Download Wire Feed Welding Equipment Power Sources

120103a
Welder
Wire Feed Welding Equipment
Power Sources
GMAW, FCAW, MCAW and SAW
First Period
Table of Contents
Objective One ............................................................................................................................................... 2 Wire Feed Welding ................................................................................................................................... 2 Safety ........................................................................................................................................................ 2 Gas Metal Arc Welding (GMAW)............................................................................................................ 4 Flux Cored Arc Welding (FCAW)............................................................................................................ 7 Metal-Cored Arc Welding (MCAW) ...................................................................................................... 10 Objective Two............................................................................................................................................. 12 Equipment for Wire Feed Welding ......................................................................................................... 12 Objective Three ........................................................................................................................................... 15 Power Source Selection Guidelines ........................................................................................................ 15 Objective Four ............................................................................................................................................ 19 GMAW Advantages and Disadvantages ................................................................................................. 19 FCAW Advantages and Disadvantages .................................................................................................. 20 MCAW Advantages and Disadvantages ................................................................................................. 21 Self-Test ...................................................................................................................................................... 22 Self-Test Answers ....................................................................................................................................... 26 Wire Feed Welding Equipment Power
Sources
NOTES
Rationale
Why is it important for you to learn this skill?
To work safely and efficiently with wire feed welding processes, you need to be familiar
with their power sources.
Outcome
When you have completed this module, you will be able to:
Select wire feed welding equipment.
Objectives
1.
2.
3.
4.
Describe the principles of operation of wire feed welding equipment.
Identify the components of a wire feed welding equipment set-up.
Describe wire process welding equipment power sources and wire feeders.
Identify advantages and disadvantages of wire feed processes.
Introduction
In this module, you will learn about the power sources involved in wire feed welding
processes. You will learn about the principles of operation and history of gas metal arc
welding (GMAW) and flux cored arc welding (FCAW), the components of a wire feed
set-up and the modes of metal transfer. You will also learn about the advantages and
disadvantages of GMAW, FCAW and metal-cored arc welding (MCAW) and relevant
safety principles.
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Objective One
When you have completed this objective, you will be able to:
Describe the principles of operation of wire feed welding equipment.
Wire Feed Welding
Wire feed welding has been developed primarily for increasing production efficiency and
quality in welding on a variety of construction materials. You will use wire feed
processes extensively for fabricating metals of many types and thicknesses.
Safety
Always consider the safe operating characteristics of the process you use before you start
any welding operation with wire feed equipment. Read the operator's manuals supplied
with your equipment and comply with all safety recommendations.
Electrical Hazards
When you work with welding equipment, electrical shock hazards are always present.
You must ensure that all electrical equipment is properly grounded and, if possible, use
ground fault interrupters in the electrical circuit. Never bypass or disconnect grounding
connections.
You must check all electrical connections to ensure that they are connected correctly and
are secure. The work lead (ground cable) should be firmly connected to the work.
To avoid the possibility of electric shocks, ensure that the following conditions exist.
 All electrical connections and insulation on electrode holder (gun) handles and
cables are in good condition.
 Clear the work area and rid it of obstacles and possible hazards, such as extra
hoses and cables.
 Protect hoses and cables from hot falling objects.
 Solder cable lugs or solidly attach them to provide full electrical contact.
 Put all protective covers in place and ensure that the welding machine is in
proper working order.
 Ensure that all connections are leak-tight if you use a water-cooled gun.
 Insulate your body from the work piece and the gun.
 Wear dry gloves that are in good condition, as perspiration can cause gloves to
become electrical conductors.
 Protect yourself from electrical shock hazards by wearing rubber boots with dry
socks and rubber gloves when you expose yourself to wet weather conditions or
stand on wet surfaces.
Toxic Fumes
Ozone and other harmful fumes may become toxic in large concentrations during welding
operations. Ozone forms when oxygen in the atmosphere comes in contact with the
intense ultraviolet light radiation that the arc produces. Air-supplied face shields or
respirators may be necessary in confined areas that are not adequately ventilated.
