Download Gas Tungsten Arc Welding

Gas Tungsten Arc Welding
Objectives
• Describe the gas tungsten arc welding process
– List other terms used to describe it
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What makes tungsten a good electrode
Eliminate tungsten erosion
Shape and clean a tungsten electrode
Grind a point on a tungsten electrode
Objectives (continued)
• Remove a contaminated tungsten end
• Melt the end of the tungsten electrode into the
desired shape
• Compare water-cooled GTA welding torches to
air-cooled torches
• The purpose of the three hoses connecting a
water-cooled torch to the welding machine
Objectives (continued)
• Choose an appropriate nozzle
• How to get an accurate reading on a flowmeter
• Compare the three types of welding current used
for GTA welding
• Shielding gases used in the GTA welding process
Objectives (continued)
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Define preflow and postflow
Problems resulting from an incorrect gas flow rate
Properly set up a GTA welder
Establish a GTA welding arc
Introduction
• The Gas Tungsten Arc Welding (GTAW) process
is sometimes referred to as a TIG or Heliarc
– TIG is short for tungsten inert gas
• An arc is established between a non-consumable
tungsten electrode (heating element) and the
base metal
• The inert gas provides the needed arc
characteristics and protects the molten weld pool
• When Argon became plentiful, the GTA process
became more common
Power Source Basics
Tungsten Electrodes
Tungsten
• Tungsten has the following properties:
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High tensile strength
Hardness
High melting temperature
High boiling temperature
Good electrical conductivity
Tungsten (continued)
• Tungsten is the best choice for a non consumable
electrode
– High melting temperature
– Good electrical conductivity
• As the tungsten electrode becomes hot the arc
between the electrode and the work stabilizes
– But a clean and correctly ground tungsten is
needed
• Because of the intense heat some erosion of the
electrode will occur
Figure 15-1 Some tungsten will erode and be transferred across the arc.
Tungsten (continued)
• Ways to limit erosion:
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Good mechanical and electrical contact
Use as low a current as possible
Use a water-cooled torch
Use as large a tungsten electrode as possible
Use DCEN current
Use as short an electrode extension as possible
Use the proper shape electrode
Use an alloyed tungsten electrode
Torch Build Out
Torch Build Out
Tungsten (continued)
• The collet is the cone-shaped sleeve that holds
the electrode in the torch
• Large-diameter electrodes conduct more current
• The current-carrying capacity at DCEN is about
ten times greater than at DCEP
• The preferred electrode shape impacts the
temperature and erosion of the tungsten
• With alternating current, the tip is subjected to
more heat than with DCEN
Figure 15-3 The smooth surface of a centerless ground tungsten electrode. Courtesy of
Larry Jeffus.
Types of Tungsten Electrodes
• Pure tungsten is an excellent nonconsumable
electrode
• Pure tungsten can be improved by adding:
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Cerium
Lanthanum
Thorium
Zirconium
Tungsten Electrodes
Table 15-1 Tungsten Electrode Types and Identification.
Shaping the Tungsten
• To obtain the desired end shape:
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Grinding (for MS and SS)
Breaking (not recommended due to cost)
Re melting the end (Aluminium welding)
Using chemical compound (doesn’t work that well)
Grinding
• Often used to clean a contaminated tungsten or
to point the end
• Should have a fine, hard stone
– A coarse grinding stone with result in more
tungsten breakage
• Should be used for grinding tungsten only
– Metal particles will quickly break free when the arc
is started, causing contamination
Figure 15-8 Correct way of holding a tungsten when grinding. Courtesy of Larry Jeffus.
Breaking and Remelting
• Tungsten is hard but brittle
– If struck sharply, it will break without bending
– Try not to do this because of $$$$$$$$
• Holding against a sharp corner and hitting results
in a square break
• After breaking squarely, melt back the end
Chemical Cleaning and Pointing
• Tungsten can be cleaned and pointed using one
of several compounds
• Heated by shorting it against the work
• Dipped in the compound
• When the tungsten is removed, cooled, and
cleaned, the end will be tapered to a fine point
• The chemical compound will dissolve the
tungsten, allowing the contamination to fall free
Pointing and Remelting
• Tapered tungsten with a balled end is made by
first grinding or chemically pointing
• The ball should be made large enough so that the
color of the end stays dull red and bright red
• Increase ball size by applying more current
• Surface tension pulls the molten tungsten up onto
the tapered end
Figure 15-14 Melting the tungsten end shape.
