SINGLE V, DOUBLE V, SINGLE U, DOUBLE U, SINGLE BEVEL BUTT, DOUBLE BEVEL BUTT, SINGLE J BUTT, DOUBLE J BUTT, STUD, BEAD(EDGE OR SEAL), PLUG, SPOT, SEAM, MASHED SEAM, STITCH, PROJECTION, FLASH, UPSET etc. (REFER sketches supplied) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 41NITC Standard location of elements of weld symbol G- Grind C- Chip F-File M-Machine Size Specification process. R- Rolling Length of weld Unwelded length Finish symbol S Weld all around L P No tailSMAW Field weld Reference line Other side of arrow Near side of Arrow 5/23/2017 Arrow connecting reference line to arrow side of joint /to edge prepared /member or both Dr. N. RAMACHANDRAN, NITC 42NITC Groove face GROOVE ANGLE Joint angle ROOT 5/23/2017 Dr. N. RAMACHANDRAN, NITC Root Face 43NITC WELD POSITIONS • • • • FLAT HORIZONTAL VERTICAL OVERHEAD 5/23/2017 WELD MOVEMENTS •H •O •C •J •U • ZIGZAG Dr. N. RAMACHANDRAN, NITC 44NITC WELDING TERMINOLOGY Slide 2 of 18 ELECTRODE COATING INGREDIENTS • Slag forming ingredients- silicates of sodium, potassium, Mg, Al, iron oxide, China clay, mica etc. • Gas shielding- cellulose, wood, starch, calcium carbonate • De-oxidising elements- ferro manganese, ferro silicon- to refine molten metal • Arc stabilizing – calcium carbonate, potassium silicate, titanates, Mg silicate etc. • .Alloying elements- ferro alloys, Mn, Mo., to impart special properties • Iron powder- to improve arc behaviour, bead appearance • Other elements - to improve penetration, limit spatter, improve metal deposition rates, 5/23/2017 Dr. N. RAMACHANDRAN, NITC 46 PURPOSE OF COATING • Gives out inert or protective gas- shields • Stabilizes the arc- by chemicals • Low rate consumption of electrode- directs arc and molten metal • Removes impurities and oxides as slag • Coatings act as insulators- so narrow grooves welded • Provide means to introduce alloying elements Bare electrodes - carbon- more conductive- slow consumption in welding 5/23/2017 Dr. N. RAMACHANDRAN, NITC 47 WELDING TECHNIQUES FOREHAND BACKHAND THIN Same direction torch Heat concentrated away from bead THICK Opposite direction torch Heat concentrated on bead 5/23/2017flow, rippled design Dr. N. RAMACHANDRAN, NITC Even Broad bead 48 WELD MOVEMENTS I L O 5/23/2017 STRAIGHT Z ZIGZAG Dr. N. RAMACHANDRAN, NITC 49 ASME P Material Numbers Explained ASME has adopted their own designation for welding processes, which are very different from the ISO definitions adopted by EN24063. Designation Description OFW Oxyfuel Gas Welding SMAW Shielded Metal Arc Welding (MMA) SAW Submerged Arc Welding GMAW Gas Metal Arc Welding (MIG/MAG) FCAW Flux Cored Wire GTAW Gas Tungsten Arc Welding (TIG) PAW Plasma Arc Welding Straight polarity = Electrode -ve Reverse polarity = Electrode +ve 5/23/2017 Dr. N. RAMACHANDRAN, NITC 50 ASME F Numbers F Number General Description 1 Heavy rutile coated iron powder electrodes :- A5.1 : E7024 2 Most Rutile consumables such as :- A5.1 : E6013 3 Cellulosic electrodes such as :- A5.1 : E6011 4 Basic coated electrodes such as : A5.1 : E7016 and E7018 5 High alloy austenitic stainless steel and duplex :- A5.4 : E316L-16 6 Any steel solid or cored wire (with flux or metal) 2X Aluminium and its alloys 3X Copper and its alloys 4X Nickel alloys 5X Titanium 6X Zirconium 7X Hard Facing Overlay 5/23/2017 Note:- X represents any number 0 to 9 Dr. N. RAMACHANDRAN, NITC 51 ASME A Numbers These refer to the chemical analysis of the deposited weld and not the parent material. They only apply to welding procedures in steel materials. A1 Plain unalloyed carbon manganese steels. A2 to A4 Low alloy steels containing Moly and Chrome Moly A8 Austenitic stainless steels such as type 316. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 52 ASME Welding Positions Note the welding progression, (vertically upwards or downwards), must always be stated and it is an essential variable for both procedures and performance qualifications. Welding Positions For Groove welds:Test Position ISO and EN Flat 1G PA Horizontal 2G PC Vertical Upwards Progression 3G PF Vertical Downwards Progression 3G PG Overhead 4G PE Pipe Fixed Horizontal 5G PF Pipe Fixed @ 45 degrees Upwards 6G HL045 Pipe Fixed @ 45 degrees Downwards 6G JL045 Welding Position 5/23/2017 Dr. N. RAMACHANDRAN, NITC 53 G for Groove Welds F 5/23/2017 Dr. N. RAMACHANDRAN, NITC for Fillet Welds 54 G for Groove Welds F for Fillet Welds 5/23/2017 Dr. N. RAMACHANDRAN, NITC 55 5/23/2017 Dr. N. RAMACHANDRAN, NITC 56 Welding Positions For Fillet welds:Test Position ISO and EN Flat (Weld flat joint at 45 degrees) 1F PA Horizontal 2F PB 2FR PB Vertical Upwards Progression 3F PF Vertical Downwards Progression 3F PG Overhead 4F PD Pipe Fixed Horizontal 5F PF Welding Position Horizontal Rotated 5/23/2017 Dr. N. RAMACHANDRAN, NITC 57 5/23/2017 Dr. N. RAMACHANDRAN, NITC 58 5/23/2017 Dr. N. RAMACHANDRAN, NITC 59 5/23/2017 Dr. N. RAMACHANDRAN, NITC 60 Multiple-pass layers. 5/23/2017 Weld layer sequence Dr. N. RAMACHANDRAN, NITC 61 Welding Positions QW431.1 and QW461.2 Basically there are three inclinations involved. Flat, which includes from 0 to 15 degrees inclination 15 - 80 degrees inclination Vertical, 80 - 90 degrees For each of these inclinations the weld can be rotated from the flat position to Horizontal to overhead. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 62 Effects of expansion and contraction 5/23/2017 Dr. N. RAMACHANDRAN, NITC 63 CONTROLLING DISTORTION 5/23/2017 Dr. N. RAMACHANDRAN, NITC 64 HEAT AFFECTED ZONE 5/23/2017 Dr. N. RAMACHANDRAN, NITC 65 5/23/2017 Dr. N. RAMACHANDRAN, NITC 66 LIQUID STATE PROCESSES • Partial melting and fusion of joint • Physical and mechanical changes taking place • Can be with application of pressure or by addition of filler material • Prior to joining, PREPARATION TO BE DONE STANDARDS- AWS; ASTMTYPES OF GROOVES, JOINTS 5/23/2017 Dr. N. RAMACHANDRAN, NITC NITC 67 OXY ACETYLENE WELDING (OAW) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 68 Oxyacetylene Welding (OAW) The oxyacetylene welding process uses a combination of oxygen and acetylene gas to provide a high temperature flame. Oxyacetylene Welding (OAW) • OAW is a manual process in which the welder must personally control the the torch movement and filler rod application • The term oxyfuel gas welding outfit refers to all the equipment needed to weld. • Cylinders contain oxygen and acetylene gas at extremely high pressure. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 70 Typical Oxyacetylene Welding (OAW) Station 5/23/2017 Dr. N. RAMACHANDRAN, NITC 71 5/23/2017 Dr. N. RAMACHANDRAN, NITC 72 STEPS for OAW 1. PREPARE THE EDGES AND MAINTAIN PROPER POSITION………………………….(USE OF FIXTURES, CLAMPS) 2. OPEN ACETYLENE AND IGNITE 3. OPEN OXYGEN AND ADJUST FLAME 4. HOLD TORCH AT ABOUT 45O AND FILLER METAL AT 30 TO 40 O 5. TOUCH FILLER ROD TO JOINT AND CONTROL MOVEMENT 6. SINGLE BEAD MADE 5/23/2017 Dr. N. RAMACHANDRAN, NITC 73 • FOR DEEP JOINTS, MULTIPLE PASSES • CLEANING EACH WELD BEAD IS IMPORTANT • EQUIPMENT- WELDING TORCHVARIOUS SIZES AND SHAPES • CYLINDERS DIFFERENT THREADS, ANCHORED AND NOT DROPPED 5/23/2017 Dr. N. RAMACHANDRAN, NITC 74 CAPABILITIES • LOW COST. MANUAL AND HENCE SLOW • PORTABLE, VERSATILE AND ECONOMICAL FOR LOW QUANTITY AND REPAIR WORKS • FOR ALL FERROUS AND NONFERROUS METALS LIMITATIONS THICKNESS < 6 MM • SKILL ESSENTIAL---FOR PIPE, PRESSURE VESSELS, LOAD BEARING STRUCTURAL MEMBERS 5/23/2017 Dr. N. RAMACHANDRAN, NITC 75 Oxygen Cylinders • Oxygen is stored within cylinders of various sizes and pressures ranging from 20002640 PSI. (Pounds Per square inch) • Oxygen cylinders are forged from solid armor plate steel. No part of the cylinder may be less than 1/4” thick. • Cylinders are then tested to over 3,300 PSI using a (NDE) hydrostatic pressure test. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 76 Oxygen Cylinders • Cylinders are regularly re-tested using hydrostatic (NDE) while in service • Cylinders are regularly chemically cleaned and annealed to relieve “jobsite” stresses created by handling . 5/23/2017 Dr. N. RAMACHANDRAN, NITC 77 Cylinder Transportation • Never transport cylinders without the safety caps in place • Never transport with the regulators in place • Never allow bottles to stand freely. Always chain them to a secure cart or some other object that cannot be toppled easily. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 78 Oxygen Cylinders • Oxygen cylinders incorporate a thin metal “pressure safety disk” made from stainless steel and are designed to rupture prior to the cylinder becoming damaged by pressure. • The cylinder valve should always be handled carefully 5/23/2017 Dr. N. RAMACHANDRAN, NITC 79 Pressure Regulators for Cylinders • Reduce high storage cylinder pressure to lower working pressure. • Most regulators have a gauge for cylinder pressure and working pressure. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 80 Pressure Regulators for Cylinders • Regulators are shut off when the adjusting screw is turn out completely. • Regulators maintain a constant torch pressure although cylinder pressure may vary • Regulator diaphragms are made of stainless steel 5/23/2017 Dr. N. RAMACHANDRAN, NITC 81 Pressure Regulators Gauges Using a “Bourdon” movement • Gas entering the gauge fills a Bourdon tube • As pressure in the semicircular end increases it causes the free end of the tube to move outward. • This movement is transmitted through to a curved rack which engages a pinion gear on the pointer shaft ultimately showing pressure. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 82 Regulator Hoses • Hoses are are fabricated from rubber • Oxygen hoses are green in color and have right hand thread. • Acetylene hoses are red in color with left hand thread. • Left hand threads can be identified by a grove in the body of the nut and it may have “ACET” stamped on it 5/23/2017 Dr. N. RAMACHANDRAN, NITC 83 Check Valves & Flashback Arrestors • Check valves allow gas flow in one direction only • Flashback arrestors are designed to eliminate the possibility of an explosion at the cylinder. • Combination Check/ Flashback Valves can be placed at the torch or regulator. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 84 Acetylene Gas • Virtually all the acetylene distributed for welding and cutting use is created by allowing calcium carbide (a man made product) to react with water. • The nice thing about the calcium carbide method of producing acetylene is that it can be done on almost any scale desired. Placed in tightly-sealed cans, calcium carbide keeps indefinitely. For years, miners’ lamps produced acetylene by adding water, a drop at a time, to lumps of carbide. • Before acetylene in cylinders became available in almost every community of appreciable size produced their own gas from calcium 5/23/2017 carbide. Dr. N. RAMACHANDRAN, NITC 85 Acetylene Cylinders • Acetylene is stored in cylinders specially designed for this purpose only. • Acetylene is extremely unstable in its pure form at pressure above 15 PSI (Pounds per Square Inch) • Acetone is also present within the cylinder to stabilize the acetylene. • Acetylene cylinders should always be stored in the upright position to prevent the acetone form escaping thus causing the acetylene to become unstable. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 86 Acetylene Cylinders • Cylinders are filled with a very porous substance “monolithic filler” to help prevent large pockets of pure acetylene form forming • Cylinders have safety (Fuse) plugs in the top and bottom designed to melt at 212° F (100 °C) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 87 Acetylene Valves 5/23/2017 • Acetylene cylinder shut off valves should only be opened 1/4 to 1/2 turn • This will allow the cylinder to be closed quickly in case of fire. • Cylinder valve wrenches should be left in place on cylinders that do not have a hand wheel. Dr. N. RAMACHANDRAN, NITC 88 Oxygen and Acetylene Regulator Pressure Settings • Regulator pressure may vary with different torch styles and tip sizes. • PSI (pounds per square inch) is sometimes shown as PSIG (pounds per square inch -gauge) • Common gauge settings for cutting – 1/4” material Oxy 30-35psi Acet 3-9 psi – 1/2” material Oxy 55-85psi Acet 6-12 psi – 1” material Oxy 110-160psi Acet 7-15 psi • Check the torch manufactures data for optimum pressure settings NITC 5/23/2017 Dr. N. RAMACHANDRAN, 89 Regulator Pressure Settings • The maximum safe working pressure for acetylene is 15 PSI ! 5/23/2017 Dr. N. RAMACHANDRAN, NITC 90 Typical torch styles • A small welding torch, with throttle valves located at the front end of the handle. Ideally suited to sheet metal welding. Can be fitted with cutting • attachment in place of the welding head shown. Welding torches of this general design are by far the most widely used. They will handle any oxyacetylene welding job, can be fitted with multiflame (Rosebud) heads for heating applications, and accommodate cutting attachments that will cut steel 6 in. thick. • 5/23/2017 A full-size oxygen cutting torch which has all valves located in its rear body. Another style of cutting torch, with oxygen valves located at the front end of its handle. Dr. N. RAMACHANDRAN, NITC 91 Typical startup procedures • Verify that equipment visually appears safe IE: Hose condition, visibility of gauges • Clean torch orifices with a “tip cleaners” (a small wire gauge file set used to clean slag and dirt form the torch tip) • Crack (or open) cylinder valves slightly allowing pressure to enter the regulators slowly • Opening the cylinder valve quickly will “Slam” the regulator and will cause failure. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 92 Typical startup procedures • Never stand directly in the path of a regulator when opening the cylinder • Check for leaks using by listening for “Hissing” or by using a soapy “Bubble” solution • Adjust the regulators to the correct operating pressure • Slightly open and close the Oxygen and Acetylene valves at the torch head to purge any atmosphere from the system. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 93 Typical startup procedures • Always use a flint and steel spark lighter to light the oxygen acetylene flame. • Never use a butane lighter to light the flame 5/23/2017 Dr. N. RAMACHANDRAN, NITC 94 Flame Settings • There are three distinct types of oxy-acetylene flames, usually termed: – Neutral – Carburizing (or “excess acetylene”) – Oxidizing (or “excess oxygen” ) • The type of flame produced depends upon the ratio of oxygen to acetylene in the gas mixture which leaves the torch tip. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 95 TYPES of FLAMES • Neutral- with inner cone(30400C-33000C), outer envelope, (21000C near inner cone, 12600C at tip)- high heating • Reducing- Bright luminous inner cone, acetylene feather, blue envelope – Low temperature, good for brazing, soldering, flame hardening Hydrogen, methyl acetylene, propadiene also used as fuel. • Oxidising- pointed inner cone, small and narrow outer envelope – Harmful for steels, good for Cu- Cu based alloys 5/23/2017 Dr. N. RAMACHANDRAN, NITC 96NITC OXY ACETYLENE WELDING (OAW) Types of Flames Neutral Reducing high heating low temperature 5/23/2017 Dr. N. RAMACHANDRAN, NITC Oxidising good for Cu- Cu alloys 97 Pure Acetylene and Carburizing Flame profiles 5/23/2017 Dr. N. RAMACHANDRAN, NITC 98 Neutral and Oxidizing Flame Profiles 5/23/2017 Dr. N. RAMACHANDRAN, NITC 99 Flame definition • The neutral flame is produced when the ratio of oxygen to acetylene, in the mixture leaving the torch, is almost exactly one-to-one. It’s termed ”neutral” because it will usually have no chemical effect on the metal being welded. It will not oxidize the weld metal; it will not cause an increase in the carbon content of the weld metal. • The excess acetylene flame as its name implies, is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. Used on steel, it will cause an increase in the carbon content of the weld metal. • The oxidizing flame results from burning a mixture which contains more oxygen than required for a neutral flame. It will oxidize or ”burn” some of the metal being welded. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 100 Quiz time • The regulator diaphragm is often made from _______? A: reinforced rubber B: malleable iron C: tempered aluminum D: stainless steel 5/23/2017 Dr. N. RAMACHANDRAN, NITC 101 Quiz time • The hose nuts for oxygen and acetylene differ greatly, because the acetylene hose nut has. A: a left hand thread. B: has a grove cut around it. C: may have ACET stamped on it. D: All of the above. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 102 Quiz time • An oxygen cylinder must be able to withstand a ________ pressure of 3300 psi (22753 kPa) to be qualified for service. A: atmospheric B: hydrostatic C: hydroscopic D: vapor 5/23/2017 Dr. N. RAMACHANDRAN, NITC 103 Quiz time • Why is the area above 15 psig often marked with a red band on a acetylene low pressure regulator ? • Answer – Acetylene pressure above 15 psig is unstable and should not be used 5/23/2017 Dr. N. RAMACHANDRAN, NITC 104 Quiz time • True or False ? – A flint and steel spark lighter is the generally used to light the oxyacetylene flame. • Answer: True 5/23/2017 Dr. N. RAMACHANDRAN, NITC 105 Quiz time • Acetylene cylinder fuse plugs melt at a temperature of ________° F or 100°C • Answer – 212°F 5/23/2017 Dr. N. RAMACHANDRAN, NITC 106 Quiz time • What is the maximum safe working gauge pressure for acetylene gas? A: 8 psig (55 kPa) B: 15 psig (103 kPa) C: 22 psig (152 kPa) D: 30 psig (207 kPa) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 107 Quiz time • The color of and oxygen hose on a oxyacetylene welding outfit is ______? • Answer – Green/Blue 5/23/2017 Dr. N. RAMACHANDRAN, NITC 108 Quiz time • The type of safety device is used on a oxygen cylinder. A: A fusible plug B: A check valve C: A pressure safety disk D: A spring loaded plug 5/23/2017 Dr. N. RAMACHANDRAN, NITC 109 Quiz time • True or False ? – The regulator is closed when the adjusting screw is turned out. • Answer: True 5/23/2017 Dr. N. RAMACHANDRAN, NITC 110 Quiz time • The color of acetylene hose on a oxyacetylene welding outfit is ______? • Answer – Red 5/23/2017 Dr. N. RAMACHANDRAN, NITC 111 Quiz time • No part of an oxygen cylinder walls may be thinner than _______? A: 1/4”in (6.4 mm) B: 3/8”in (9.5 mm) C: 3/16”in (4.8 mm) D: 7/32”in (5.6 mm) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 112 Quiz time • To prevent the occurrence of flashbacks, a ________ should be installed between either the torch and hoses or regulators and hoses. A: a two way check valve. B: flame screen. C: flashback arrestor. D: three way check valve. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 113 Quiz time • What type of safety device is used on a acetylene cylinder. A: A spring loaded plug B: A pressure safety disk C: A fusible plug D: A check valve 5/23/2017 Dr. N. RAMACHANDRAN, NITC 114 Quiz time • Mixing _______ and water will produce acetylene gas. A: calcium carbide B: potassium carbonate C: carbon dioxide D: acetylene carbide 5/23/2017 Dr. N. RAMACHANDRAN, NITC 115 LIQUID STATE PROCESS PARTIAL MELTING BY STRIKING AN ARC AFTER THE INVENTION OF ELECTRICITY HOW ARC STRUCK? ARC COLUMN THEORY 5/23/2017 Dr. N. RAMACHANDRAN, NITC 116 ARC WELDING 5/23/2017 Dr. N. RAMACHANDRAN, NITC 118 • ARC WELDING ELECTRIC ARC WITHOUT ADDITIONAL AUTOGENEOUS EXTERNAL SOURCE NONCONSUMABLE- CONSUMABLE CARBON ARC WELDING (CAW) - OLDEST METALLIC ARC WELDING (MAW) COATING MATERIALS ARC TO BE CREATED BY ELECTRICITY WHEN? WITH THE INVENTION OF AC DYNAMO IN 1877 5/23/2017 Dr. N. RAMACHANDRAN, NITC 119 BEGINNING IN 1881- TO CONNECT PLATES OF STORAGE BATTERY 1886- BUTT WELDING TECHNIQUE WAS DEVELOPED BUTTED, CLAMPED HIGH CURRENT PASSED AT THE JOINT, RESISTANCE OF METAL TO ELECTRIC CURRENT PRODUCES HIGH HEAT- PIECES FUSED 5/23/2017 Dr. N. RAMACHANDRAN, NITC 120 ARC WELDING- MELTING AND FUSING OF METAL BY ELECTRODES 1ST BY N.V. BERNADO USING CARBON ELECTRODES CONSISTANTLY IMPROVED 1895 N.G. SLAVIANOFF USED METALLIC ELECTRODES 1905 BARE ELECTRODES COATED—SHIELDING--- (SAW) PORTABLE AND AUTOMATIC WELDING MACHINES 5/23/2017 Dr. N. RAMACHANDRAN, NITC 121 ARC WELDING PROCESSES USE OF CONSUMABLE ELECTRODES SHIELDED METAL ARC WELDING (SMAW) • SIMPLEST AND MOST VERSATILE • ABOUT 50% OF INDUSTRIAL WELDING BY THIS PROCESS • CURRENT- 50 TO 300 A, < 10 KW • AC/DC USED • FOR THICKNESSES UPTO 19 –20 MM 5/23/2017 Dr. N. RAMACHANDRAN, NITC 122 SHIELDED METAL ARC WELDING (SMAW) •Shielded metal arc welding (SMAW), •Also known as Manual Metal Arc (MMA) welding • Informally as stick welding is a manual arc welding process that uses a consumable electrode coated in flux to lay the weld. •An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 124 ARC COLUMN THEORY ELECTRICAL / IONIC THEORY IONS FROM ANODE TO CATHODE, AS METAL IONS ARE +VE CHARGED •TOUCH AND THEN ESTABLISH A GAP TO BALANCE THE ATOMIC STRUCTURE ANODE + •IONS COLLIDE WITH GAS MOLECULES •PRODUCES A THERMAL IONISATION LAYER DC CATHODE - •IONISED GAS COLUMN – AS HIGH RESISTANCE CONDUCTOR •ON STRIKING CATHODE, HEAT GENERATED •TERMED AS IONIC THEORY •NOT COMPLETE IN EXPLAINING ARC COLUMN THEORY •THUS, ELECTRON THEORY ELECTRON THEORY ARC COLUMN THEORY IONS FROM ANODE TO CATHODE AS METAL IONS ARE +VE CHARGED -VELY CHARGED ELECTRONS DISSOCIATED FROM CATHODE MOVE OPPOSITE WITH HIGH VELOCITY ANODE + DC CATHODE - (MASS- 9.1x 10-28 gm) CAUSES HEAT IN ARC COLUMN RELEASES HEAT ENERGY IN STRIKING THE ANODE CALLED ELECTRON IMPINGEMENT AND IONIC BOMBARDMENT 5/23/2017 Dr. N. RAMACHANDRAN, NITC 126 ANODE+ HIGH HEAT ELECTRON IMPINGEMENT LOW HEAT MEDIUM HEAT IONIC BOMBARDMENT CATHODE 5/23/2017 - Dr. N. RAMACHANDRAN, NITC 127 MAGNETIC FLUX THEORY • THE COLUMN NOT FLAIRING DUE TO THE FLUX LINES AROUND THE ARC COLUMN. (Right hand Thumb Rule) THIS COMPLETES THE ARC COLUMN THEORY 5/23/2017 Dr. N. RAMACHANDRAN, NITC 128 POLARITY AC 1. 2. 3. 4. 5. 6. 5/23/2017 Currents higher than those of DCRP can be employed (400 A to 500 Afor 6 mm electrode) Arc cleaning of the base metal Normal penetration Equal heat distribution at electrode and job Electrode tip is colder as compared to that in DCRP Average arc voltage in argon atmosphere is 16V Dr. N. RAMACHANDRAN, NITC 129 DCRP 1. 2. 3. 4. 5. 6. 5/23/2017 Currents generally less than 125 amps (upto 6 mm dia electrodes) to avoid overheating 2/3rd heat at electrode and 1/3rd at the job Least penetration Average arc voltage on argon atmosphere is 19V Chances of electrode overheating, melting and losses Better arc cleaning action Dr. N. RAMACHANDRAN, NITC 130 DCSP 1. 2. 3. 4. 5. 6. 5/23/2017 Welding currents upto 1000 amps can be employed for 6 mm electrodes 33.33% heat is generated at the electrode and 66.66% at the job. Deep penetration Average arc voltage in an argon atmsphere is 12 V Electrode runs colder as compared to AC or DCRP No arc cleaning of base metal Dr. N. RAMACHANDRAN, NITC 131 METALLURGY OF WELDING During joining, localized heating occurs. This leads to metallurgical and physical changes in materials welded. Hence, study of: . 1. Nature of welded joint 2. 3. 4. 5. 6. Quality and property of welded joint Weldability of metals Methods of testing welds Welding design Process selection- important (3) Heat Affected Zone (HAZ) (2) Fusion Zone 1) Base Metal ( Structures: (1) SMALL (2) MEDIUM (3) LARGE Properties of (2) and (3) important 5/23/2017 Dr. N. RAMACHANDRAN, NITC 133 • Cooling of Beadsimilar to a casting in mould, which is metallic here. Cooling is slow Hence the structure is coarse and Strength toughness and ductility low. But use of proper electrodes improves these. • The purpose of coating the electrode is to achieve the improved properties. If without, nitrides and oxides of base metal form and these result in weak and brittle nature. • With coating, properties comparable with base metal achieved. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 134 Gas shield Arc column makes CRATER on striking the surface- Temperature above 1500 C Flux + impurities- less dense. Floats as SLAG Slag prevents heat loss- makes an evenly distribution of heat radiation. Preheating to receive the molten metal at an elevated temperature and modify the structure. Not for M.S. Locked in stresses due to heating and cooling- to be relieved by PEENING, or other heat treatment processes. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 135 MAGNETIC ARC BLOW -- FOR AC SUPPLY. Current through conductor- magnetic Flux lines perpendicular to current flow- apply Right hand Thumb Rule. Three areas of magnetic field 1. Arc; 2. Electrode; 3. Work piece, when ground. Forward pull of Arc column results, called as Magnetic Arc Blow. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 136 EQUIPMENT 5/23/2017 Dr. N. RAMACHANDRAN, NITC 137 5/23/2017 Dr. N. RAMACHANDRAN, NITC 138 • As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination. • Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is one of the world's most popular welding processes. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 139 PURPOSE OF COATING • Gives out inert or protective gas- shields • Stabilizes the arc- by chemicals • Low rate consumption of electrode- directs arc and molten metal • Removes impurities and oxides as slag • Coatings act as insulators- so narrow grooves welded • Provide means to introduce alloying elements Bare electrodes - carbon- more conductive- slow consumption in welding 5/23/2017 Dr. N. RAMACHANDRAN, NITC 140 ELECTRODE COATING INGREDIENTS • Slag forming ingredients- silicates of sodium, potassium, Mg, Al, iron oxide, China clay, mica etc. • Gas shielding- cellulose, wood, starch, calcium carbonate • De-oxidising elements- ferro manganese, ferro silicon- to refine molten metal • Arc stabilizing – calcium carbonate, potassium silicate, titanates, Mg silicate etc. • .Alloying elements- ferro alloys, Mn, Mo., to impart special properties • Iron powder- to improve arc behaviour, bead appearance • Other elements - to improve penetration, limit spatter, improve metal deposition rates, 5/23/2017 Dr. N. RAMACHANDRAN, NITC 141 WELD POSITIONS • FLAT 5/23/2017 HORIZONTAL VERTICAL Dr. N. RAMACHANDRAN, NITC OVERHEAD 142NITC WELD MOVEMENTS I L O 5/23/2017 STRAIGHT Z ZIGZAG Dr. N. RAMACHANDRAN, NITC 143 WELDING TECHNIQUES FOREHAND BACKHAND THIN Same direction torch Heat concentrated away from bead THICK Opposite direction torch Heat concentrated on bead 5/23/2017flow, rippled design Dr. N. RAMACHANDRAN, NITC Even Broad bead 144 • It dominates other welding processes in the maintenance and repair industry, used extensively in the construction of steel structures and in industrial fabrication. • The process is used primarily to weld iron and steels (including stainless steel) but aluminum, nickel and copper alloys can also be welded with this method. • Flux-Cored Arc Welding (FCAW) , a modification to SMAW is growing in popularity 5/23/2017 Dr. N. RAMACHANDRAN, NITC 145 Various andNITC an electrode holder 5/23/2017 welding electrodes Dr. N. RAMACHANDRAN, 146 SAFETY PRECAUTIONS • Uses an open electric arc, so risk of burns – to be prevented by protective clothing in the form of heavy leather gloves and long sleeve jackets. •The brightness of the weld area can lead arc eye, in which ultraviolet light causes the inflammation of the cornea and can burn the retinas of the eyes. •Welding helmets with dark face plates to be worn to prevent this exposure 5/23/2017 Dr. N. RAMACHANDRAN, NITC 147 • New helmet models have been produced that feature a face plate that self-darkens upon exposure to high amounts of UV light • To protect bystanders, especially in industrial environments, transparent welding curtains often surround the welding area. • These are made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc, but should not be used to replace the filter glass used in helmets. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 148 ARC EYE Arc eye, also known as arc flash or welder's flash or corneal flash burns, is a painful condition sometimes experienced by welders who have failed to use adequate eye protection. It can also occur due to light from sunbeds, light reflected from snow (known as snow blindness), water or sand. The intense ultraviolet light emitted by the arc causes a superficial and painful keratitis. Symptoms tend to occur a number of hours after exposure and typically resolve spontaneously within 36 hours. It has been described as having sand poured into the eyes. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 149 Signs Intense lacrimation Blepharospasm Photophobia Fluorescein dye staining will reveal corneal ulcers under blue light Management • Instill topical anaesthesia • Inspect the cornea for any foreign body • Patch the worse of the two eyes and prescribe analgesia • Topical antibiotics in the form of eye drops or eye ointment or both should be prescribed for prophylaxis against infection 5/23/2017 Dr. N. RAMACHANDRAN, NITC 150 SUBMERGED ARC WELDING (SAW) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 151 5/23/2017 CONTROL PANEL Dr. N. RAMACHANDRAN, NITC 152 Submerged Arc Welding (SAW) • Is a common arc welding process. • A continuously fed consumable solid or tubular (metal cored) electrode used. • The molten weld and the arc zone are protected from atmospheric contamination by being “submerged” under a blanket of granular fusible flux. • When molten, the flux becomes conductive, and provides a current path between the electrode and the work 5/23/2017 Dr. N. RAMACHANDRAN, NITC 153 • Normally operated in the automatic or mechanized mode. • Semi-automatic (hand-held) SAW guns with pressurized or gravity flux feed delivery are available. • The process is normally limited to the 1F, 1G, or the 2F positions (although 2G position welds have been done with a special arrangement to support the flux). Deposition rates approaching 45 kg/h have been reported — this compares to ~5 kg/h (max) for shielded metal arc welding. • Currents ranging from 200 to 1500 A are commonly used; currents of up to 5000 A have been used (multiple arcs). 5/23/2017 Dr. N. RAMACHANDRAN, NITC 154 • Single or multiple (2 to 5) electrode wire variations of the process exist • SAW strip-cladding utilizes a flat strip electrode (e.g. 60 mm wide x 0.5 mm thick). • DC or AC power can be utilized, and combinations of DC and AC are common on multiple electrode systems. • Constant Voltage welding power supplies are most commonly used, however Constant Current systems in combination with a voltage sensing wire-feeder are available. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 155 5/23/2017 Dr. N. RAMACHANDRAN, NITC 156 5/23/2017 Dr. N. RAMACHANDRAN, NITC 157 5/23/2017 Dr. N. RAMACHANDRAN, NITC 158 5/23/2017 Dr. N. RAMACHANDRAN, NITC 159 5/23/2017 Dr. N. RAMACHANDRAN, NITC 160 SAW • • • • • • • • • Fusion Welding Process Automatic / Semi Automatic Arc Between Consumable Electrode And Work Arc Covered Under granular Flux Wire / Electrode Continuously Fed To Weld Pool Wire / Arc Under Flux Moves Along The Groove Wire, BM & Flux Close to Arc Melt Under Flux On Cooling Weld Metal Solidifies Molten Flux Forms Thick Slag Coating On Weld SAW Hopper Flux Wire Flux Power Source + + Slag Weld •••••••••••••••• ••• Arc Base Metal – Flux For SAW • • • • • Sodium Chloride Potassium Chloride Titanium Dioxide Sodium Silicate Deoxidizing Agents Types Of Flux • Fused Flux • Agglomerated Flux » Neutral Flux » Active Flux Types Of Flux • Neutral Flux -Wire compatible to base metal - Single flux suitable for several material • Active Flux - Single flux suitable for specific application - Wire may be different from basemetal - To be welded within the recommended parameters Function Of Flux In SAW • • • • • • • • • • Stabilizes Arc Prevents contamination of weld metal Cleans the weld from unwanted impurities Increases Fluidity of molten metal Generates inert gas shielding while metal transfers Forms slag after melting & covers weld Allows deposited metal to cool slowly Compensates alloying elements Within the weld Eliminates spatter generation Helps in even & uniform bead finish Baking Requirements For Flux • • • • • • Spread the loose Flux in a Tray Of baking Oven Identify The Tray With The Quality/Grade Of Flux Bake Tray in an Oven Between 300° C to 350° C Baking Time 2 Hrs to 3 Hrs Reduce the temperature to 100 ° C to 150 ° C Hold the Flux at this temperature till use Why Baking Flux? • To remove the moisture (H2O) • To avoid possible cracking of weld due to H2 How Does Moist Flux Generate Crack Within Weld? • Moist Flux introduce atomic hydrogen at high temperature in weld • On cooling, atomic hydrogen try to form molecules • The reaction results in stresses and fine cracks • Cracks occur within hardened metal - HAZ • Known as “Hydrogen Embrittlement” or “Under Bead Crack” or Delayed Crack Reuse Of Flux • Flux May Be Reused Provided - Weld Not Highly Critical In Impact / Chemistry - Reuse Limited To Maximum Twice - All Slag Particles Are sieved & Removed - Rebaked If not Remained In Hot - Minimum 50% Fresh Flux Well Mixed - Customer Spec. Doesn't Prohibit The Same Types Of Power Source • Thyrester – DC • Rectifier – DC • Motor Generator – DC • Transformer - AC Characteristic Of Power Source Machine welding Drooping – Cons. A Linear – Cons. V V V V1 V1 V2 V2 A1 A2 A A1 A2 A SAW Wire - Electrode • • • • • • • Consumable Electrode / Wire Layer Wound On Spool / Coil CS & LAS Wires Coated with Cu Conducts Current and generates Arc Chemistry Compatible To Base Metal Grade Of Flux Can Be Same For CS & LAS Wire melts & deposited as filler in joint Typical Welding Parameter Sr no Wire Ø mm Current A Voltage V Speed mm/min Dep. Rate Per Arc Hr 1 1.6 200-300 22-26 750-1500 3 – 4 kgs 2 2 250-350 24-26 750-1250 3- 4.5 kgs 4 2.5 300-350 25-27 750-1250 4 –4.5 kgs 5 3 400-500 28-30 500-100 5 – 5.5 kgs 6 4 550-650 30-32 400-750 5.5 - 7 kgs 7 5 600-800 30-34 350-700 6 - 8 kgs Wire & Flux CS wire + Neutral Flux Important Terminology used in Critical SAW • • • • • Preheating Post Heating or Dehydrogenation Intermediate Stress leaving Inter pass Temperature Post Weld Heat Treatment What Is Preheating? • Heating the base metal along the weld joint to a predetermined minimum temperature immediately before starting the weld. • Heating by Oxy fuel flame or electric resistant coil • Heating from opposite side of welding wherever possible • Temperature to be verified by thermo chalks prior to starting the weld Why Preheating? • Preheating eliminates possible cracking of weld and HAZ • Applicable to -Hardenable low alloy steels of all thickness -Carbon steels of thickness above 25 mm. -Restrained welds of all thickness • Preheating temperature vary from 75°C to 200°C depending on hardenability of material, thickness & joint restrain How does Preheating Eliminate Crack? • Preheating promotes slow cooling of weld and HAZ • Slow cooling softens or prevents hardening of weld and HAZ • Soft material not prone to crack even in restrained condition What Is Post Heating? • Raising the pre heating temperature of the weld joint to a predetermined temperature range (250° C to 350° C) for a minimum period of time (3 Hrs) before the weld cools down to room temperature. • Post heating performed when welding is completed or terminated any time in between. • Heating by Oxy fuel flame or electric resistant coil • Heating from opposite side of welding wherever possible • Temperature verified by thermo chalks during the period Why Post Heating? • Post heating eliminates possible delayed cracking of weld and HAZ • Applicable to -Thicker hardenable low alloy steels -Restrained hardenable welds of all thickness • Post heating temperature and duration depends on hardenability of material, thickness & joint restrain How does Post Heating Eliminate Crack? • SAW introduces hydrogen in weld metal • Entrapped hydrogen in weld metal induces delayed cracks unless removed before cooling to room temperature • Retaining the weld at a higher temperature for a longer duration allows the hydrogen to come out of weld What Is Intermediate Stress Relieving? • Heat treating a subassembly in a furnace to a predetermined cycle immediately on completion of critical restrained weld joint / joints without allowing the welds to go down the pre heat temperature. Rate of heating, Soaking temperature, Soaking time and rate of cooling depends on material quality and thickness • Applicable to Highly restrained air hardenable material Why Intermediate Stress Relieving? • Restrained welds in air hardenable steel highly prone to crack on cooling to room temperature. • Cracks due to entrapped hydrogen and built in stress • Intermediate stress relieving relieves built in stresses and entrapped hydrogen making the joint free from crack prone What Is Inter- Pass Temperature? • The temperature of a previously layed weld bead immediately before depositing the next bead over it • Temperature to be verified by thermo chalk prior to starting next bead • Applicable to Stainless Steel Carbon Steel & LAS with minimum impact Why Inter Pass Temperature? • Control on inter pass temperature avoids over heating, there by -Refines the weld metal with fine grains -Improves the notch toughness properties -Minimize the loss of alloying elements in welds -Reduces the distortion What Is Post Weld Heat Treatment? • Heat treating an assembly on completion of all applicable welding, in an enclosed furnace with controlled heating/cooling rate and soaking at a specific temperature for a specific time. • Rate of heating, Soaking temperature, Soaking time and rate of cooling depends on material quality and thickness • Applicable to -All type of CS & LAS Material applications • Carbon steels (structural and vessel construction); • Low alloy steels; • Stainless Steels; • Nickel-based alloys; • Surfacing applications (wearfacing, buildup, and corrosion resistant overlay of steels). 