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Where you use of chlorinated solvents (carbon tetrachloride, trichlorethylene or
tetrachlorethylene) for cleaning or degreasing, an additional hazard is present. The high
ultraviolet radiation from the arc in GMAW can convert the chlorine to deadly phosgene
gas. (Phosgene gas has a threshold limit of 1 part in 10 million). This reaction can occur
up to tens of metres (hundreds of feet) away from the arc source. You must store
containers of these solvents and use them in an isolated area far from welding operations.
NOTES
Ventilation
By placing a low-velocity suction hood a short distance away from the welding operation,
you can have adequate ventilation that does not disturb the gas shield. Air movement or
drafts of over 1415 L/min (50 cfm) could blow the protective gas envelope away from the
weld zone.
In addition to toxic fume hazards, shielding gases in sufficient quantities displace enough
air to render the work area oxygen deficient. Dizziness or euphoria is usually the first
signs of oxygen deficiency.
NOTE
Always take precautions to ensure the supply of adequate breathing
air.
Personal Protection
You are responsible for ensuring that you wear the proper personal protection for the
work you must perform.
Protective Clothing and Footwear
Protective clothing varies with the nature and location of the work. The electric arc from
the wire feed welding processes emits intense visible, ultraviolet and infrared rays that
are extremely dangerous to the eyes and skin. For this reason, you require full body
protection. For clothing and footwear selection, you should consider the following points.
 Wear 100% cotton clothing that is dark in colour to reduce reflected light.
 Wear tight-knit clothing to prevent penetration of light radiation.
 Wear trousers without cuffs and make sure that all clothing is in good condition
without frayed edges.
 Keep clothing free of oil and grease.
 Wear head and hair protection suitable for arc welding operations.
 Wear welders' gauntlet gloves.
 Wear steel-toed safety boots as standard footwear.
Eye and Face Protection
For eye and face protection, use a recommended welder's helmet, complete with suitable
filter plates to protect against harmful light rays and the intense heat that the arc
produces.
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The filter plates you use in the helmets should conform to American Welding Society
(AWS) and Canadian Standards Association (CSA) recommendations.
 Shade #10 is the AWS and CSA recommended shade as a minimum.
 Shade #11 is the suggested shade when you weld in the range of 60 - 160
amperes.
 Shade #2 (tinted safety glasses), complete with side shields, is the recommended
shade for the welder and other personnel in the welding area.
Hearing Protection
When you weld in an area with a high noise level, hearing protection is mandatory to
avoid hearing loss.
Gas Metal Arc Welding (GMAW)
Introduced in the 1920s, gas metal arc welding (GMAW) was not commercially available
until 1948. The development of GMAW was originally to eliminate the use of fluxes
when welding aluminum and magnesium alloys. The fluxes caused corrosion problems
when the welder subjected the joint to heat and/or acid service conditions. The welding of
aluminum and magnesium alloys required an inert shielding gas such as argon or helium
for proper shielding. The new process was called metal inert gas (MIG). Later the MIG
process was adapted to weld carbon and low alloy steels using carbon dioxide (CO2)
shielding gas.
Carbon dioxide is not an inert gas; it breaks down in the arc to form compounds with
other elements. Active gases react chemically in the arc atmosphere. Some publications
use the term metal active gas (MAG) to describe a process similar to MIG welding, but
with active gas shielding. These terms are quite restrictive and often technically incorrect.
The general term gas metal arc welding (GMAW) has been adopted to describe this
process, regardless of the type of gas used.
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Figure 1 shows a typical equipment set-up for GMAW.
Figure 1 - Equipment set-up used for GMAW.
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Principles of Operation
Gas metal arc welding (GMAW), commonly called MIG welding, heats metals with an
electric arc established between a continuously fed filler metal (consumable) electrode
and the workpiece to join the metals. The operator makes the appropriate settings to
maintain a constant burn-off rate of the wire electrode. Depending upon the power source
and the wire drive system the operator uses, the arc length maintains itself automatically.