GTAW Equipment
GTA Welding Equipment
“Cadillac Stick Welder”
• GTA welding torches are water- or air-cooled
• Water-cooled GTA welding torch is more efficient
• Water-cooled torch has three hoses connecting it
to the welding machine
• Nozzle directs the shielding gas directly on the
welding zone
• Flowmeter regulates the rate of gas flow
Figure 15-21 Schematic of a GTA welding setup with a water-cooled torch.
Types of Welding Current
• DCEN concentrates about 2/3 of its welding heat
on the work
– Max penetration
– High Freq. – start only
• DCEP concentrates about 1/3 of its welding heat
on the work
– Max cleaning action
– 2/3 of heat at tungsten – primarily used for balling
tungsten for aluminium welding
– High Freq. – start only
Types of Welding Current
• AC concentrates its heat at 50/50
– Sign wave provides for DCRP (cleaning action)
and DCSP (penetration action)
– Square wave technology allows for adjusting the
cleaning or penetration cycle.
– High Freq. is on Continuous so there is equal
firing of both sides of sign wave.
– DC Component will take place if there is no High
Freq.
Figure 15-29 Electrons collect under the oxide layer during the DCEP portion of the cycle.
Figure 15-30 Sine wave of alternating current at 60 cycle.
Shielding Gas
Shielding Gases
• Shielding gases used for GTA welding process:
– Argon (Ar)
– Helium (He)
– Or a mixture of two or more gases
Shielding Gases (continued)
• Argon effectively shields welds in deep grooves in
flat positions
• Helium offers the advantage of deeper
penetration
Shielding Gases (continued)
• Hot start allows a surge of welding current
• Preflow is the time gas flows to clear out air in the
nozzle
– Some machines do not have preflow
• Postflow is the time the gas continues flowing
after the welding current has stopped
Shielding Gases (continued)
• Ionization Potential
– Amount of voltage needed to “kick start” the arc
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The ionization potential, or ionization energy, of a gas atom is the energy required to strip it of an electron. That is
why a shielding gas such as helium, with only 2 electrons in its outer shell, requires more energy (higher voltage
parameters) for welding. The ionization potential of a shielding gas also establishes how easily an arc will initiate and
stabilize. A low ionization potential means the arc will start relatively easy and stabilize quite well. A high ionization
potential has difficulty initiating and may have difficulty keeping the arc stable.
• Argon
– 15.7 electron volts
• Helium
– 24.4 electron volts
– More penetration
Figure 15-35 Too steep an angle between the torch and work may draw in air.
Remote Controls
Foot or Finger
Remote Controls
• Can be used to:
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Start the weld
Increase the current
Decrease the current
Stop the weld
• Remote can be foot-operated or hand-operated
device
Welding Techniques
Objectives
• Applications using the gas tungsten arc welding
process
• Effects on the weld of varying torch angles
• Why and how the filler rod is kept inside the
protective zone of the shielding gas
• How tungsten contamination occurs and what to
do
• Causes of change in welding amperage
• Correct settings for the minimum and maximum
welding current
Objectives (continued)
• Types and sizes of tungsten and metal
• Factors affecting gas preflow and postflow times
• Minimum and maximum gas flow settings:
– Nozzle size
– Tungsten size
– Amperage setting
• Characteristics of low carbon and mild steels,
stainless steel, and aluminum
• Metal preparation for GTA welding
• Make GTA welds in all positions
Introduction
• Gas tungsten arc is also called GTA welding
• GTA welding can be used to for nearly all types
and thicknesses of metal
• GTA welding is fluxless, slagless, and
smokeless
• Welders have fine control of the welding process
• GTA welding is ideal for close-tolerance welds
• Some GTA welds make the critical root pass
• GTA used when appearance is important
Introduction (continued)
• Setup of GTA equipment affects weld quality
– Charts give correct settings
• Field conditions affect the variables in the charts
• Experiments designed to evaluate the
appearance of a weld
• After welding in the lab, troubleshooting field
welding problems is easier
• To make a weld is good: to solve a welding
problem is better
Torch Angle
• As close to perpendicular as possible
• May be angled 0-15 degrees from perpendicular
for better visibility
• As the gas flows out it forms a protective zone
around the weld
• Too much tilt distorts protective shielding gas
zone
Figure 16-5 Filler being remelted as the weld is continued. Courtesy of Larry Jeffus.