5/23/2017 Dr. N. RAMACHANDRAN, NITC 187 Advantages of SAW • High deposition rates (over45 kg/h) have been reported; • High operating factors in mechanized applications; • Deep weld penetration; • Sound welds are readily made (with good process design and control); • High speed welding of thin sheet steels at over 2.5 m/min is possible; • Minimal welding fume or arc light is emitted. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 188 Limitations of SAW • Limited to ferrous (steel or stainless steels) and some nickel based alloys; • Normally limited to the 1F, 1G, and 2F positions; • Normally limited to long straight seams or rotated pipes or vessels; • Requires relatively troublesome flux handling systems; • Flux and slag residue can present a health & safety issue; • Requires inter-pass and post weld slag removal. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 189 Key SAW process variables • • • • • Wire Feed Speed (main factor in welding current control); Arc Voltage; Travel Speed; Electrical Stick-Out (ESO) or Contact Tip to Work (CTTW); Polarity and Current Type (AC or DC). Other factors • • • • Flux depth/width; Flux and electrode classification and type; Electrode wire diameter; Multiple electrode configurations. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 190 GAS TUNGSTEN ARC WELDING (GTAW) GTAW 5/23/2017 Dr. N. RAMACHANDRAN, NITC 192 GTAW • Fusion Welding Process • Arc Between Non-Consumable Tungsten Rod And Work • Arc & Weld Pool Shielded By Argon/Gas • Filler Wire Separately Added To Weld Pool • Welding Torch & Tungsten Rod Cooled by Flow OF Argon / Cooling Water 5/23/2017 Dr. N. RAMACHANDRAN, NITC 193 GAS TUNGSTEN ARC WELDING (GTAW) • ELECTRODE NOT CONSUMED • TUNGSTEN ELECTRODES USED • ARGON- HEAVIER FOR NARROW AND LIMITED EXPANSION,WIDER, DEEPER PUDDLE • HELIUM FOR EVEN EXPANSIONLIMITED STRESS BUILDUP • MORE He, MORE HEAT IN ARC • Ar-He MIX FOR AUTOMATIC GTAW • Ar- CO2 FOR CARBON STEELS, ECONIMICAL, INCREASES WETTING ACTION • GTAW TORCH- WATER OR AIR COOLED CONSTANT CURRENT SOURCE.(IIIr TO SMAW) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 194 GTAW Equipment & Accessories • Power Source – Inverter, Thyrister, Rectifier, • • • • • • • Generator High Frequency Unit Water Cooling System Welding Torch- (Ceramic Cup, Tungsten Rod, Collet, Gas-lens) Pedal Switch Argon Gas Cylinder Pressure Gauge, Regulator, Flow Meter Earthing Cable With Clamp 5/23/2017 Dr. N. RAMACHANDRAN, NITC 195 Equipment & Accessories Pressure Regulator Flow Meter Tungsten Rod Argon Gas In Cooling Water In Solenoid Valve Argon Cylinder Gas Lens Ceramic Cup Welding Cable & Cooling Water In Tube Cooling Water Out Argon Shielding Arc + HF Unit & Water Cooling System High Frequency Connection Work Pedal Switch 5/23/2017 Power Source Dr. N. RAMACHANDRAN, NITC – + 196 Equipment GTAW torch with various electrodes, cups, collets and gas diffusers 5/23/2017 GTAW torch, disassembled Dr. N. RAMACHANDRAN, NITC 197 Gas tungsten arc welding (GTAW), commonly known as Tungsten Inert Gas (TIG) welding • Is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. • The weld area is protected from atmospheric contamination by a shielding gas (usually an inert gas such as argon), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. • A constant current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 198 • Most commonly used to weld thin sections of stainless steel and light metals such as aluminum, magnesium, and copper alloys. • The process grants the operator greater control over the weld than competing procedures such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. • GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. • A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 199 GTAW system setup 5/23/2017 Dr. N. RAMACHANDRAN, NITC 200 Applications • • • • • • Aerospace industry is one of the primary users of gas tungsten arc welding, the process is used in a number of other areas. Many industries use GTAW for welding thin workpieces, especially nonferrous metals. It is used extensively in the manufacture of space vehicles, and is also frequently employed to weld small-diameter, thin-wall tubing. Is often used to make root or first pass welds for piping of various sizes. In maintenance and repair work, the process is commonly used to repair tools and dies, especially components made of aluminum and magnesium. Because the welds it produces are highly resistant to corrosion and cracking over long time periods, GTAW is the welding procedure of choice for critical welding operations like sealing spent nuclear fuel canisters before burial. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 201 GTAW ranks the highest in terms of the quality of weld produced. Operation must be with free from oil, moisture, dirt and other impurities, as these cause weld porosity and consequently a decrease in weld strength and quality. To remove oil & grease, alcohol or similar commercial solvents used, while a stainless steel wire brush or chemical process remove oxides from the surfaces of metals like aluminum. Rust on steels removed by first grit blasting the surface and then using a wire brush to remove imbedded grit. These steps important when DCEN used, because this provides no cleaning during the welding process, unlike DCEPor AC. To maintain a clean weld pool during welding, the shielding gas flow should be sufficient and consistent so that the gas covers the weld and blocks impurities in the atmosphere. GTA welding in windy or drafty environments increases the 5/23/2017 Dr. N.toRAMACHANDRAN, NITC amount of shielding gas necessary protect the weld, increasing the cost and 202 making the process unpopular outdoors. Quality • Because of GTAW's relative difficulty and the importance of proper technique, skilled operators are employed for important applications. • Low heat input, caused by low welding current or high welding speed, can limit penetration and cause the weld bead to lift away from the surface being welded. • If there is too much heat input, the weld bead grows in width while the likelihood of excessive penetration and spatter increase. • If the welder holds the welding torch too far from the workpiece, shielding gas is wasted and the appearance of the weld worsens. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 203 • If the amount of current used exceeds the capability of the electrode, tungsten inclusions in the weld may result. Known as tungsten spitting, it can be identified with radiography and prevented by changing the type of electrode or increasing the electrode diameter. • If the electrode is not well protected by the gas shield or the operator accidentally allows it to contact the molten metal, it can become dirty or contaminated. This often causes the welding arc to become unstable, requiring that electrode be ground with a diamond abrasive to remove the impurity. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 204 • GTAW welding torches designed for either automatic or manual operation and are equipped with cooling systems using air or water. The automatic and manual torches are similar in construction, but the manual torch has a handle while the automatic torch normally comes with a mounting rack. • The angle between the centerline of the handle and the centerline of the tungsten electrode, known as the head angle, can be varied on some manual torches according to the preference of the operator. • Air cooling systems are most often used for lowcurrent operations (up to about 200 A), while water cooling is required for high-current welding (up to about 600 A). • The torches are connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 205 • The internal metal parts of a torch are made of hard alloys of copper or brass in order to transmit current and heat effectively. • The tungsten electrode must be held firmly in the center of the torch with an appropriately sized collet, and ports around the electrode provide a constant flow of shielding gas. • The body of the torch is made of heat-resistant, insulating plastics covering the metal components, providing insulation from heat and electricity to protect the welder. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 206 GTAW TORCH Torch Handle Cap with collet For Holding Tungsten Cooling Water Outlet Argon Gas Inlet Cooling Water Inlet Tube with cable Ceramic Cup Argon Shielding Gas Tungsten Rod Base Metal Earthing Cable 5/23/2017 Arc Dr. N. RAMACHANDRAN, NITC 207 • The size of the welding torch nozzle depends on the size of the desired welding arc, and the inside diameter of the nozzle is normally at least three times the diameter of the electrode. • The nozzle must be heat resistant and thus is normally made of alumina or a ceramic material, but fused quartz, a glass-like substance, offers greater visibility. • Devices can be inserted into the nozzle for special applications, such as gas lenses or valves to control shielding gas flow and switches to control welding current. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 208 Power supply 5/23/2017 • GTAW uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. • This is important because most applications of GTAW are manual or semiautomatic, requiring that an operator hold the torch. • Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult. Dr. N. RAMACHANDRAN, NITC 209 • The preferred polarity of the GTAW system depends largely on the type of metal being welded. • DCEN is often employed when welding steels, nickel, titanium, and other metals. It can also be used in automatic GTA welding of aluminum or magnesium when helium is used as a shielding gas. The negatively charged electrode generates heat by emitting electrons which travel across the arc, causing thermal ionization of the shielding gas and increasing the temperature of the base material. The ionized shielding gas flows toward the electrode, not the base material, and this can allow oxides to build on the surface of the weld. • DCEP is less common, and is used primarily for shallow welds since less heat is generated in the base material. Instead of flowing from the electrode to the base material, as in DCEN, electrons go the other direction, causing the electrode to reach very high temperatures. To help it maintain its shape and prevent softening, a larger electrode is often used. As the electrons flow toward the electrode, ionized shielding gas flows back toward the base material, cleaning the weld by removing oxides and other impurities and thereby improving its quality and appearance. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 210 • AC commonly used when welding aluminum and magnesium manually or semi-automatically, combines the two direct currents by making the electrode and base material alternate between positive and negative charge. This causes the electron flow to switch directions constantly, preventing the tungsten electrode from overheating while maintaining the heat in the base material. This makes the ionized shielding gas constantly switch its direction of flow, causing impurities to be removed during a portion of the cycle. • Some power supplies enable operators to use an unbalanced alternating current wave by modifying the exact percentage of time that the current spends in each state of polarity, giving them more control over the amount of heat and cleaning action supplied by the power source. • In addition, operators must be wary of rectification, in which the arc fails to reignite as it passes from straight polarity (negative electrode) to reverse polarity (positive electrode). • To remedy the problem, a square wave power supply can be used, as can high frequency voltage to 5/23/2017 Dr. N. RAMACHANDRAN, NITC 211 encourage ignition. Tungsten Rod Tungsten Rod • Non Consumable Electrode. • Maintains Stable Arc • Tip to be Ground to a cone Shape of 60º to 30º angle • Thoriated Tungsten for General Application, Zerconiated Tungsten for Aluminium Welding • Sizes :- 2, 2.4 & 3 mm Ø 5/23/2017 Dr. N. RAMACHANDRAN, NITC Ground to 50º ankle 212 •The electrode used in GTAW is ISO made of tungsten or a tungsten alloy, ISO Color Class because tungsten has the highest melting temperature among metals, at 3422 °C. WP Green • The electrode is not consumed WC20 Gray during welding, though some erosion WL10 Black (called burn-off) can occur. WL15 Gold •Electrodes can have either a clean finish or a ground finish—clean finish WL20 Sky-blue electrodes have been chemically WT10 Yellow cleaned, while ground finish WT20 Red electrodes have been ground to a WT30 Violet uniform size and have a polished surface, making them optimal for WT40 Orange heat conduction. WY20 Blue •The diameter of the electrode can WZ3 Brown vary between 0.5 mm and 6.4 mm, White and their length can range from 75 to WZ8 610 mm . 5/23/2017 Dr. N. RAMACHANDRAN, NITC AWS Class AWS Color Alloy [18] EWP Green None EWCe-2 Orange ~2% CeO2 EWLa-1 Black ~1% LaO2 EWLa-1.5 Gold ~1.5% LaO2 EWLa-2 Blue ~2% LaO2 EWTh-1 Yellow ~1% ThO2 EWTh-2 Red ~2% ThO2 ~3% ThO2 ~4% ThO2 ~2% Y2O3 EWZr-1 Brown ~0.3% ZrO2 ~0.8% ZrO2 213 • A number of tungsten alloys have been standardized by the International Organization for Standardization and the American Welding Society in ISO 6848 and AWS A5.12, respectively, for use in GTAW electrodes- refer table • Pure tungsten electrodes (classified as WP or EWP) are general purpose and low cost electrodes. Cerium oxide (or ceria) as an alloying element improves arc stability and ease of starting while decreasing burn-off. Using an alloy of lanthanum oxide (or lanthana) has a similar effect. Thorium oxide (or thoria) alloy electrodes were designed for DC applications and can withstand somewhat higher temperatures while providing many of the benefits of other alloys. However, it is somewhat radioactive, and as a replacement, electrodes with larger concentrations of lanthanum oxide can be used. Electrodes containing zirconium oxide (or zirconia) increase the current capacity while improving arc stability and starting and increasing electrode life. • • • Electrode manufacturers may create alternative tungsten alloys with specified metal additions, and these are designated with the classification EWG under the AWS system. Filler metals are also used in nearly all applications of GTAW, the major exception being the welding of thin materials. Filler metals are available with different diameters and are made of a variety of materials. In most cases, the filler metal in the form of a rod is added to the weld pool manually, but some applications call for an automatically fed filler metal, which is fed from 5/23/2017 Dr. N. RAMACHANDRAN, NITC 214 rolls. shielding gases • Necessary in GTAW to protect the welding area from atmospheric gases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weld metal embrittlement if they come in contact with the electrode, the arc, or the welding metal. The gas also transfers heat from the tungsten electrode to the metal, and it helps start and maintain a stable arc. • The selection of a shielding gas depends on several factors, including the type of material being welded, joint design, and desired final weld appearance. • Argon is the most commonly used shielding gas for GTAW, since it helps prevent defects due to a varying arc length. When used with alternating current, the use of argon results in high weld quality and good appearance. • Another common shielding gas, helium, is most often used to increase the weld penetration in a joint, to increase the welding speed, and to weld conductive metals like copper and aluminum. • A significant disadvantage is the difficulty of striking an arc with helium gas, and the decreased weld quality associated with a varying arc length. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 215 Shielding Gas • • • • • • • • Inert Gas - Argon , Helium Common Shielding Gas – Argon When Helium Is Used – Called Heli – Arc Welding When Argon Is Used – Called Argon Arc Welding Inert Gas Prevents Contamination Of Molten Metal It Prevents Oxidation Of Tungsten Rod It Ionizes Air Gap and Stabilizes Arc It Cools Welding Torch & Tungsten Rod 5/23/2017 Dr. N. RAMACHANDRAN, NITC 216 Shielding Gas • Argon - Purity 99.95% • Impure Argon Results In Porosities • Purity Verified by Fusing BQ CS plate • Leakage of Argon in Torch Results in Porosity. • Check Leakage by Closing the Ceramic Cup With Thump 5/23/2017 Dr. N. RAMACHANDRAN, NITC 217 Argon Gas Cylinder • Light Blue In Colour • Full Cylinder Pressure: 1800 psi ( 130 Kgs / Cm2 ) • Volume Of Argon In Full Cylinder: 7.3 M3 • Commercial Argon (99.99%) Cost: Rs 70/- Per M3 • High Purity Argon (99.999) Cost: Rs 87/- Per M3 5/23/2017 Dr. N. RAMACHANDRAN, NITC 218 Back Purging Purging Gas Commercial Argon or• Applicable to Single Nitrogen Sided full penetration • Prevents oxidation of Filler Wire Welding Torch root pass from opposite side of weld • Essential for high alloy steels, nonferrous Purging Purging Gas In Gas Out metals and alloys Root Pass Purging • Desirable For All chamber Material 5/23/2017 Dr. N. RAMACHANDRAN, NITC 219 • Argon-helium mixtures are also frequently utilized in GTAW, since they can increase control of the heat input while maintaining the benefits of using argon. Normally, the mixtures are made with primarily helium (often about 75% or higher) and a balance of argon. These mixtures increase the speed and quality of the AC welding of aluminum, and also make it easier to strike an arc. • Argon-hydrogen, is used in the mechanized welding of light gauge stainless steel, but because hydrogen can cause porosity, its uses are limited. • Nitrogen can sometimes be added to argon to help stabilize the austenite in austentitic stainless steels and increase penetration when welding copper. Due to porosity problems in ferritic steels and limited benefits, however, it is not a popular shielding gas additive. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 220 Materials • Most commonly used to weld stainless steel and nonferrous materials, such as aluminum and magnesium, but it can be applied to nearly all metals, with notable exceptions being lead and zinc. • Its applications involving carbon steels are limited not because of process restrictions, but because of the existence of more economical steel welding techniques, such as gas metal arc welding and shielded metal arc welding. • GTAW can be performed in a variety of otherthan-flat positions, depending on the skill of the welder and the materials being welded. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 221 A TIG weld showing an accentuated AC etched zone 5/23/2017 Closeup view of an aluminium TIG weld AC etch zone Dr. N. RAMACHANDRAN, NITC 222 • Aluminum and magnesium are most often welded using alternating current, but the use of direct current is also possible, depending on the properties desired. Before welding, the work area should be cleaned and may be preheated to 175-200 °C for aluminum or to a maximum of 150 °C for thick magnesium workpieces to improve penetration and increase travel speed. • AC current can provide a self-cleaning effect, removing the thin, refractory aluminium oxide (sapphire) layer that forms on aluminium metal within minutes of exposure to air. This oxide layer must be removed for welding to occur. When alternating current is used, pure tungsten electrodes or zirconiated tungsten electrodes are preferred over thoriated electrodes, as the latter are more likely to "spit" electrode particles across the welding arc into the weld. • Blunt electrode tips are preferred, and pure argon shielding gas should be employed for thin workpieces. Introducing helium allows for greater penetration in thicker workpieces, but can make arc starting difficult. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 223 • Direct current of either polarity, positive or negative, can be used to weld aluminum and magnesium as well. • DCEN allows for high penetration, and is most commonly used on joints with butting surfaces, such as square groove joints. Short arc length (generally less than 2 mm or 0.07 in) gives the best results, making the process better suited for automatic operation than manual operation. Shielding gases with high helium contents are most commonly used with DCEN, and thoriated electrodes are suitable. • DCEP is used primarily for shallow welds, especially those with a joint thickness of less than 1.6 mm. While still important, cleaning is less essential for DCEP than DCEN, since the electron flow from the workpiece to the electrode helps maintain a clean weld. A large, thoriated tungsten electrode is commonly used, along with a pure argon shielding gas. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 224 Steels • For GTA welding of carbon and stainless steels, the selection of a filler material is important to prevent excessive porosity. Oxides on the filler material and workpieces must be removed before welding to prevent contamination, and immediately prior to welding, alcohol or acetone should be used to clean the surface. • Preheating is generally not necessary for mild steels less than one inch thick, but low alloy steels may require preheating to slow the cooling process and prevent the formation of martensite in the heat-affected zone. • Tool steels should also be preheated to prevent cracking in the heat-affected zone. Austenitic stainless steels do not require preheating, but martensitic and ferritic chromium stainless steels do. A DCEN power source is normally used, and thoriated electrodes, tapered to a sharp point, are recommended. Pure argon is used for thin workpieces, but helium can be introduced as thickness increases. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 225 Dissimilar metals • Welding dissimilar metals often introduces new difficulties to GTA welding, because most materials do not easily fuse to form a strong bond. Welds of dissimilar materials have numerous applications in manufacturing, repair work, and the prevention of corrosion and oxidation. In some joints, a compatible filler metal is chosen to help form the bond, and this filler metal can be the same as one of the base materials (eg:, using a stainless steel filler metal stainless steel and carbon steel as base materials), or a different metal (such as the use of a nickel filler metal for joining steel and cast iron). Very different materials may be coated or "buttered" with a material compatible with a particular filler metal, and then welded. In addition, GTAW can be used in cladding or overlaying dissimilar materials. • When welding dissimilar metals, the joint must have an accurate fit, with proper gap dimensions and bevel angles. Care should be taken to avoid melting excessive base material. Pulsed current is particularly useful for these applications, as it helps limit the heat input. The filler metal should be added quickly, and a large weld pool should be avoided to prevent dilution of the base materials. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 226 Process variations Pulsed-current • In the pulsed-current mode, the welding current rapidly alternates between two levels. • The higher current state is known as the pulse current, while the lower current level is called the background current. • During the period of pulse current, the weld area is heated and fusion occurs. Upon dropping to the background current, the weld area is allowed to cool and solidify. • Pulsed-current GTAW has a number of advantages, including lower heat input and consequently a reduction in distortion and warpage in thin workpieces. In addition, it allows for greater control of the weld pool, and can increase weld penetration, welding speed, and quality. A similar method, manual programmed GTAW, allows the operator to program a specific rate and magnitude of current variations,Dr.making it useful 5/23/2017 N. RAMACHANDRAN, NITCfor specialized 227 applications. Dabber • The Dabber variation is used to precisely place weld metal on thin edges. The automatic process replicates the motions of manual welding by feeding a cold filler wire into the weld area and dabbing (or oscillating) it into the welding arc. It can be used in conjunction with pulsed current, and is used to weld a variety of alloys, including titanium, nickel, and tool steels. Common applications include rebuilding seals in jet engines and building up saw blades, milling cutters, drill bits, and mower blades 5/23/2017 Dr. N. RAMACHANDRAN, NITC 228 Heat-affected zone The cross-section of a welded butt joint, with the darkest gray representing the weld or fusion zone, the medium gray the heat affected zone, and the lightest gray the base material. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 229 • The heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic, which has had its microstructure and properties altered by welding. The heat from the welding process and subsequent recooling causes this change in the area surrounding the weld. The extent and magnitude of property change depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input by the welding process. • The thermal diffusivity of the base material plays a large role – if the diffusivity is high, the material cooling rate is high and the HAZ is relatively small. Alternatively, a low diffusivity leads to slower cooling and a larger HAZ. The amount of heat inputted by the welding process plays an important role as well, as processes like oxyfuel welding use high heat input and increase the size of the HAZ. Processes like laser beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ. Arc welding falls between these two extremes, with the individual processes varying somewhat in heat input 5/23/2017 Dr. N. RAMACHANDRAN, NITC 230 • To calculate the heat input for arc welding procedures, the formula used is: where Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed (mm/min). The efficiency is dependent on the welding process used, with shielded metal arc welding having a value of 0.75, gas metal arc welding and submerged arc welding, 0.9, and gas tungsten arc welding, 0.8. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 231 Types Of GTAW Power Source • Inverter- DC • Thyrister – DC • Motor Generator – DC • Rectifier – DC • Transformer – AC (For Aluminium Welding Only) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 232 Power Source • Provides Electric Energy – Arc – Heat • Drooping Characteristic • OCV – Appx. 90V, • Current Range 40 A to 300 A ( Capacity Of M/s) • Arc Voltage 18V to 26V 5/23/2017 Dr. N. RAMACHANDRAN, NITC 233 Characteristic Of GTAW Power Source Drooping – Constant Current V V1 V2 Vertical Curve A 5/23/2017 A1 A2 Dr. N. RAMACHANDRAN, NITC 234 High Frequency Unit • Provides High Voltage Electric Energy With Very high Frequency – 10000 Cycles / Sec. • Initiates low energy Arc / Spark & Ionize Air Gap. • Electrically charges Air Gap For welding Current to Jump Across the Tungsten Tip & BM to Form An Arc. • HF Gets Cut Off, Once Welding Arc Struck. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 235 Water Cooling System • Provides Cooling Water To Welding Torch. • Cools Tungsten Rod, Torch handle & Welding Cable. • Cooling Water Returns through Flexible Tube Which Carries welding cable within. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 236 Pedal Switch When Pedal Pressed • Solenoid valve opens, Argon gas flows • High Frequency current jumps from tungsten rod generating sparks • Welding current flows generating an Switches system arc across tungsten rod and work. on And off in sequence • High frequency gets cut off from the system & welding continues. When Pedal Released 1 Current gets cut off, Arc extinguishes 2 Gas flow remains for few more seconds before it stops. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 237 Argon Gas Cylinder- Pressure Regulator + Flow Meter Cylinder Valve Pressure gauges Flow Meter Flow Regulator • Cylinder Stores Argon At High Pressure • Regulator Regulates Cylinder Pressure to Working Pressure Pressure Regulator Connection To Torch • Flow Meter Controls Flow Rate Argon Cylinder 5/23/2017 Dr. N. RAMACHANDRAN, NITC 238 Tools For GTAW • Head Screen • Hand gloves • Chipping Hammer • Wire Brush • Spanner Set 5/23/2017 Dr. N. RAMACHANDRAN, NITC 239 Filler Wire • Added Separately to the weld pool. • Compatible to base metal • Used in cut length for manual welding. • Used from layer wound spool for automatic welding. • Sizes :- 0.8, 1, 1.2, 1.6, 2, 2.4 & 3 mm 5/23/2017 Dr. N. RAMACHANDRAN, NITC 240 ASME Classification Of Filler Wire SS Filler Wire: SFA-5.9, ER 308, 308L, 316, 316L, 347, 309 LAS Filler Wire: SFA 5.28, ER 70S A1, ER 80S B2, ER90S D2, ER 80S Ni2 CS Filler Wire: SFA- 5.18 , ER 70S2 C = 0.07%, Mn = 0.9% – 1.4%, Si = 0.4 – 0.7%, P = 0.025%, S = 0.035% 5/23/2017 Dr. N. RAMACHANDRAN, NITC 241 Dos & Don'ts In GTAW Don’ts Dos • Always Connect Electrode – Ve • Keep Always Flow Meter Vertical • Check & Confirm Argon Purity • Clean Groove & Filler wire With Acetone • Grind Tungsten Tip to Point 5/23/2017 • Don’t Strike Arc With Electrode + Ve • Don’t strike Arc Without Argon Flow • Don’t Strike Arc By touching Tungsten Rod • Don’t Touch Weld Pool With Tungsten Rod • Don’t Lift and break Arc Dr. N. RAMACHANDRAN, NITC 242 Dos & Don'ts In GTAW Don’ts Dos • Break The Arc Only By Pedal Switch • Lift The Torch only After 5 Sec Of Arc Break. • Ensure Pre Purging & Post Purging of 5Sec • Ensure Argon Flow & Water Circulation To Torch 5/23/2017 • When Arc is Stopped Don’t Lift Torch immediately. • Don’t Weld With Blend Tungsten Rod • Don’t Weld With Argon Leaking Torch • Don’t Weld Without Water Circulation Dr. N. RAMACHANDRAN, NITC 243 Dos & Don'ts In GTAW Don’ts Dos • Provide Back Purging For Single Sided Full Penetration Welds • Use N2 or Argon as Back Purging Gas For CS & LAS • Use Argon As Back Purging Gas For SS & Non Ferrous Alloys 5/23/2017 • Don’t Weld Single Sided Full Penetration Welds Without Back Purging • Don’t Use N2 As Back Purging Gas For Non Ferrous Alloys • Don’t Empty Ag Cylinders Fully. Dr. N. RAMACHANDRAN, NITC 244 Defects In GTAW 1. Cracks 2. Lack Of Fusion 3. Porosity 4. Undercut 5.Lack Of Penetration 6. Excess Penetration 7.Overlap 8. Suck Back 9. Under Flush 10. Burn Through 11. Tungsten Inclusion 11.Stray Arcing 5/23/2017 Dr. N. RAMACHANDRAN, NITC 245 Crack 1) 2) 3) 4) Cause Wrong Consumable Wrong Procedure Improper Preheat Inadequate Thickness In Root Pass 1) 2) 3) 4) Remedy Use Right Filler Wire Qualify Procedure Preheat Uniformly Add More Filler Wire in root Pass crack 5/23/2017 Dr. N. RAMACHANDRAN, NITC 246 Lack Of Fusion Cause Remedy 1) Inadequate Current 1) Use Right Current 2) Wrong Torch angle 2) Train /Qualify welder 3) Improper bead placement 3) Train/Qualify Welder Lack Of Fusion 5/23/2017 Dr. N. RAMACHANDRAN, NITC 247 Porosity Cause 1) Impure Argon Gas 2) Argon Leak Within Torch 3) Defective Filler Wire 4) Wet surface of BM 5) Rusted / Pitted Filler wire 6) Improper Flow Of Argon Porosity 5/23/2017 Remedy 1) Replace Argon Cylinder 2) Replace Leaking Torch 3) Replace Filler Wire 4) Clean & Warm BM 5) Clean Filler Wire 6) Provide Gas lens . . Dr. N. RAMACHANDRAN, NITC 248 Undercut Cause 1) Excess Current 2) Excess Voltage 3) Improper Torch angle Remedy 1) Reduce the Current 2) Reduce Arc length 3) Train & Qualify the Welder Under cut 5/23/2017 Dr. N. RAMACHANDRAN, NITC 249 Lack Of Penetration* Cause 1) Excess Root Face 2) Inadequate Root opening 3) Over size Filler Wire 4) Wrong Direction of Arc 5) Improper bead placement 6) Improper weaving technique Remedy 1) Reduce Root Face 2) Increase Root Opening 3) Reduce Filler Wire size 4) Train / Qualify Welder 5) Train / Qualify Welder 6) Train & Qualify Welder * Applicable to SSFPW 5/23/2017 LOPDr. N. RAMACHANDRAN, NITC 250 Excess Penetration* Cause 1)Excess root opening 2) Excess Current 3) Inadequate root face 4) Excess Weaving 5) Wrong Direction Of Arc 1) 2) 3) 4) 5) Remedy Reduce root gap Reduce Current Increase Root face Train Welder Train Welder * Applicable to SSFPW Excess Penetration Dr. N. RAMACHANDRAN, NITC 5/23/2017 251 Overlap Cause 1) Wrong Direction Of Arc 2) Inadequate Current 3) Excess Filler Wire Remedy 1) Train & Qualify Welder 2) Increase Current 3) Reduce Filler Metal Overlap 5/23/2017 Dr. N. RAMACHANDRAN, NITC 252 Suck Back* Cause Remedy 1) Excess weaving in root 2) Excess Current 3) Inadequate root face 4) Wrong Electrode angle 1) Reduce weaving 2) Reduce Current 3) Increase root face 4) Train / Qualify Welder * Applicable to SSFPW in 4G, 3G & 2G Suck Back 5/23/2017 Dr. N. RAMACHANDRAN, NITC 253 Under flush Cause Remedy 1) Weld some more beads 1) Inadequate weld beads in final layer in final layer 2) Inadequate understanding on 2) Train / Qualify welder weld reinforcement 3) Wrong selection of filler wire 3) Train / Qualify Welder size Under flush 5/23/2017 Dr. N. RAMACHANDRAN, NITC 254 Burn through* Cause 1) Excess Current 2) Excess Root opening 3) Inadequate Root face 4) Improper weaving Remedy 1) Reduce the Current 2) Reduce root opening 3) Increase root face 4) Train / Qualify Welder *Applicable to root pass Burn trough 5/23/2017 Dr. N. RAMACHANDRAN, NITC 255 Tungsten Inclusion Cause 1) Ineffective HF 2) Improper Starting of Arc 3) Tungsten Tip Comes in Contact With Weld Remedy 1) Rectify HF Unit 2) Never Touch Weld With Tungsten Rod 3) Train / Qualify welder Tungsten Inclusion 5/23/2017 Dr. N. RAMACHANDRAN, NITC 256 Stray Arcing Cause Remedy 1) HF Not In Operation 1) Rectify HF Unit 2) Inadequate Skill of Welder 2) Train the Welder Arc Strikes 5/23/2017 Dr. N. RAMACHANDRAN, NITC 257 Gas Metal Arc Welding What Is GMAW ? • A Fusion Welding Process – Semi Automatic • Arc Between Consumable Electrode &Work • Arc