An externally applied shielding gas (Figure 2) prevents atmospheric contamination.
Figure 2 - Gas metal arc welding process.
Generally GMAW uses direct current electrode positive (DCEP). Filler metal transfers to
the base metal through one of three main modes of transfer. The mode you choose
depends on factors such as:
 the position of welding,
 the current and voltage range,
 the shielding gas and
 the type of metal being welded.
The GMAW process is used for machine, semi-automatic and fully automatic welding of
these materials:
 carbon and alloy steels,
 stainless steels and
 several classes of non-ferrous metals, including copper alloys and aluminum.
These processes are used in the pressure vessel and piping industry, on power generation
equipment, construction equipment, agricultural equipment and in many other areas that
include welded metal fabrications.
Gas metal arc welding has taken over much of the work that more tradition processes
such as oxyacetylene welding (OAW) and stick welding (SMAW) had done. As GMAW
equipment and consumables improve, their use has increased over time.
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Figure 3 shows a typical shop setting where GMAW is being used in the semi-automatic
mode.
NOTES
Figure 3 - GMAW being used to weld a pressure vessel nozzle neck.
(Courtesy Dacro Industries Ltd.)
Flux Cored Arc Welding (FCAW)
The creation of the flux cored arc welding (FCAW) occurred because of the development
of CO2 shielding combined with a flux-filled tubular electrode. It became commercially
available in the mid-1950s. The FCAW process was immediately favoured for its high
weld metal deposition rates, but it did not always produce the required mechanical
properties for some weldments. It also had poor operator appeal because of the heat and
the high fume emissions from welding. Several refinements to this process have occurred
in recent years, making it a recognized high-production process for welded fabrication
projects.
Applications
The FCAW process is used for machine, semi-automatic and fully automatic welding of
carbon and alloy steels, stainless steels and hard surfacing applications. It is used in the
pressure vessel and piping industry, on power generation equipment, construction
equipment, agricultural equipment and in many other areas that include welded metal
fabrications.
Much of the work that the more traditional processes such as OAW and SMAW did in
the past, FCAW has taken over. As FCAW equipment and consumables improve, their
use has increased over time.
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Figure 4 shows a typical shop setting where FCAW is being used on a bridge component
structure in the semi-automatic mode.
Figure 4 - FCAW semi-automatic application.
Principles of Operation
Flux cored arc welding uses an arc established between a continuously fed consumable
electrode and the workpiece. Appropriate settings that the operator makes maintain a
constant burn-off rate of the wire electrode. Depending upon the power source and wire
drive system you use, the arc length maintains itself automatically.
Shielding gas can be externally applied and/or obtained from a flux contained within the
hollow electrode core (depending upon the type of flux cored wire used). The designation
FCAW-G indicates the use of an externally supplied shielding gas.
Gas-Shielded FCAW
When you use gas-shielded FCAW, a mixture of CO2 and argon or only CO2 protects the
molten weld pool from atmospheric contamination. The filler metals are formulated with
deoxidizers to combine with any small amounts of oxygen that may be produced from the
breakdown of CO2 gas during welding.
The gas-shielded method is well suited to the production of narrow, deeply penetrating
welds. The use of short electrode extensions and high welding currents is for all wire
diameters. The electrode extension cannot be excessive with the gas-shielded method
because of adverse effects on shielding coverage.
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Figure 5 shows the gas shielded flux cored arc welding process.
Figure 5 - Gas shielded flux cored arc welding process.
Self-Shielded FCAW
Self-shielded FCAW uses an electrode containing a flux that generates all the shielding
gas required. The self-shielded method obtains shielding from vaporized flux ingredients
that displace atmospheric air and by slag compositions that cover the molten metal.
Self-shielded FCAW is a common choice for field fabrications or repairs because selfshielded electrodes tolerate windy conditions better than gas shielded electrodes. This is
due to the production of CO2 and the introduction of deoxidizing and denitriding agents
from flux ingredients at the surface of the weld pool.