Torch Angle (continued)
• Velocity of shielding gas affects protective zone
• Low-pressure area develops behind the cup
when velocity increases
• Sharper angle and higher flow rate increases
contamination
Filler Rod Manipulation
• Filler rod must be kept inside the protective zone
• If filler rod is removed from the gas protection, it
oxidizes rapidly
– Oxide is added to the molten weld pool
• When a weld is temporarily stopped, the
shielding gas must be kept flowing
Filler Rod Manipulation (continued)
• If the rod tip becomes oxidized, if should be cut
off before restarting
• The rod should enter the shielding gas as close
to the base metal as possible
• An angle less than 15 degrees prevents air from
being pulled in the welding zone
Figure 16-2 The hot filler rod end is well within the protective gas envelope. Courtesy of
Larry Jeffus.
Figure 16-7 Too much filler rod angle has caused oxides to be formed on the filler rod
end. Courtesy of Larry Jeffus.
Tungsten Contamination
• Most frequent problem is tungsten
contamination
• Tungsten becomes contaminated if it touches:
– Molten weld pool
– Filler metal
• Surface tension pulls the contamination up onto
the hot tungsten
• Extreme heat causes some of the metal to
vaporize and form a large oxide layer
Tungsten Contamination (continued)
• Contamination caused by the tungsten touching
the molten pool or filler metal forms a weak weld
• The weld and tungsten must be cleaned before
any more welding can be done
• Tiny tungsten particles will show up if the weld is
x-rayed
• Contamination can be knocked off quickly by
flipping the torch head
• This procedure should never be used with heavy
contamination or in the field
Figure 16-8 Contaminated tungsten. Courtesy of Larry Jeffus.
Current Setting
• Amperage on the machine's control is the same
at the arc when:
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Power to the machine is exactly correct
Lead length is very short
All cable connections are perfect
Arc length is exactly right
Remote current control is in the full on position
Figure 16-10 Melting first occurring. Courtesy of Larry Jeffus.
Figure 16-12 Oxides forming due to inadequate gas shielding. Courtesy of Larry Jeffus.
Gas Flow
• Gas preflow and postflow times depend upon:
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Wind or draft speed
Tungsten size used
Amperage
Joint design
Welding position
Type of metal welded
• Maximum flow rates must never be exceeded
– Air can be sucked into the weld zone
Practice Welds
• Practice welds are grouped according to the
weld position and type of joint
• Mild steel is inexpensive and requires the least
amount of cleaning
• With aluminum, cleanliness is a critical factor
• Try each weld with each metal to determine
which metal will be easier to master
Low Carbon and Mild Steels
• Low carbon and mild steel are two basic steel
classifications
• Small pockets of primary carbon dioxide gas
become trapped
• Porosity most likely when not using a filler metal
• Most filler metals have some alloys, called
deoxidizers
Stainless Steel
• Setup and manipulation are nearly the same as
for low carbon and mild steels
• Most welds on stainless steels show effects of
contamination
• Most common problem is the bead color after
the weld
• Using a low arc current with faster travel speeds
is important
Aluminum
• Molten aluminum weld pool has high surface
tension
• Preheat the base metal in thick sections
• Preheat temperature is around 300° Fahrenheit
• Cleaning and keeping the metal clean is time
consuming
• Aluminum rapidly oxidizes at welding
temperatures
Metal Preparation
• Base and filler metals must be thoroughly
cleaned
• Contamination will be deposited into the weld
• Oxides, oil, and dirt are the most common
• Contaminants can be removed mechanically or
chemically
Figure 16-15 Aluminum filler being correctly added to the molten weld pool. Courtesy of
Larry Jeffus.
Figure 16-16 Filler rod being melted before it is added to the molten pool. Courtesy of
Larry Jeffus.
Figure 16-18 Surfacing weld. Courtesy of Larry Jeffus.
Figure 16-20 Establish a molten weld pool and dip the filler rod into it. Courtesy of Larry
Jeffus.
Figure 16-21 Note the difference in the weld produced when different size filler rods are
used. Courtesy of Larry Jeffus.
Figure 16-22 Move the electrode back as the filler rod is added. Courtesy of Larry Jeffus.
Figure 16-34 Be sure both the top and bottom pieces are melted. Courtesy of Larry
Jeffus.
Figure 16-35 Oxides form during tack welding. Courtesy of Larry Jeffus.
Figure 16-36 A notch indicates the root was not properly melted and fused. Courtesy of
Larry Jeffus.
Figure 16-37 Watch the leading edge of the molten weld pool. Courtesy of Larry Jeffus.
Summary
• Positioning yourself to control the electrode filler
metal and to see the joint is critical
• Experienced welders realize they need to see
only the leading edge of the weld pool
• Good idea to gradually reduce your need for
seeing 100% of the weld pool
– Increasing this skill is significant advantage in the
field
• Welding in the field may have to be done out of
position