Generated by Electric Energy From a Rectifier / Thyrester / Inverter • Filler Metal As Electrode Continuously fed From Layer Wound Spool. • Filler Wire Driven to Arc By Wire Feeder through Welding Torch • Arc & Molten Pool Shielded by Inert Gas through Torch / Nozzle Gas Metal Arc Welding • MIG – Shielding Gas Ar / Ar + O2 / Ar + Co2 • MAG – Shielding Gas Co2 • FCAW – Shielding Gas Co2 With Flux cored Wire Note:- Addition of 1 – 5% of O2 or 5 – 10% of Co2 in Ar. increases wetting action of molten metal Power Source For MIG / MAG • • • • Inverter- DC Thyrister – DC Motor Generator – DC Rectifier – DC Characteristic Of GMAW Power Source Constant V / Linear Characteristic V Appx. Horizontal Curve V1 V2 A1 A2 A Current & Polarity DC- Electrode +Ve Stable Arc Smooth Metal Transfer Relatively Low Spatter Good Weld Bead Characteristics – DC- Electrode Ve, Seldom Used AC- Commercially Not In use Accessories Of GMAW • • • • • • Power Source Wire Feed Unit Shielding Gas Cylinder, Pressure gauges/ Regulator, Flow meter (Heater For Co2 ) Welding Torch Water Cooling System (For Water cooled Torch) Earthing Cable With Clamp Tools For GMAW • • • • • • • • • Head Screen With DIN 13 / 14 Dark Glass Hand Wire Brush / Grinder With Wire Wheel Cutting Pliers Hand Gloves Chipping Hammer / Chisel & hammer Spanner Set Cylinder Key Anti-spatter Spray Earthing Cable With Clamp GMAW Torch On / Off Switch Shielding Gas Torch Handle Spring Conduit Gas Cup Arc Nozzle Tip Filler Wire - Electrode Job Equipment & Accessories Pressure Regulator Flow Meter Shielding Gas Switch Heater (Only For Co2) Solenoid Valve Shielding Gas Cylinder Copper Cup Electrode / Wire Arc – Welding Torch Wire Inside Spring Lining Contact Tip Argon / Co2 Shielding Work Torch With Cable Max. 3Mtr Wire Feeder Wire Spool Power Source With Inductance + – GAS METAL ARC WELDING (GMAW) ALMOST REPLACING SMAW, FASTER, INTRODUCED IN 1940’S, DCRP GENERALLY EMPLOYED, CONTINUOUS WIRE FEEDING MODES OF METAL TRANSFER 1 2 3 4 5 SPRAY SHORT GLOBULAR BURIED ARC PULSED CIRCUIT ARC HIGH VOLTAGE HIGH AMPERAGE (WIRE FEED) VERY LOW VOLTAGE MODERATE WIRE FEED DROPLETSDEEP Penet. FOR THICK COOLEST MODE, LEAST Penetration. ARGON ST. (FOR NARROW) 75 % Ar + 25% CO2 5/23/2017 BETWEEN 1&2 FOR CARBON STEELS, 6 TO 12 MM UNIQUE IN GMAW, HIGHER WIRE FEED PULSING BETWEEN MODES HIGH SPPED, LOW SPATTER, DEEP Penet., FOR MS AND SS NO GUN OSCILLATI ON 90%Ar + 7.5% CO2 +2.5% He FOR THICK TO THIN, Dr. N. RAMACHANDRAN, NITC DISSIMILAR 268 GASES • PUROPOSE1.TO SHIELD MOLTEN PUDDLE FROM CONTAMINATION 2.CREATE A SMOOTH ELECTRICAL CONDUCTION PATH FOR ELECTRONS IN ARC • SOME GASES (ARGON)MAKE SMOOTH PATH, BUT SOME RESISTS (CO2) PATH. • STRAIGHT ARGON FOR NARROW BEADS • 98% Ar+ 2 OXYGEN FOR SPRAY, • He FOR COPPER, THICK Al (WITH Ar). • 75 % Ar + 25% CO2 FOR SHORT CIRCUIT., • STRAIGHT CO2 ECONOMICAL, BUT SPATTERING. • 90%Ar + 7.5% CO2 +2.5% He FOR BURIED ARC, SS. • 90% Ar + 10% He FOR AUTOMATIC V, WIRE FEED SYSTEMS • A CONSTANT VOLTAGE POWER SOURCE USED. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 269 ABOUT THE POWER SOURCE • DCRP, DCSP, ACHF USED • ELECTRODES OF 0.25 mm TO 6.4 mm FOR DIFFERENT APPLICATIONS • ELECTRODES CODED, WITH COLOR STRIPS • BEST FOR ALUMINIUM, SINCE OXIDE FILM BREAKS BY PENETRATION Frequent cleaning and shaping of electrode tip to be done 5/23/2017 Dr. N. RAMACHANDRAN, NITC 270 + POINTS OF GMAW • HIGH WELDING SPEED • NO NEED TO CHANGE ELECTRODES (ONLY WIRE SPOOL IN GMAW) • HAZ SMALL • VERY LITTLE SMOKE AND VERY LIGHT SiO2 SLAG(CALLED GLASS SLAG) • LEAST DISTORTION • EASE OF OPERATION (QUICK LEARNING) • GUN MANIPULATION EASIER • MOST FLEXIBLE PROCESS- VERSATILE • VERY FEW MACHINE ADJUSTMENTS FOR THICK TO THIN CHANGE • MS, MCS, TOOL STEEL GRADES, SS, COPPER, Al, Mg WELDED • FCAW, SAW, ESW- OTER FORMS OF GMAW 5/23/2017 Dr. N. RAMACHANDRAN, NITC 271 Types Of Wire Feeding In GMAW • Push Type – Wire fed in to The torch by Pushing through Flexible Conduit From A Remote Spool • Pull Type – Feed Rollers Mounted on The Torch Handle Pulls the Wire From A Remote spool • Self Contained – Wire Feeder & The Spool On the Torch Function Of Shielding Gas In GMAW • Prevents Air contamination of weld Pool • Prevents Contamination During Metal Transfer • Increases fluidity of molten metal • Minimizes the spatter generation • Helps in even & uniform bead finish Shielding Gases For GMAW • MIG: • • • • Argon Or Helium For SS, CS, LAS & Non-ferrous Mt & Al MIG: Ar + 1 to 2 % O2, Wire With Add. Mn & Si For SS, CS, LAS & Non-ferrous Mt & Al MIG: Ar + 5 to 20 % Co2 Wire With Add. Mn & Si For SS, CS, LAS & Non-ferrous Mt & Al MAG: Co2 With Solid Wire For CS & LAS FCAW: Co2 With Flux Cored Wire For CS, LAS & SS Overlay ASME Classification For CS GMAW Wire • SFA 5.18 : - CS Solid Wire ER 70 S – 2, ER 70 S – 3 ER 70 S – 6, ER 70 S – 7 • SFA 5.20 :- CS Flux Cored Wire E 71 T-1, E 71 T-2 ( Co2 Gas ) E 71 T-1M, E 71 T-2M ( Ar + Co2 Mix) GMAW CS Wire • Generally Copper Coated – Prevents Oxidation / rusting in Storage – Promotes Electric Conductivity in Arcing • Available In Solid & Flux Cored – Size in mm 0.8, 1, 1.2, 1.6, 2, 2.4, 3 • Manganese & Silicon ( Mn 1 – 2 %, Si Max 1%) – Act As Deoxidizing Agents – Eliminate Porosity – Increase Wetting Of Molten Pool Metal Transfer In MIG • Short-Circuiting / Dip Transfer • Globular Transfer • Spray Transfer Metal Transfer In MIG Up to 120A CS Solid Wire 1.2 mm Φ 120 to 250A 14 – 22V Dip/Short Circuiting Co2 or Ar 16 – 24 V Globular Co2 or Ar Above230A 24 – 35 V Spray Only Ar / Ar+O2 Short-Circuiting / Dip Transfer • Wire In Contact With Molten Pool 20 to 200 times per Second • Operates in Low Amps & Volts – Less Deposition • Best Suitable for Out of Position Welding • Suitable for Welding Thin Sheets • Relatively Large opening of Root Can be Welded • Less Distortion • Best Suitable for Tacking in Set up • Prone to Get Lack of Fusion in Between Beads Globular Transfer • Metal transferred in droplets of Size grater than wire diameter • Operates in Moderate Amps & Volts – Better Deposition • Common in Co2 Flux Cored and Solid Wire • Suitable for General purpose Welding Spray Transfer • • • • • Metal transferred in multiples of small droplets 100 to 1000 Droplets per Second Metal Spray Axially Directed Electrode Tip Remains pointed Applicable Only With Inert Gas Shielding – Not With Co2 • Operates in Higher Amps & Volts – Higher Deposition Rate • Not Suitable for Welding in Out of Position. • Suitable for Welding Deep Grooves Pulsed Spray Welding • Power Source Provides Two different Current Levels“Background” and “Peak”at regular interval • “Background” & “Peak” are above and below the Average Current • Best Suitable for Full Penetration Open Root Pass Welding • Good Control on Bead Shape and Finish Synergic Pulse GMAW • Parameters of Pulsed Current (Frequency, Amplitude, Duration, Background Current) Related to Wire feed Rate • One Droplet detaches with each pulse • An Electronic Control unit synchronizes wire feed Rate with Pulse Parameters • Best Suitable for Most Critical Full Penetration Open Root Pass Welding • Good Control on Open Root penetration, Bead Shape and Finish GMAW Process Variables • • • • • • • • Current Voltage Travel Speed Stick Out / Electrode Extension Electrode Inclination Electrode Size Shielding Gas & Flow Rate Welding Position Parameter For 1.2 ф FC Wire • • • • • • Current – 200 to 240 A Voltage – 22-24 Travel Speed 150 to 250 mm / min Stick Out / Electrode Extension – 15 to 20 mm Electrode Inclination – Back Hand Technique Shielding Gas – Co2, 12 L/Min Parameter For 1.2 ф Solid Wire • • • • • • Current – 180 to 220 A Voltage – 20-22 Travel Speed 150 to 200 mm / min Stick Out / Electrode Extension – 10 to 20 mm Electrode Inclination – Back Hand Technique Shielding Gas – Co2 – 12 L/Min Results In Change Of Parameters • Increase In Current – More deposition, More Penetration, More BM Fusion • Increase In Voltage – More Weld Bead Width, Less Penetration, Less Reinforcement, Excess Spatter • Increase In Travel Speed – Decrease in Penetration, Decrease in Bead Width, • Decrease In Gas Flow rate – Results In porosity • Long Stick Out / Electrode Extension – Excess Weld Deposit With Less Arc intensity, Poor Bead Finish, Shallow Penetration Common Defects In GMAW 1. Porosity 3. Lack Of Fusion 5. Over Lap 7. Crack 9. Burn Through 11. Unstable Arc 2. Spatters 4. Under Cut 6. Slag 8. Lack Of Penetration 10. Convex Bead 12. Wire Stubbing Porosity Cause Remedy 1) Less Mn & Si In Wire 2) Rusted / Unclean BM / Groove 3) Rusted wire 4) Inadequate Shielding Gas 1) Use High Mn & Si Wire 2) Clean & warm the BM 3) Replace the Wire 4) Check & Correct Flow Rate Porosity . . Spatters Cause Remedy 1) Low Voltage 2) Inadequate Inductance 3) Rusted BM surface 4) Rusted Core wire 5) Quality Of Gas 1) Increase Voltage 2) Increase Inductance 3) Clean BM surface 4) Replace By Rust Free wire 5) Change Over To Ar + Co2 Spatters • •• Lack Of Fusion Cause Remedy 1) Inadequate Current 1) Use Right Current 2) Inadequate Voltage 3) Wrong Polarity 4) Slow Travel Speed 5) Excessive Oxide On Joint 2) Use Right Voltage 3) Connect Ele. + Ve 4) Increase Travel speed 5) Clean Weld Joint Lack Of Fusion Undercut Cause 1) Excess Voltage 2) Excess Current 3) Improper Torch angle 4) Excess Travel Speed Under cut Remedy 1) Reduce Voltage 2) Reduce Current 3) Train & Qualify the Welder 4) Reduce Travel Speed Overlap Cause Remedy 1) Too Long Stick Out 1) Reduce Stick Out 2) Inadequate Voltage 2) Increase the Voltage Overlap Slag Cause 1) Inadequate Cleaning 2) Inadequate Current 3) Wrong Torch angle 4) Improper bead placement Slag Remedy 1) Clean each bead 2) Use Right Current 3) Train / Qualify welder 4) Train / Qualify Welder Crack Cause Remedy 1) Incorrect Wire Chemistry 1) Use Right Wire 2) Increase wire Feed 2) Too Small Weld Bead 3) Preheat Uniformly 3) Improper Preheat 4) Post heating or ISR 4) Excessive Restrain crack Lack Of Penetration* Cause 1) Too Narrow Groove Angle 2) Inadequate Root opening 3) Too Low Welding current 4) Wrong Torch angle 5) Puddle Roll In Front Of Arc 6) Long Stick Out * Applicable to SSFPW LOP Remedy 1) Widen The Groove 2) Increase Root Opening 3) Increase Current 4) Train / Qualify Welder 5) Correct Torch Angle 6) Reduce Stick Out Burn through* Cause 1) Excess Current 2) Excess Root opening 3) Inadequate Root face 4) Too Low Travel Speed 5) Quality Of Gas Burn trough Remedy 1) Reduce the Current 2) Reduce root opening 3) Increase root face 4) Increase Speed 5) Use Ar + Co2 *Applicable to root pass Convex Bead Finish Cause 1) Low Current 2) Low Voltage 3) Low Travel Speed 4) Low Inductance 5) Too Narrow Groove Uneven bead finish Remedy 1) Increase Current 2) Increase Voltage 3) Increase Travel Speed 4) Increase Inductance 5) Increase Groove Width Unstable arc Cause 1) Improper Wire Feed 2) Improper Gas Flow 3) Twisted Torch Conduit Remedy 1) Check Wire Feeder 2) Check Flow Meter 3) Straighten Torch Cab Wire Stubbing Cause 1) Too Low Voltage 2) Too High Inductance 3) Excess Slope 4) Too Long Stick Out Remedy 1) Increase Voltage 2) Reduce Inductance 3) Adjust Slope 4) Reduce Stick Out Important Terminology used in Critical Welding • • • • • Preheating Post Heating or Dehydrogenation Intermediate Stress leaving Inter pass Temperature Post Weld Heat Treatment What Is Preheating? • Heating the base metal along the weld joint to a predetermined minimum temperature immediately before starting the weld. • Heating by Oxy fuel flame or electric resistant coil • Heating from opposite side of welding wherever possible • Temperature to be verified by thermo chalks prior to starting the weld Why Preheating? • Preheating eliminates possible cracking of weld and HAZ • Applicable to Hardenable low alloy steels of all thickness Carbon steels of thickness above 25 mm. Restrained welds of all thickness • Preheating temperature vary from 75°C to 200°C depending on hardenability of material, thickness & joint restrain How does Preheating Eliminate Crack? • Preheating promotes slow cooling of weld and HAZ • Slow cooling softens or prevents hardening of weld and HAZ • Soft material not prone to crack even in restrained condition What Is Post Heating? • Raising the pre heating temperature of the weld joint to a predetermined temperature range (250° C to 350° C) for a minimum period of time (3 Hrs) before the weld cools down to room temperature. • Post heating performed when welding is completed or terminated any time in between. • Heating by Oxy fuel flame or electric resistant coil • Heating from opposite side of welding wherever possible • Temperature verified by thermo chalks during the period Why Post Heating? • Post heating eliminates possible delayed cracking of weld and HAZ • Applicable to Thicker hardenable low alloy steels Restrained hardenable welds of all thickness • Post heating temperature and duration depends on hardenability of material, thickness & joint restrain How does Post Heating Eliminate Crack? • SMAW introduces hydrogen in weld metal • Entrapped hydrogen in weld metal induces delayed cracks unless removed before cooling to room temperature • Retaining the weld at a higher temperature for a longer duration allows the hydrogen to come out of weld What Is Intermediate Stress Relieving? • Heat treating a subassembly in a furnace to a predetermined cycle immediately on completion of critical restrained weld joint / joints without allowing the welds to go down the pre heat temperature. Rate of heating, Soaking temperature, Soaking time and rate of cooling depends on material quality and thickness • Applicable to Highly restrained air hardenable material Why Intermediate Stress Relieving? • Restrained welds in air hardenable steel highly prone to crack on cooling to room temperature. • Cracks due to entrapped hydrogen and built in stress • Intermediate stress relieving relieves built in stresses and entrapped hydrogen making the joint free from crack prone What Is Inter- Pass Temperature? • The temperature of a previously layed weld bead immediately before depositing the next bead over it • Temperature to be verified by thermo chalk prior to starting next bead • Applicable to Stainless Steel Carbon Steel & LAS with minimum impact Why Inter Pass Temperature? • Control on inter pass temperature avoids over heating, there by Refines the weld metal with fine grains Improves the notch toughness properties Minimize the loss of alloying elements in welds Reduces the distortion What Is Post Weld Heat Treatment? • Heat treating an assembly on completion of all applicable welding, in an enclosed furnace with controlled heating/cooling rate and soaking at a specific temperature for a specific time. • Rate of heating, Soaking temperature, Soaking time and rate of cooling depends on material quality and thickness • Applicable to All type of CS & LAS Why Post Weld Heat Treatment? • Welded joints retain internal stresses within the structure • HAZ of welds remains invariably hardened • Post Weld Heat Treatment relieves internal stresses and softens HAZ. This reduces the cracking tendency of the equipment in service Weldability • The weldability of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others. It greatly influences weld quality and is an important factor in choosing which welding process to use. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 314 • Steels • The weldability of steels is inversely proportional to a property known as the hardenability of the steel, which measures the ease of forming martensite during heat treatment. The hardenability of steel depends on its chemical composition, with greater quantities of carbon and other alloying elements resulting in a higher hardenability and thus a lower weldability. In order to be able to judge alloys made up of many distinct materials, a measure known as the equivalent carbon content is used to compare the relative weldabilities of different alloys by comparing their properties to a plain carbon steel. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 315 • The effect on weldability of elements like chromium and vanadium, while not as great as carbon, is more significant than that of copper and nickel, for example. As the equivalent carbon content rises, the weldability of the alloy decreases. The disadvantage to using plain carbon and low-alloy steels is their lower strength— there is a trade-off between material strength and weldability. High strength, low-alloy steels were developed especially for welding applications during the 1970s, and these generally easy to weld materials have good strength, making them ideal for many welding applications. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 316 • Stainless steels, because of their high chromium content, tend to behave differently with respect to weldability than other steels. Austenitic grades of stainless steels tend to be the most weldable, but they are especially susceptible to distortion due to their high coefficient of thermal expansion. Some alloys of this type are prone to cracking and reduced corrosion resistance as well. Hot cracking is possible if the amount of ferrite in the weld is not controlled—to alleviate the problem, an electrode is used that deposits a weld metal containing a small amount of ferrite. Other types of stainless steels, such as ferritic and martensitic stainless steels, are not as easily welded, and must often be preheated and welded with special electrodes. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 317 • Aluminum • The weldability of aluminum alloys varies significantly, depending on the chemical composition of the alloy used. Aluminum alloys are susceptible to hot cracking, and to combat the problem, welders increase the welding speed to lower the heat input. Preheating reduces the temperature gradient across the weld zone and thus helps reduce hot cracking, but it can reduce the mechanical properties of the base material and should not be used when the base material is restrained. The design of the joint can be changed as well, and a more compatible filler alloy can be selected to decrease the likelihood of hot cracking. Aluminum alloys should also be cleaned prior to welding, with the goal of removing all oxides, oils, and loose particles from the surface to be welded. This is especially important because of an aluminum weld's susceptibility to porosity due to hydrogen and dross due to oxygen. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 318 • References • Lincoln Electric (1994). The Procedure Handbook of Arc Welding. Cleveland: Lincoln Electric. ISBN 9994925822. • Residual stresses are stresses that remain after the original cause of the stresses has been removed. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment. Heat from welding may cause localized expansion, which is taken up during welding by either the molten metal or the placement of parts being welded. When the finished weldment cools, some areas cool and contract more than others, leaving residual stresses. Castings may also have large residual stresses due to uneven cooling. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 319 • While un-controlled residual stresses are undesirable, many designs rely on them. For example, toughened glass and pre-stressed concrete depend on them to prevent brittle failure. Similarly, a gradient in martensite formation leaves residual stress in some swords with particularly hard edges (notably the katana), which can prevent the opening of edge cracks. In certain types of gun barrels made with two tubes forced together, the inner tube is compressed while the outer tube stretches, preventing cracks from opening in the rifling when the gun is fired. Parts are often heated or dunked in liquid nitrogen to aid assembly. • Press fits are the most common intentional use of residual stress. Automotive wheel studs, for example are pressed into holes on the wheel hub. The holes are smaller than the studs, requiring force to drive the studs into place. The residual stresses fasten the parts together. Nails are another example. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 320 5/23/2017 Dr. N. RAMACHANDRAN, NITC 322 Resistance Welding Commonly used resistance welding processes: • Resistance Spot Welding (RSW), • Resistance Seam Welding (RSEW),& • Resistance Projection Welding (PW) or (RPW) • Resistance welding uses the application of electric current and mechanical pressure to create a weld between two pieces of metal. Weld electrodes conduct the electric current to the two pieces of metal as they are forged together. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 323 • The welding cycle must first develop sufficient heat to raise a small volume of metal to the molten state. This metal then cools while under pressure until it has adequate strength to hold the parts together. The current density and pressure must be sufficient to produce a weld nugget, but not so high as to expel molten metal from the weld zone. • High Frequency Resistance Welding (HFRW) Percussion Welding (PEW) and Stud Welding (SW), too. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 324 H = I2 R t K Electrode K- energy losses through radiation & conduction Weld Nugget •resistances of the electrodes •electrode- w/p contact resistance •resistance of the individual parts to be welded •w/p-w/p contact resistance (maintained high) Resistance Welding Benefits • High speed welding • Easily automated • Suitable for high rate production • 5/23/2017 Economical HAZ Electrode Dr. N. RAMACHANDRAN, NITC 325 • Resistance Welding Limitations • Initial equipment costs • Lower tensile and fatigue strengths • Lap joints add weight and material Common Resistance Welding Concerns •Optimize welding process variables. •Evaluate current welding parameters and techniques. •And thus eliminate common welding problems and 5/23/2017 Dr. N. RAMACHANDRAN, NITC 326 discontinuities - such as Resistance Welding Problems and Discontinuities • • • • • • • • • Cracks Electrode deposit on work Porosity or cavities Pin holes Deep electrode indentation Improper weld penetration Surface appearance Weld size Irregular shaped welds 5/23/2017 Dr. N. RAMACHANDRAN, NITC 327 RESISTANCE SPOT WELDING • AIR 5/23/2017 OPERATED ROCKER ARM SPOT WELDING MACHINE Dr. N. RAMACHANDRAN, NITC 328 RESISTANCE SPOT WELDING ELECTRODE DESIGNS FOR EASY ACCESS INTO COMPONENTS 5/23/2017 Dr. N. RAMACHANDRAN, NITC 329 RESISTANCE SEAM WELDING 5/23/2017 Dr. N. RAMACHANDRAN, NITC 330 RESISTANCE PROJECTION WELDING 5/23/2017 Dr. N. RAMACHANDRAN, NITC 331 HIGH FREQUENCY BUTT WELDING OF TUBES 5/23/2017 Dr. N. RAMACHANDRAN, NITC 332 FLASH WELDING POOR FOR SOLID RODS & TUBES 5/23/2017 GOOD DESIGN GUIDELINES Dr. N. RAMACHANDRAN, NITC 333 RESISTANCE STUD WELDING 5/23/2017 Dr. N. RAMACHANDRAN, NITC 334 DISTORTION • Welding involves highly localized heating of the metal being joined together. • The temperature distribution in the weldment is nonuniform. • Normally, the weld metal and the heat affected zone (HAZ) are at temperatures substantially above that of the unaffected base metal. • Upon cooling, the weld pool solidifies and shrinks, exerting stresses on the surrounding weld metal and HAZ. • If the stresses produced from thermal expansion and contraction exceed the yield strength of the parent metal, localized plastic deformation of the metal occurs. • Plastic deformation results in lasting change in the component dimensions and distorts the structure. This causes distortion of weldments. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 335 Types of distortion • • • • • • Longitudinal shrinkage Transverse shrinkage Angular distortion Bowing Buckling Twisting 5/23/2017 Dr. N. RAMACHANDRAN, NITC 336 Factors affecting distortion • If a component were uniformly heated and cooled distortion would be minimized. However, welding locally heats a component and the adjacent cold metal restrains the heated material. This generates stresses greater than yield stress causing permanent distortion of the component. Some of the factors affecting the distortion are: 1. Amount of restraint 2. Welding procedure 3. Parent metal properties 4. Weld joint design 5. Part fit up 5/23/2017 Dr. N. RAMACHANDRAN, NITC 337 • Restraint - to minimize distortion. Components welded without any external restraint are free to move or distort in response to stresses from welding. It is not unusual for many shops to clamp or restrain components to be welded in some manner to prevent movement and distortion. This restraint does result in higher residual stresses in the components. • Welding procedure impacts the amount of distortion primarily due to the amount of the heat input produced. The welder has little control on the heat input specified in a welding procedure. This does not prevent the welder from trying to minimize distortion. While the welder needs to provide adequate weld metal, the welder should not needlessly increase the total weld metal volume added to a weldment. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 338 • Parent metal properties, which have an effect on distortion, are coefficient of thermal expansion and specific heat of the material. The coefficient of thermal expansion of the metal affects the degree of thermal expansion and contraction and the associated stresses that result from the welding process. This in turn determines the amount of distortion in a component. • Weld joint design will effect the amount of distortion in a weldment. Both butt and fillet joints may experience distortion. However, distortion is easier to minimize in butt joints. • Part fit up should be consistent to fabricate foreseeable and uniform shrinkage. Weld joints should be adequately and consistently tacked to minimize movement between the parts being joined by welding. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 339 Welding Discontinuities Some examples of welding discontinuities are shown below. Evaluation of the discontinuity will determine if the discontinuity is a defect or an acceptable condition 5/23/2017 Incomplete Fusion - A weld discontinuity in which fusion did not occur between weld metal Dr. N. RAMACHANDRAN, NITC and fusion faces or adjoining weld beads. 340 Undercut - A groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal. Overlap - The protrusion of weld metal beyond the weld toe or weld root. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 341 Underfill - A condition in which the weld face or root surface extends below the adjacent surface of the base metal. Incomplete Joint Penetration - A joint root condition in a groove weld in which weld metal does not extend through the joint thickness •Partial joint penetration groove welds are commonly specified in lowly loaded structures. However, incomplete joint penetration when a full penetration joint is required, as depicted above, would be cause for rejection. A fix for an incomplete penetration joint would be to back gouge and weld from the other side. Another acceptable partial penetration joint is shown below. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 342 Partial penetration joint on the left without discontinuities is an acceptable condition. Appropriate engineering decisions need to be applied to determine what type of joint should be specified for a given application. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 343 Several different representations of weld Cracking 5/23/2017 Dr. N. RAMACHANDRAN, NITC 344 Representation of a convex fillet weld without discontinuities 5/23/2017 Dr. N. RAMACHANDRAN, NITC 345 SOLID STATE PROCESSES • Joining without fusion of work pieces • No liquid (molten ) phase present in joint • Principle: If two clean surfaces are brought into atomic contact with each other - made with sufficient pressure -(in the absence of oxide film and other contaminents) they form bonds and produce strong joint • To improve strength, heat and some movement of mating surfaces by plastic deformation employed. Eg: USW, Friction Welding (FRW) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 346 FORGE WELDING (FOW) • Both elevated temperature and pressure applied to form strong bond between members • Components heated and pressed/ hammered with tools, dies or rollers • Local plastic deformation at interface breaks up the oxide films – improves bond strength. • Not for high load bearing applications. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 347 COLD WELDING (CW) • Pressure applied to work pieces either through dies or rolls • One (or both) of the mating parts must be ductile • Interface cleaned prior to welding- brushing etc. Rolling metal Roll 5/23/2017 Dr. N. RAMACHANDRAN, NITC Bare metal 348 Explosive welding • Solid state bonding process • Joining by the cohesive force between atoms of two intimate contact surfaces High pressure waves- thousands of MPa created• To weld dissimilar metals, thick to thin, high difference in Melting Point metals. • Not a costly process • Extremely large surfaces can be joined (2m X 10 m) • Welding of heat treated metals without affecting the process • No HAZ • Incompatible metals joined(thin foils to heavy plates) 5/23/2017 severe deformation needed for joining. Dr. N. RAMACHANDRAN, NITC 349 • Principle: Explosive Impulse used to produce extremely high normal pressure and a slight shear or sliding pressure ( uses a detonator for this) Two properly laid metal surfaces brought together with high relative velocity at high pressure and with proper orientation Large amount of plastic interaction between surfaces results. TWO WAYS 5/23/2017 Dr. N. RAMACHANDRAN, NITC 350 (1)Contact technique (2) Impact technique • (1). Plastic interaction by positioning explosive charge to deliver shock waves at an oblique angle to parts to be welded- Less frequently used. • (2). Two pieces explosively projected towards each other. • Impact with high velocity (200 – 400 m/s) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 351 (1)Contact technique •Plastic interaction by positioning explosive charge to deliver shock waves at an oblique angle to parts to be welded- Less frequently used. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 352 (2) Impact technique Two pieces explosively projected towards each other. Impact with high velocity (200 – 400 m/s) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 353 • Detonation velocity approx. 7000 m/s in the detonation front. • Produces pressure at interface 7000 to 70,000 atms. Parts driven at an angle Velocity of impact and angle of collapse selected. Joining as s result of intense plastic flow at the surface called “surface jetting” • For good joint, surface to be free from contaminants • Pressure sufficient to bring surfaces within interatomic distances of each other [ In a range of speed and angle of impact, a high velocity metal jet forms. Removes surface contamination. Speed, angle(10 to 100) of detonation important] 5/23/2017 Dr. N. RAMACHANDRAN, NITC 354 • Bond as strong as the weaker of the two obtained. 100 % efficient joint, (eg. In sheet forming in aerospace industries) • At the interface, microhardness slightly increased. (because of plastic deformation and strain hardening- a very thin hardness zone) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 355 • Titanium cladding common • Others- Ni, SS(50 mm), tantalum, carbon steels, for heat exchangers, tubes, pressure vessels, etc. • No change in chemical and physical properties of parent metal • But, not for brittle alloys. Metal must possess some ductility. • [Quantity of charge, detonation velocity, and deformation characteristics of flyer plate decide the weld] • Also spot welding by small charge. Handy explosive spot welding sets available (for 10mm to 12 mm spots) 5/23/2017 Dr. N. RAMACHANDRAN, NITC 356 • Minus points: Severe deformation needed for joining (minimum 40 to 60%), as welding is by pressure. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 357 THERMIT WELDING • THERMITE- based on Therm, meaning heat • Involves exothermic reactions between metal oxides and metallic reducing agents • Heat of reaction used for welding. • • Fine particles of iron oxide, aluminium oxide, iron & aluminium • Reactions are: (3/4) Fe3 O4 + 2 Al --- (9/4) Fe + Al2O3 + Heat 3 FeO + 2 Al --- 3 Fe + Al2O3 + Heat Fe2O3 + 2Al --- 2Fe + Al2O3+ Heat 5/23/2017 Dr. N. RAMACHANDRAN, NITC 358 THERMIT WELDING THERMIT WELDING Slide 13 of 18 5/23/2017 Dr. N. RAMACHANDRAN, NITC 360 • Mixture is non explosive. Produces temperature of 32000 C within a minute • Practically about 22000- 24000 C. Other materials to impart special properties added. Applying a Mg fuse of special compounds of peroxides, chlorates/ chromates. • Welding copper, brasses, bronzes and copper alloys to steel using oxides of copper, nickel, aluminium, manganese – temperatures of 50000 C obtained 5/23/2017 Dr. N. RAMACHANDRAN, NITC 361 PLASMA WELDING 5/23/2017 Dr. N. RAMACHANDRAN, NITC 363 5/23/2017 Dr. N. RAMACHANDRAN, NITC 364 5/23/2017 Dr. N. RAMACHANDRAN, NITC 365 Gas MIG/TIG Weldi ng Plasma Arc Weldi ng Laser Laser Weldi ng Cuttin g Plasma Cuttin g Acetylene Oxy-Fuel Cuttin g X Air Alumaxx Plus X Argon X X Argon/hydrogen TIG X Carbon dioxide MAG X X X X X X X X X X Carbon monoxide X Cooling X Ferromaxx Plus MAG Ferromax 15 MAG Ferromaxx 7 MAG Helium TIG X X X Hydrogen X Inomaxx Plus MAG Inomaxx 2 MAG Inomaxx TIG TIG X Nitrogen X Nitrogen/hydrogen mixes X Oxygen X X Propane Propylene 5/23/2017 Thermal Spraying Dr. N. RAMACHANDRAN, NITC X X X X X X X 366 Arc Spraying Arc spraying is the highest productivity thermal spraying process. A DC electric arc is struck between two continuous consumable wire electrodes which form the spray material. Compressed gas (usually air) atomises the molten spray material into fine droplets and propels them towards the substrate The process is simple to operate- Can be used manually or in an automated manner. Possible to spray a wide range of metals, alloys and metal matrix composites (MMCs) in wire form. A limited range of cermet coatings (with tungsten carbide) can also be sprayed in cored wire form, where the hard ceramic phase is packed into a metal sheath as a fine powder. The combination of high arc temperature (6000 K) and particle velocities in excess of 100 m.sec-1 gives arc sprayed coatings superior bond strengths and lower porosity levels when compared with flame sprayed coatings. However, air for dropletNITC atomization and propulsion 5/23/2017 the use of compressed Dr. N. RAMACHANDRAN, 367 gives rise to high coating oxide content. PLASMA SPRAYING PROCESS •Uses a DC electric arc to generate a stream of high temperature ionised plasma gas, which acts as the spraying heat source. •The arc is struck between two nonconsumable electrodes, a tungsten cathode and a copper anode within the • torch. •The torch is fed with a continuous flow of inert gas, which is ionised by • the DC arc, and is compressed and accelerated by the torch nozzle so that it issues from the torch as a high velocity (in excess of 2000 m/sec), high temperature (12000–16000 K) plasma jet. • •The coating material, in powder form, is carried in an inert gas stream into the plasma jet where it is heated and propelled towards the substrate. 5/23/2017 Because of the high temperature and high thermal energy of the plasma jet, materials with high melting points can be sprayed. Plasma spraying produces a high quality coating by a combination of a high temperature, high energy heat source, a relatively inert spraying medium and high particle velocities, typically 200–300 m.sec-1. However, inevitably some air becomes entrained in the spray stream and some oxidation of the spray material may occur. The surrounding atmosphere also cools and slows the spray stream. Dr. N. RAMACHANDRAN, NITC 368 Applications • Plasma spraying is widely applied in the production of high quality sprayed coatings. • Spraying of seal ring grooves in the compressor area of aeroengine turbines with tungsten carbide/cobalt to resist fretting wear. • Spraying of zirconia-based thermal barrier coatings (TBCs) onto turbine combustion chambers. • Spraying of wear resistant alumina and chromium oxide ceramic onto printing rolls for subsequent laser and diamond engraving/etching. • Spraying of molybdenum alloys onto diesel engine piston rings. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 369 HIGH VELOCITY OXYFUEL SPRAYING The most recent addition to the thermal spraying family, high velocity oxyfuel spraying (HVOF SPRAYING) has become established as an alternative to the proprietary, detonation (D-GUN) flame spraying and the lower velocity, air plasma spraying processes for depositing wear resistant tungsten carbide-cobalt coatings. This differs from conventional flame spraying in that the combustion process is internal, and the gas flow fates and delivery pressures are much higher than those in the atmospheric burning flame spraying processes. The combination of high fuel gas and oxygen flow rates and high pressure in the combustion chamber leads to the generation of a supersonic flame with characteristic shock diamonds. Flame speeds of 2000ms-1 and particle velocities of 600–800ms-1 are claimed by HVOF equipment suppliers. A5/23/2017 range of gaseous fuels is currently used, including Dr. N. RAMACHANDRAN, NITC propylene, propane, 370 hydrogen and acetylene. • Although similar in principle, potentially significant details, such as powder feed position, gas flow rates and oxygen to fuel ratio, are apparent between each system. • The HVOF process produces exceptionally high quality cermet coatings (e.g., WC-Co), but it is now also used to produce coatings of metals, alloys and ceramics. Not all HVOF systems are capable of producing coatings from higher melting point materials, e.g., refractory metals and ceramics. The capability of the gun is dependent upon the range of fuel gases used and the combustion chamber design. • A liquid fuel (kerosene) HVOF system, has just been launched, which is capable of much higher deposition rates than the conventional 5/23/2017 Dr. N. RAMACHANDRAN, NITC 371 gas-fuelled units. Applications HVOF spraying is a very recent process development, yet the high quality of the coatings produced at competitive cost has already seen its introduction in a number of very significant industries. Potential applications overlap with plasma and D-gun spraying, particularly for WC-Co coatings. Tungsten carbide-cobalt coatings for fretting wear resistance on aeroengine turbine components. Wear resistant cobalt alloys onto fluid control valve seating areas. Tungsten carbide-cobalt coatings on gate valves. Various coatings for printing rolls, including copper, alumina, chromia. NiCrBSi coatings (unfused) for glass plungers. NiCr coatings for high temperature oxidation/corrosion resistance. Alumina and alumina-titania dielectric coatings. Biocompatible hydroxylapatite coatings for prostheses. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 372 Schematic of High Velocity Oxyfuel (HVOF) Spraying System Process Particle Velocity (m/s) Adhesion (MPa) Oxide Content (%) Porosity (%) Deposition Rate (kg/hr) Typical Deposit Thicknes s (mm) Flame 40 <8 10–15 10–15 1–10 0.2–10 Arc 100 10–30 10–20 5–10 6–60 0.2–10 Plasma 200–300 20–70 1–3 1–8 1–5 0.2–2 HVOF 600–800 >70 1–2 1–2 1–5 5/23/2017 Dr. N. RAMACHANDRAN, NITC 373 Comparison of Thermal Spraying Processes and Coating Characteristics Typical Deposit Thickness (mm) Particle Velocity (m/s) Adhesion (MPa) Oxide Content (%) Porosity (%) Deposition Rate (kg/hr) Flame 40 <8 10–15 10–15 1–10 0.2–10 Arc 100 10–30 10–20 5–10 6–60 0.2–10 Plasma 200–300 20–70 1–3 1–8 1–5 0.2–2 HVOF 600–800 >70 1–2 1–2 1–5 Process 5/23/2017 Dr. N. RAMACHANDRAN, NITC 374 Thermal Spraying Gases Process Fuels that can be used Other gases HVOF Acetylene, hydrogen, propylene, propane, or liquid kerosene depending on material type Oxygen and argon Arc spraying Flame spraying Normally compressed air but can use nitrogen or argon Mainly acetylene, but sometimes propane depending on material Plasma spraying 5/23/2017 Oxygen Argon and hydrogen Dr. N. RAMACHANDRAN, NITC 375 ELECTROGAS WELDING Slide 14 of 18 5/23/2017 Dr. N. RAMACHANDRAN, NITC 376 ELECTRON BEAM WELDING •The electron beam gun has a tungsten filament which is heated, freeing electrons. •The electrons are accelerated from the source with high voltage potential between a cathode and anode. •The stream of electrons then pass through a hole in the anode. The beam is directed by magnetic forces of focusing and deflecting coils. This beam is directed out of the gun column and strikes the workpiece. •The potential energy of the electrons is transferred to heat upon impact of the workpiece and cuts a perfect hole at the weld joint. Molten metal fills in behind the beam, • The electron beam stream and workpiece are manipulated by means of precise, computer driven controls, within a vacuum welding chamber, therefore eliminating oxidation, contamination. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 378 • How an Electron Beam Machine Works The EB system is composed of an electron beam gun, a power supply, control system, motion equipment and vacuum welding chamber. Fusion of base metals eliminates the need for filler metals. The vacuum requirement for operation of the electron beam equipment eliminates the need for shielding gases and fluxes. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 379 ELECTRON BEAM WELDING Slide 15 of 18 5/23/2017 Dr. N. RAMACHANDRAN, NITC 380 ELECTRON BEAM WELDING Slide 16 of 18 5/23/2017 Dr. N. RAMACHANDRAN, NITC 381 Electron Beam Welding • Electron Beam Welding joins ferrous metals, light metals, precious metals, and alloys, to themselves or each other. • Multi-axis EB control • High ratio of depth-to-width • Maximum penetration with minimal distortion • Exceptional weld strength • Ability to weld components up to 10 feet in diameter • High precision and repeatability with virtually 0% scrap • Versatility from .002" depth to 3.00" depth of penetration 5/23/2017 Dr. N. RAMACHANDRAN, NITC 382 Electron Beam Welding Facts • Electron Beam Welding Advantages • Maximum amount of weld penetration with the least amount of heat input reduces distortion • Electron beam welding often reduces the need for secondary operations • Repeatability is achieved through electrical control systems • A cleaner, stronger and homogeneous weld is produced in a vacuum • The electron beam machine's vacuum environment eliminates atmospheric contaminates in the weld • Exotic alloys and dissimilar materials can be welded • Extreme precision due to CNC programming and magnification of operator viewing • Electron beam welding frequently yields a 0% scrap rate • The electron beam process can be used for salvage and repair of new and used components 5/23/2017 Dr. N. RAMACHANDRAN, NITC 383 Electron Beam Welding Speeds/Depth of Penetration 5/23/2017 Dr. N. RAMACHANDRAN, NITC 384 • Electron Beam Welding Limitations • The necessity of an electron beam welding vacuum chamber limits the size of the workpiece — EBTEC's maximum chamber size is 11' 4" wide x 9' 2" high x 12' deep 5/23/2017 Dr. N. RAMACHANDRAN, NITC Electron Beam Welding Speeds/Depth of Penetration 385 LASER BEAM WELDING(LBW) • LASER- Light Amplification by Stimulated Emission of Radiation • Focusing of narrow monochromatic light into extremely concentrated beams (0.001 mm even) • Used to weld difficult to weld materials, hard to access areas, extremely small components, In medical field to weld detached retinas back into place • Laser Beam- coherent Laser production- complex process. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 386 The LASER, an acronym for "Light Amplification by Stimulated Emission of Radiation," is a device that produces a concentrated, coherent beam of light by stimulating molecular or electronic transitions to lower energy levels, causing the emission of photons. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 387 Al2O3 + 0.05% Chromium • solid state RubyLaser- Neon flash tube emits light into specially cut ruby crystals- absorbs light electrons of chromium atoms get stimulated• Increase in stimulation ---- electrons increase from normal(ground) orbit to an exited orbit. More energy input- energy absorbed exceeds thermal energy- no longer to heat energy. • Electrons drop back to intermediate orbit- emits PHOTONS (light) called spontaneous emission • With continued emission, released photons stimulate other exited electrons to release photonscalled stimulated emission • Causes exited electrons to emit photons of same wave length. Dr. N. RAMACHANDRAN, NITC 5/23/2017 388 • Power intensities > 10 kw/cm2 • No physical contact between work and welding equipment • 2 mirrors- coherent light reflected back and forth, becomes dense, penetrates partially reflective mirror, focused to the exact point • Very little loss of beam energy • Solid state, liquid, semiconductor and gas lasers used. • Solid state uses light energy to stimulate electrons Ruby, Neodymium, YAG • Gas lasers use electrical charge to stimulate electrons Gas lasers- higher wattage outputs. Used for thicker sections - CO2, N2, He • 5/23/2017 Liquid- nitrobenzene; Gas- based on gallium arsenide Dr. N. RAMACHANDRAN, NITC 389 Laser Welding Facts • Laser Welding Advantages • Processes high alloy metals without difficulty • Can be used in open air • Can be transmitted over long distances with a minimal loss of power • Narrow heat affected zone • Low total thermal input • Welds dissimilar metals • No filler metals necessary • No secondary finishing necessary • Extremely accurate • Welds high alloy metals without difficulty • CO2 Laser Welding Speeds 5/23/2017 Dr. N. RAMACHANDRAN, NITC 390 • The solid-state laser utilizes a single crystal rod with parallel, flat ends. Both ends have reflective surfaces. A highintensity light source, or flash tube surrounds the crystal. When power is supplied by the PFN (pulse-forming network), an intense pulse of light (photons) will be released through one end of the crystal rod. The light being released is of single wavelength, thus allowing for minimum divergence 5/23/2017 Dr. N. RAMACHANDRAN, NITC 391 • One hundred percent of the laser light will be reflected off the rear mirror and thirty to fifty percent will pass through the front mirror, continuing on through the shutter assembly to the angled mirror and down through the focusing lens to the workpiece. • The laser light beam is coherent and has a high energy content. When focused on a surface, laser light creates the heat used for welding, cutting and drilling. • The workpiece and the laser beam are manipulated by means of robotics. The laser beam can be adjusted to varying sizes and heat intensity from .004 to .040 inches. The smaller size is used for cutting, drilling and welding and the larger, for heat treating 5/23/2017 Dr. N. RAMACHANDRAN, NITC 392 Laser Welding Limitations • Rapid cooling rate may cause cracking in certain metals • High capital cost • Optical surfaces easily damaged • High maintenance cost 5/23/2017 Dr. N. RAMACHANDRAN, NITC 393 LASER WELDING Slide 17 of 18 5/23/2017 Dr. N. RAMACHANDRAN, NITC 394 LASER WELDING Slide 18 of 18 5/23/2017 Dr. N. RAMACHANDRAN, NITC 395 Laser beam cutting • Along with beam, oxygen used to help cutting. Ar, He, N, CO2 also for steel, alloys etc. Two ways to weld 1. Work piece rotated or moved past beam 2. Many pulses of laser (10 times/sec)used. Narrow HAZ., speeds of 40 mm/sec to 1.5 m/sec Cooling system to remove the heatgas and liquid cooling used 5/23/2017 Dr. N. RAMACHANDRAN, NITC 396 • Klyston tubes (glass to metal sealing), capacitor bank, triggering device, flash tube, focusing lens, etc. in the setup. • Cathode of molybdenum, tantalum or titanium used. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 397 1987 Laser research begins a unique method for depositing complex metal alloys (Laser Powder Fusion). 2002 From Linde Gas in Germany, a Diode laser using process gases and "active-gas components" is investigated to enhance the "keyholing" effects for laser welding. The process gas, Argon-CO2, increases the welding speed and in the case of a diode laser, will support the transition of heat conductivity welding to a deep welding, i.e., 'key-holing'. Adding active gas changes the direction of the metal flow within a weld pool and produces narrower, highquality weld. CO2 Lasers are used to weld polymers. The Edison Welding Institute is using through-transmission lasers in the 230-980 nm range to readily form welded joints. Using silicon carbides embedded in the surfaces of the polymer, the laser is capable of melting the material leaving a near invisible joint line. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 398 Soldering and Brazing •Soldering and Brazing are joining processes where parts are joined without melting the base metals. •Soldering filler metals melt below 450 °C. •Brazing filler metals melt above 450 °C. (De)soldering a contact from a wire •Soldering is commonly used for electrical connection or mechanical joints, but brazing is only used for mechanical joints due to the high temperatures involved Soldering • A method of joining metal parts using an alloy of low melting point (solder) below 450 °C (800 °F). • Heat is applied to the metal parts, and the alloy metal is pressed against the joint, melts, and is drawn into the joint by capillary action and around the materials to be joined by 'wetting action'. • After the metal cools, the resulting joints are not as strong as the base metal, but have adequate strength, electrical conductivity, and water-tightness for many uses. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 400 Soldering and Brazing Benefits • • • • • • Economical for complex assemblies Joints require little or no finishing Excellent for joining dissimilar metals Little distortion, low residual stresses Metallurgical bond is formed Sound electrical component connections 5/23/2017 Dr. N. RAMACHANDRAN, NITC 401 Soldering can be done in a number of ways Including passing parts over a bulk container of melted solder, using an infrared lamp, or by using a point source such as an electric soldering iron, a brazing torch, or a hot-air soldering tool. A flux is usually used to assist in the joining process. Flux can be manufactured as part of the solder in single or multi-core solder, in which case it is contained inside a hollow tube or multiple tubes that are contained inside the strand of solder. Flux can also be applied separately from the solder, often in the form of a paste. In some fluxless soldering, a forming gas that is a reducing atmosphere rich in hydrogen can also serve much the same purpose as traditional flux, and provide the benefits of traditional flux in re-flow ovens through which electronic parts placed on a circuit card are transported for a carefully timed period of 402 5/23/2017 Dr. N. RAMACHANDRAN, NITC time. • One application of soldering is making connections between electronic parts and printed circuit boards. • Another is in plumbing. Joints in sheet-metal objects such as cans for food, roof flashing, and drain gutters were also traditionally soldered. • Jewelry and small mechanical parts are often assembled by soldering. 5/23/2017 Dr. N. RAMACHANDRAN, NITC Soldering can also be used as a repair technique to patch a leak in a container or cooking vessel. 403 • Soldering is distinct from welding in that the base materials to be joined are not melted, though the base metal is dissolved somewhat into the liquid solder much as a sugar cube into coffee - this dissolution process results in the soldered joint's mechanical and electrical strengths. • A "cold solder joint" with poor properties will result if the base metal is not warm enough to melt the solder and cause this dissolution process to occur. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 404 • Due to the dissolution of the base metals into the solder, solder should never be reused • Once the solder's capacity to dissolve base metal has been achieved, the solder will not properly bond with the base metal and a cold solder joint with a hard and brittle crystalline appearance will usually be the result. • It is good practice to remove solder from a joint prior to resoldering - desoldering wicks or vacuum desoldering equipment can be used. • Desoldering wicks contain plenty of flux that will lift the contamination from the copper trace and any device leads that are present. This will leave a bright, shiny, clean junctionNITC to be resoldered. 405 5/23/2017 Dr. N. RAMACHANDRAN, • The lower melting point of solder means it can be melted away from the base metal, leaving it mostly intact through the outer layer. • It will be "tinned" with solder. • Flux will remain which can easily be removed by abrasive or chemical processes. • This tinned layer will allow solder to flow into a new joint, resulting in a new joint, as well as making the new solder 5/23/2017 Dr. N. RAMACHANDRAN, NITC 406 flow very quickly and easily. Common joining problems and discontinuities: • • • • • • No wetting Excessive wetting Flux entrapment Lack of fill (voids, porosity) Unsatisfactory surface appearance Base metal erosion 5/23/2017 Dr. N. RAMACHANDRAN, NITC 407 • Basic electronic soldering techniques All solder pads and device terminals must be clean for good wetting and heat transfer. The soldering iron or gun must be clean, otherwise components may heat up excessively due to poor heat transfer. The devices must then be mounted on the circuit board properly. One technique is to elevate the components from the board surface (a few millimeters) to prevent heating of the circuit board during circuit operation. After device insertion, the excess leads can be cut leaving only a length equal to the radius of the pad. Plastic mounting clips or holders are used for large devices to reduce mounting stresses. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 408 • Heat sink the leads of sensitive devices to prevent heat damage. • Apply soldering iron or gun to both terminal lead and copper pad to equally heat both. • Apply solder to both lead and pad but never directly to the tip of soldering iron or gun. • Direct contact will cause the molten solder to flow over the gun and not over the joint. • The moment the solder melts and begins to flow, remove the solder supply immediately. • Do not remove the iron yet. The remaining solder will then flow over the junction of the lead and pad, assuming both are free of dirt. • Let the iron heat the junction until the solder flows and then remove the iron tip. This will ensure a good solid junction. • Remove the iron from the junction and let the junction cool. Solder flux will remain and should NITC be removed. 5/23/2017 Dr. N. RAMACHANDRAN, 409 • Be sure not to move the joint while it is cooling. Doing so will result in a fractured joint. • Do not blow air onto the joint while it is cooling; Instead, let it cool naturally, which will occur fairly rapidly. • A good solder joint is smooth and shiny. The lead outline should be clearly visible. Clean the soldering iron tip before you begin on a new joint. It is absolutely important that the iron tip be free of residual flux. • Excess solder should be removed from the tip. This solder on the tip is known as keeping the tip tinned. It aids in heat transfer to the joint. • After finishing all of the joints, remove excess flux residue from the board using alcohol, acetone, or other organic solvents. • Individual joints can be cleaned mechanically. • The flux film fractures easily with a small pick and can be blown away with canned air. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 410 • In solder formulations with water-soluble fluxes, • Traditional solder for electronic joints is a 60/40 Tin/Lead mixture with a rosin based flux that requires solvents to clean the boards of flux. • Environmental legislation in many countries, and the whole of the European Community area, have led to a change in formulation. • Water soluble non-rosin based fluxes have been increasingly used since the 1980's so that soldered boards can be cleaned with water or water based cleaners. This eliminates hazardous solvents from the production environment, and effluent. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 411 Lead-free electronic soldering • More recently environmental legislation has specifically targeted the wide use of lead in the electronics industry. The directives in Europe require many new electronic circuit boards to be lead free by 1st July 2006, mostly in the consumer goods industry, but in some others as well. • Many new technical challenges have arisen, with this endeavour. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 412 • For instance, traditional lead free solders have a significantly higher melting point than lead based solders, which renders them unsuitable for use with heat sensitive electronic components and their plastic packaging. To overcome this problem solder alloys with a high silver content and no lead have been developed with a melting point slightly lower than traditional solders. • Not using lead is also extended to components pins and connectors. Most of those pins were using copper frames, and either lead, tin, gold or other finishes. Tinfinishes is the most popular of lead-free finishes. However, this poses nevertheless the question of tinwhiskers. Somehow, the current movement brings the electronic industry backs to the problems solved 40 years ago by adding lead. • A new classification to help lead-free electronic manufacturers decide what kind of provisions they want to take against whiskers, depending upon their application criticity. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 413 Stained glass soldering • Historically soldering tips were copper, placed in braziers. One tip was used; when the heat had transferred from the tip to the solder (and depleted the heat reserve) it was placed back in the brazier of charcoal and the next tip was used. • Currently, electric soldering irons are used; they consist of coil or ceramic heating elements, which retain heat differently, and warm up the mass differently, internal or external rheostats, and different power ratings - which change how long a bead can be run. • Common solders for stained glass are mixtures of tin and lead, respectively: • 60/40: melts between 361°-376°F • 50/50: melts between 368°-421°F • 63/37: melts between 355°-365°F • lead-free solder (useful in jewelry, eating containers, and other environmental uses): melts around 490°F 5/23/2017 Dr. N. RAMACHANDRAN, NITC 414 Pipe/Mechanical soldering • Sometimes it is necessary to use solders of different melting points in complex jobs, to avoid melting an existing joint while a new joint is made. • Copper pipes used for drinking water should be soldered with a lead-free solder, which often contains silver. Leaded solder is not allowed for most new construction, though it is easier to create a solid joint with that type of solder. The immediate risks of leaded solder are minimal, since minerals in municipal or well water supplies almost immediately coat the inside of the pipe, but lead will eventually find its way into the environment. • Tools required for pipe soldering include a blowtorch (typically propane), wire brushes, a suitable solder alloy and an acid paste flux, typically based on zinc chloride. Such fluxes should never be used on electronics or with electronics tools, since they will cause corrosion of the delicate electronic part. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 415 Soldering defects • Soldering defects are solder joints that are not soldered correctly. • These defects may arise when solder temperature is too low. • When the base metals are too cold, the solder will not flow and will "ball up", without creating the metallurgial bond. • An incorrect solder type (for example, electronics solder for mechanical joints or vice versa) will lead to a weak joint. • An incorrect or missing flux can corrode the metals in the joint. Without flux the joint may not be clean. • A dirty or contaminated joint leads to a weak bond. A lack of solder on a joint will make the joint fail. • An excess of solder can create a "solder bridge" which is a short circuit. Movement of metals being soldered before the solder has cooled will make the solder appear grainy and may cause a weakened joint. • Soldering defects in electronics can lead to short circuits, high resistance in the joint, intermittent connections, components overheating, and damaged circuit boards. Flux left around integrated circuits' leads will lead to inter-lead leakage. •5/23/2017 It is a big issue on surface mount components and causes 416 Dr. N. RAMACHANDRAN, NITC improper device operation as moisture absorption rises. In Soldering processes • • • • • • • • • • Wave soldering Reflow soldering Infrared soldering Induction soldering Ultrasonic soldering Dip soldering Furnace soldering Iron soldering Resistance soldering Torch soldering Silver soldering/Brazing 5/23/2017 Dr. N. RAMACHANDRAN, NITC 417 Brazing • Is similar to soldering but uses higher melting temperature alloys, based on copper, as the filler metal. • "Hard soldering", or "silver soldering" (performed with high-temperature solder containing up to 40% silver) is also a form of brazing, and involves solders with melting points above 450 C. Even though the term "silver soldering" is more often used than silver brazing, it is technically incorrect. • Since lead used in traditional solder alloys is toxic, much effort in industry has been directed to adapting soldering techniques to use lead-free alloys for assembly of electronic devices and for potable water supply piping. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 418 Brazing • Brazing is a joining process whereby a non-ferrous filler metal and an alloy are heated to melting temperature (above 450°C;) and distributed between two or more close-fitting parts by capillary action. • At its liquid temperature, the molten filler metal interacts with a thin layer of the base metal, cooling to form an exceptionally strong, sealed joint due to grain structure interaction. T • he brazed joint becomes a sandwich of different layers, each metallurgically linked to each other. • Common brazements are about 1/3 as strong as the materials they join, because the metals partially dissolve each other at the interface, and usually the grain structure and joint alloy is uncontrolled. • To create high-strength brazes, sometimes a brazement can be annealed, or cooled at a controlled rate, so that the joint's grain structure and alloying is controlled. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 419 • In Braze Welding or Fillet Brazing, a bead of filler material reinforces the joint. A braze-welded tee joint is shown here. • In another common specific similar usage, brazing is the use of a bronze or brass filler rod coated with flux, together with an oxyacetylene torch, to join pieces of steel. The American Welding Society prefers to use the term Braze Welding for this process, as capillary attraction is not involved, unlike the prior silver brazing example. • Braze welding takes place at the melting temperature of the filler (e.g., 870 °C to 980 °C for bronze alloys) which is often considerably lower than the melting point of the base material (e.g., 1600 °C for mild steel). 5/23/2017 Dr. N. RAMACHANDRAN, NITC 420 • A variety of alloys of metals, including silver, tin, zinc, copper and others are used as filler for brazing processes. • There are specific brazing alloys and fluxes recommended, depending on which metals are to be joined. Metals such as aluminum can be brazed though aluminum requires more skill and special fluxes. It conducts heat much better than steel and is more prone to oxidation. • Some metals, such as titanium cannot be brazed because they are insoluble with other metals, or have an oxide layer that forms too quickly at intersoluble temperatures. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 421 • Although there is a popular belief that brazing is an inferior substitute for welding, this is false. • For example, brazing brass has a strength and hardness near that of mild steel, and is much more corrosion-resistant. • In some applications, brazing is indisputably superior. For example, silver brazing is the customary method of joining high-reliability, controlled-strength corrosion-resistant piping such as a nuclear submarine's seawater coolant pipes. • Silver brazed parts can also be precisely machined after joining, to hide the presence of the joint to all but the most discerning observers, whereas it is nearly impossible to machine welds having any residual slag present and still hide joints. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 422 • In order to work properly, parts must be closely fitted and the base metals must be exceptionally clean and free of oxides for achieving the highest strengths for brazed joints. • For capillary action to be effective, joint clearances of 0.002 to 0.006 inch (50 to 150 µm) are recommended. In braze-welding, where a thick bead is deposited, tolerances may be relaxed to 0.5 mm. • Cleaning of surfaces can be done in several ways. Whichever way is selected, it is vitally important to remove all grease, oils, and paint. For custom jobs and part work, this can often be done with fine sand paper or steel wool. • In pure brazing (not braze welding), it is vitally important to use sufficiently fine abrasive. Coarse abrasive can lead to deep scoring that interferes with capillary action and final bond strength. Residual particulates from sanding should be thoroughly cleaned from pieces. •5/23/2017 In assembly line Dr. work, a "pickling N. RAMACHANDRAN, NITCbath" is often used 423 to dissolve oxides chemically. Dilute sulfuric acid is • In most cases, flux is required to prevent oxides from forming while the metal is heated. The most common fluxes for bronze brazing are borax-based. T • he flux can be applied in a number of ways. It can be applied as a paste with a brush directly to the parts to be brazed. Commercial pastes can be purchased or made up from powder combined with water (or in some cases, alcohol). Alternatively, brazing rods can be heated and then dipped into dry flux powder to coat them in flux. • Brazing rods can also be purchased with a coating of flux. In either case, the flux flows into the joint when the rod is applied to the heated joint. Using a special torch head, special flux powders can be blown onto the workpiece using the torch flame itself. • Excess flux should be removed when the joint is completed. Flux left in the joint can lead to corrosion. • During the brazing process, flux may char and adhere to the work piece. Often this is removed by quenching the still-hot workpieceDr.inN.water (to loosen the flux scale), 424 5/23/2017 RAMACHANDRAN, NITC followed by wire brushing the remainder. • Brazing is different from welding, where even higher temperatures are used, the base material melts and the filler material (if used at all) has the same composition as the base material. • Given two joints with the same geometry, brazed joints are generally not as strong as welded joints. Careful matching of joint geometry to the forces acting on the joint, however, can often lead to very strong brazed joints. • The butt joint is the weakest geometry for tensile forces. The lap joint is much stronger, as it resists through shearing action rather than tensile pull and its surface area is much larger. To get joints roughly equivalent to a weld, a general rule of thumb is to make the overlap equal to 3 times the thickness of the pieces of metal being joined. • The "welding" of cast iron is usually a brazing operation, with a filler rod made chiefly of nickel being used although true welding with cast iron rods 5/23/2017 Dr. N. RAMACHANDRAN, NITC 425 is also available. • Vacuum brazing is another materials joining technique, • • • • one that offers extremely clean, superior, flux free braze joints while providing high integrity and strength. The process can be expensive because it is performed inside a vacuum chamber vessel however, the advantages are significant. For example, furnace operating temperatures, when using specialized vacuum vessels, can reach temperatures of 2400 °C. Other high temperature vacuum furnaces are available ranging from 1500 °C and up at a much lesser cost. Temperature uniformity is maintained on the work piece when heating in a vacuum, greatly reducing residual stresses because of slow heating and cooling cycles. This, in turn, can have a significant impact on the thermal and mechanical properties of the material, thus providing unique heat treatment capabilities. One such capability is heat treating or age hardening the work piece while performing a metal-joining process, all in a single furnace thermal cycle. Reference: M.J.Fletcher, “Vacuum Brazing”. Mills and Boon Limited: London, 1971. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 426 Advantages over welding • The lower temperature of brazing and brass-welding is less likely to distort the work piece or induce thermal stresses. For example, when large iron castings crack, it is almost always impractical to repair them with welding. In order to weld cast-iron without recracking it from thermal stress, the work piece must be hot-soaked to 1600 °F. When a large (more than fifty kilograms (100 lb)) casting cracks in an industrial setting, heat-soaking it for welding is almost always impractical. Often the casting only needs to be watertight, or take mild mechanical stress. Brazing is the premium, preferred repair method in these cases. • The lower temperature associated with brazing vs. welding can increase joining speed and reduce fuel gas consumption. • Brazing can be easier for beginners to learn than welding. • For thin workpieces (e.g., sheet metal or thin-walled pipe) brazing is less likely to result in burn-through. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 427 • Brazing can also be a cheap and effective technique for mass production. Components can be assembled with preformed plugs of filler material positioned at joints and then heated in a furnace or passed through heating stations on an assembly line. The heated filler then flows into the joints by capillary action. • Braze-welded joints generally have smooth attractive beads that do not require additional grinding or finishing. • The most common filler materials are gold in colour, but fillers that more closely match the color of the base materials can be used if appearance is important. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 428 Possible problems • A brazing operation may cause defects in the base metal, especially if it is in stress. This can be due either to the material not being properly annealed before brazing, or to thermal expansion stress during heating. • An example of this is the silver brazing of copper-nickel alloys, where even moderate stress in the base material causes intergranular penetration by molten filler material during brazing, resulting in cracking at the joint. • Any flux residues left after brazing must be thoroughly removed; otherwise, severe corrosion may eventually occur. 5/23/2017 Dr. N. RAMACHANDRAN, NITC 429 Brazing processes • • • • • • • • • • • • Block Brazing Diffusion Brazing Dip Brazing Exothermic Brazing Flow Brazing Furnace Brazing Induction Brazing Infrared Brazing Resistance Brazing Torch Brazing Twin Carbon Arc Brazing Vacuum Brazing 5/23/2017 Dr. N. RAMACHANDRAN, NITC 430