On certain types of self-shielding flux cored electrodes, direct current electrode negative
(DCEN) is used. This results in less base metal penetration. Because of the lower
penetration characteristics of DCEN, the smaller diameter electrodes have proven to be
quite successful for work on thin materials using FCAW.
Self-shielded FCAW generally employs long electrode extensions. For self-shielded
electrodes, extensions ranging from 19 mm - 95 mm (34" - 334") are generally used,
depending upon the type and diameter of the electrode. You should consult with the
electrode manufacturer for recommended electrode extensions. Increasing the electrode
extension results in a narrow bead with shallow penetration. This makes the process
suitable for light gauge materials and for bridging gaps with poor fit-ups. If the arc length
(voltage) and the welding current increase to higher settings, a longer electrode extension
increases weld metal deposition rates.
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Figure 6 shows the self-shielded flux cored arc welding process.
Figure 6 - Self-shielded flux cored arc welding process.
Metal-Cored Arc Welding (MCAW)
Metal-cored arc welding (MCAW) is very similar to FCAW. The process uses the same:
 power sources,
 wire feeders,
 gun and cable assemblies and
 shielding gases.
Since the 1970s, metal-cored wire has been a filler metal option for welding a wide range
of applications, including mild and low alloy steels. The technology has begun to gain
greater popularity in recent years and has also undergone significant advancements.
Nowadays the wire can be more readily alloyed to match different materials. When a job
calls for special electrodes, metal-cored electrodes are more economical than solid
electrodes. The manufacturing process involves blending metal powders instead of
creating a special melt of steel. As a result, small quantities are easier to produce.
The MCAW wire is a tubular wire electrode consisting of a metal outer sheath and a core
of various powdered materials. The outer sheath can be cobalt, nickel, iron or stainless
steel. The core consists of metallic powders, alloys and arc stabilizers, each of which
offer distinct benefits to the final weld.
Lowering oxidation, providing higher impact strengths and reducing silicon deposits are
variables that different companies may require in their specific applications. The outer
sheath and the internal powder composition can be formulated to meet specific
requirements.
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Spray transfer in the flat or horizontal position is an example of a common MCAW
application. All position welding can be more readily done when you use the pulsed
current (which allows higher deposition rates and better puddle control) is another
application. Another application of MCAW welding is the multiple pass robotic and
automatic welding.
NOTES
Some advantages of using MCAW are:
 fast, good quality roots using a power source equipped with modified short
circuit capabilities;
 no slag and almost no spatter and
 very little post-weld or inter-pass cleaning.
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Objective Two
When you have completed this objective, you will be able to:
Identify the components of a wire feed welding equipment set-up.
Equipment for Wire Feed Welding
The basic equipment you require for the application of wire feed welding is:
 a welding power source,
 a wire feeder compatible with the power source,
 a electrode holder (welding gun) and cable assembly,
 spooled filler wire,
 shielding gas,
 regulator/flowmeter and
 a cooling system (if required).
Figure 7 shows a typical wire feed equipment set-up.
Figure 7 - Wire feed equipment set-up.
(Courtesy ESAB Welding & Cutting Products)
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Power Source
The welding power source delivers current to the electrode and workpiece to produce an
arc. Most wire feed applications use reverse polarity (DRRP). The common types of
power sources may produce either a constant current (CC) or constant voltage (CV)
output. The power sources are:
 motor-driven generators or alternators,
 transformer-rectifiers and
 inverters.
Wire Feeder
The wire feeder houses a control system, a drive roll system and a variable-speed feed
motor. The control system is mainly responsible for regulating the speed of the feed
motor and for starting and stopping electrode feed. The feeder control system may also
contain solenoids for operating the shielding gas flow valve and water coolant flow valve
when used.
The drive roll system pushes the wire electrode forward through the cable assembly and
gun to the weld zone. An electronic governor that connects to the control system controls
the variable speed feed motor, which usually operates on direct current (dc).
Electrode Holders and Cable Assembly
Electrode holders (guns) and cable assemblies are normally designed to carry welding
current, shielding gas, control switch wires and coolant. Gun and cable assemblies are
rated by their current carrying capacity. In semi-automatic welding, the arc has to be
directed manually to the joint to be welded. In fully automatic welding, the arc is directed
or positioned to the joint by mechanical means.
Spooled Filler Wire
Chemical composition, mechanical properties, manufacturing, sizing and storage of solid
wires for GMAW are classified in AWS and CSA specifications. Many CSA and AWS
specified filler wires are available to satisfy most applications.
Shielding Gas
Shielding gases can range from argon or helium for GMAW on aluminum, to welding
grade dry CO2 or a mixture of argon and CO2 and other gases for welding carbon and
low-alloy steels with GMAW, FCAW or MCAW. Shielding the molten weld metal from
atmospheric contamination is the primary function of the shielding gas, but it also affects
other variables such as metal transfer modes, arc stability and penetration.
Regulator/Flowmeter
A combination regulator/flowmeter is often used for wire feed processes. The regulator
reduces the gas source pressure to a constant working pressure. Flowmeters control the
rate of flow of a shielding gas to the gun nozzle. Flowmeters are calibrated to indicate a
volumetric flow rate in litres per minute (L/min) or cubic feet per hour (CFH).
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Cooling System
High current applications with wire feed processes will usually require an additional
cooling system to help cool the electrode holder. Water cooling is more common when
using the fully automatic mode where high speed, high amperage welding is required.
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Objective Three
When you have completed this objective, you will be able to:
Describe wire process welding equipment power sources and wire feeders.
Power Source Selection Guidelines
Welding power sources for wire feed processes (welding machines or simply machines)
can range from rated output currents of less than 100 amperes (amps) for light duty sheet
metal work with GMAW to more than 500 amps for high production automatic use with
FCAW or MCAW.
Consider the following guidelines when selecting a welding power source and related
equipment.
 Rated output (amperage range required).
 Mode of metal transfer required.
 Duty cycle rating (intermittent or continuous welding).
 Primary power compatibility with the input power requirements of the welding
power source.
 Portability (welding in the shop or in the field).
 Remote control (welding near the power source or at some distance from it).
 Process capability (one process or multiple processes).
Rating Welding Power Sources
The construction of welding power sources meet specifications set by the National
Electrical Manufacturers Association (NEMA). The sources are rated based on
amperage, voltage and duty cycle. For example, a machine is rated for
300 amps with a 60% duty cycle. This means that the machine can operate at
300 amps for 6 minutes out of 10 minutes of elapsed time. The power source must be idle
for 4 minutes out of this 10 minutes to allow cooling and to prevent overheating.
Conversely, the power source can operate continuously at 60% of its maximum load
which is 180 amps.
Machines are manufactured with duty cycle ratings from 20% to 100%. Some GMAW
power sources are rated at 100% duty cycle. This is because GMAW operates in the
semi-automatic and fully-automatic modes. As such, it can potentially run continuously
at or near maximum rated output for long periods without a break.
Some of the new generation multi-process inverter design power sources have a 60%
duty cycle rating, which is more than adequate for most GMAW applications in the semiautomatic mode.
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Constant Voltage
The conventional dc constant current (CC) welding machine that works for SMAW and
GTAW is not recommended for wire feed welding. Wire feed processes require a CV
machine, sometimes called constant arc voltage (CAV) or constant potential (CP). This
is because in many applications, the filler metal transfers to the base metal by making
contact on the work surface; the wire makes a dead short circuit when it makes contact
with the base metal.
The cycle of short circuits that a welding arc follows occurs many times per second.
Think about what happens when you stick a SMAW electrode to the work using a CC
power source. The dead short is difficult to clear; the electrode usually welds itself to the
work, while the power source surges to high amperage with little or no arc voltage
present. If you could maintain voltage in the welding circuit (which controls arc striking
ease), then clearing the short circuit would be much easier to accomplish.
The CV power source maintains arc voltage during current surges when the wire short
circuits. This clears the short circuits smoothly back to the welding arc.
The CV power source must also automatically supply amperage according to the speed at
which the electrode wire is fed to the weld zone. With wire feed processes, spooled
electrode wire feeds continuously to the weld zone at a constant pre-set speed. The
requirement of the power source is to automatically increase or decrease amperage
(which controls burn-off rate) as the wire speed settings are increased or decreased. This
prevents the filler wire from stubbing in the weld puddle or burning back into the gun
contact tube.
Figure 8 illustrates a drooping volt-ampere curve typical of a constant current welding
power source with SMAW and GTAW. Fluctuations in arc voltage (a-b) result in a very
small change in amperage (c-d). For example, when the welding gun to work distance
changes, this initiates a change in arc voltage. The resulting amperage change with a
constant current machine is not great enough to increase or decrease burn-off rate of the
wire significantly. This would result in an erratic arc that causes uneven penetration and
an irregular shaped weld deposit.
Figure 8 - Constant current volt-amp curve (drooping).
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Figure 9 illustrates the relatively flat volt-ampere curve typical of a CV power source.
Comparing Figure 8 and Figure 9, a constant voltage machine gives the greatest
amperage change (c-d) with the same amount of change in arc voltage (a-b). This greater
amperage to voltage ratio requires less welder skill to maintain a steady gun-to-work
distance because the arc length becomes self-correcting.
NOTES
Figure 9 - Constant voltage volt-amp curve.
The curve does not touch the amperage line like the CC curve. When amperage surges
during short circuits, there is still voltage available to clear the short circuit smoothly.
When used with a constant speed wire feeder, a CV power source maintains a nearly
constant voltage during the short circuit metal transfer welding operation. This system
compensates for variations in the contact tube-to-work distances by instantly increasing
or decreasing the welding current.
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CV Power Source Characteristics
The characteristics that enable CV machines to produce uniform welds are as follows.
 Arc voltage remains at a nearly constant level.
 Welding current flow automatically adjusts to burn off the electrode at the same
rate at which the filler wire feeds to the work.
 A constant wire feed speed is maintained.
 Current surges instantly when the wire short circuits to the work to
re-establish the arc without sticking to the work.
 Quick recovery to normal current levels prevents burn-back of the wire to the
contact tube.
Constant voltage machines can be used with all wire feed processes including GMAW,
FCAW, MCAW and SAW.
A CV power source has two primary controls.
1. Voltage control on the power source to pre-set desired arc length.
2. Wire feed speed control on the wire feeder that directly influences amperage
when the wire feed speed is altered.
Wire feed speed controls amperage when using a CV welding power source. Wire feed
speed rate is measured in millimetres per second or inches per minute.
NOTE
Change only one variable at a time when you are setting a CV welding
power source.
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NOTES
Objective Four
When you have completed this objective, you will be able to:
Identify advantages and disadvantages of wire feed processes.
GMAW Advantages and Disadvantages
No one welding process can perform all welding operations satisfactorily. Before starting
the job, you may be given the responsibility of deciding which process to use. When
making this decision, you need to consider the capabilities of the process in question.
You should also consider secondary factors such as deposition rates, level of quality
required and total costs.
When considering GMAW for the job, look at the advantages and disadvantages of this
process (Table 1).
GMAW Advantages
Skill requirements are generally lower
than for SMAW.
Works well on most weldable metals; one
of the best processes for welding
aluminum.
Good arc control makes the manual
welding of light gauge materials possible.
Amount of spatter is low with little or no
post-weld cleaning required.
Less distortion than with SMAW due to
high deposition rates and fast welding
speeds.
High deposition rates can be achieved in
all positions due to:
- no slag removal,
- no stub end loss,
- high current density and
- continuous welding.
Very good control of penetration, leading
to a better control with poor fit-ups and
wide gaps.
GMAW Disadvantages
Special equipment must be used that is
more complex and costly to purchase.
Greater maintenance is required to keep
the equipment in good working condition.
Starts are relatively cold with the
possibility of lack of fusion (cold lap).
GMAW is less practical for welding
outdoors. GMAW requires positive
protection from strong drafts that can blow
the shielding gas away from the weld
zone, which causes porosity.
Power sources designed only for wire feed
processes are not recommended for
SMAW.
High levels of radiant energy create
physical hazards.
The wire feeder must be in close proximity
to welding.
Table 1 - Advantages and disadvantages of GMAW.
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FCAW Advantages and Disadvantages
One of the main advantages of FCAW is its ability to achieve high weld metal deposition
rates. In many instances, FCAW is capable of providing high quality weld metal at lower
costs and with less effort than SMAW.
In terms of process limitations, FCAW equipment is more expensive to purchase and
maintain than SMAW. Other disadvantages include the fact that welding cannot take
place at great distances from the wire feeder and the process can generate large amounts
of smoke and fumes.
Table 2 compares some of the advantages and disadvantages of FCAW.
Advantages of FCAW
Smooth, uniform weld appearance with
excellent contour on horizontal fillet
welds.
High quality weld metal deposits.
Suitable for welding many steel types.
Weldable over a wide range of thickness.
High operating factor that is easily
mechanized.
Up to 4 times greater deposition rate than
with SMAW.
Reduced distortion when compared to
SMAW.
Use of self-shielded electrodes
eliminates the need for flux handling or
gas apparatus and is more tolerant of
windy conditions. Self-shielded
electrodes generate shielding at the end
of the electrode, where it is required.
Higher tolerance for contaminants which
may otherwise cause weld cracking.
Skill requirements are generally lower
than for SMAW.
Resistance to underbead cracking.
Relatively high electrode deposit
efficiency.
Disadvantages of FCAW
Slag removal is necessary.
Presently limited to welding ferrous metals
and nickel-based alloys.
The equipment is more expensive to
purchase than that needed for SMAW.
The wire feeder must be in close proximity
to the welding.
More smoke and fumes are generated,
requiring suitable exhaust removal
equipment.
For gas shielded version, the external
shield may be adversely affected by drafts
and breezes.
FCAW electrodes are generally more
expensive on a weight-to-weight basis
than solid wires.
Equipment is more complex than for
SMAW, so it requires more maintenance.
Table 2 - Advantages and disadvantages of FCAW.
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MCAW Advantages and Disadvantages
The advantages and disadvantages of MCAW are similar to those of FCAW and GMAW.
Table 3 compares some of the advantages and disadvantages of MCAW.
Advantages of MCAW
High quality weld metal deposits.
No slag or interpass cleaning.
Suitable for welding many steel types.
Weldable over a wide range of thickness.
Easily adapted to mechanized welding.
Reduced distortion when compared to
SMAW.
High electrode deposit efficiency.
Skill requirements are generally lower
than for SMAW.
Resistance to underbead cracking and
lack of fusion.
Metal-cored wires can weld through mill
scale and rust and still produce very little
spatter.
Smooth, uniform weld appearance with
excellent contour on horizontal fillet
welds.
Disadvantages of MCAW
Special equipment must be used that is
more complex and costly to purchase.
Greater maintenance is required to keep
the equipment in good working condition.
MCAW is less practical for welding
outdoors. MCAW requires positive
protection from strong drafts that can blow
the shielding gas away from the weld
zone, which causes porosity.
High levels of radiant energy create
physical hazards.
The wire feeder must be in close proximity
to welding.
MCAW electrodes are generally more
expensive on a weight-to-weight basis
than solid wires.
Pulsed arc is often used for out of position
welding. for
Table 3 - Advantages and disadvantages of MCAW.
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Self-Test
1. What colour should your protective clothing be when welding with GMAW and
why?
2. Is it advisable to weld with any arc welding process under wet weather conditions?
Explain.
3. The use of shielding gases when using wire feed processes increases the need for
good:
a) lighting.
b) ventilation.
c) heat shielding.
d) eye protection.
4. Name two (2) toxic gases that can be created from the ultraviolet light produced from
welding.
a)
b)
5. How can air quality be affected when welding with a process that uses a shielding
gas?
6. High ultraviolet radiation emissions from an open arc can cause atmospheric oxygen
in the vicinity of the arc to convert to:
a) ozone.
b) carbon monoxide.
c) carbon dioxide.
d) argon.
7. A potential source of poisonous chemical fumes when welding is:
a) inert shielding gases.
b) chlorinated degreasing solvents.
c) carbon dioxide extinguishers.
d) anti-spatter nozzle sprays.
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8. Define and describe the GMAW process in detail.
9. The gas metal arc welding process is also known as:
a) MIG welding.
b) flux cored arc welding.
c) self-shielding process.
d) shielded metal arc welding.
10. GMAW is recommended for use on which of the following metals?
a) stainless steel
b) aluminum
c) copper alloys
d) carbon and low alloy steels
e) all of the above
11. Describe the operating principles of FCAW.
12. Self-shielded FCAW uses shielding gas generated from:
a) the base metal.
b) an external source.
c) the flux.
d) a paste flux applied to the base metal before welding.
13. What is the electrode extension range for self-shielded FCAW?
14. The MCAW process uses the same equipment as which process?
a) SMAW
b) GTAW
c) FCAW
d) OAW
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15. List the basic equipment components always required for the set-up of a wire feed
outfit.
a)
b)
c)
d)
e)
f)
16. How are wire feed guns rated?
a) by voltage-carrying capacity
b) by physical size
c) by current-carrying capacity
d) by shielding gas flow rate
17. The major factor in choosing between an air-cooled or a water-cooled gun is the
required:
a) shielding gas.
b) current/voltage.
c) joint design.
d) base metal alloy.
18. Welding power sources designed only for wire feed welding can be used for shielded
metal arc welding (SMAW).
a) true
b) false
19. GMAW may be used in the semi-automatic or automatic operation modes.
a) true
b) false
20. What type of welding power source is most often used for GMAW?
21. Constant voltage (CV) machines produce a:
a) drooping volt-amp curve.
b) rising volt-amp curve.
c) relatively flat volt-amp curve.
d) constantly changing volt-amp curve.
22. The power source best suited for the short-circuit mode of metal transfer is a:
a) rising arc voltage power source.
b) constant voltage power source.
c) constant current power source.
d) constant amperage power source.
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23. What type of volt-amp curve does a constant current machine produce?
a) flat
b) variable
c) rising
d) drooping
24. A welding machine that produces an almost horizontal output slope is a:
a) constant current machine.
b) constant voltage machine.
c) rising arc voltage machine.
d) high amperage potential welding machine.
25. List three (3) advantages and three (3) disadvantage of GMAW.
Advantages
a)
b)
c)
Disadvantages
a)
b)
c)
26. One of the main advantages of metal core wires is:
a) lower cost than flux core wires.
b) no external shielding gas is required.
c) designed for use on conventional CC power sources.
d) high deposition rates.
27. Name three (3) advantages and three (3) disadvantages of FCAW
Advantages
a)
b)
c)
Disadvantages
a)
b)
c)
28. The powder in a MCAW wire consists mostly of:
a) flux.
b) metal.
c) slag formers.
d) arc stabilizers.
25
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Self-Test Answers
1. Clothing should be dark to reduce reflected light radiation.
2.
When you are exposed to wet weather conditions or standing on wet surfaces,
protect yourself from electrical shock hazards by wearing rubber boots with dry
socks and rubber gloves.
3. b) ventilation.
4. a) ozone
b) phosgene gas (from chlorinated hydrocarbon degreasers)
5.
Shielding gases in sufficient quantities displace enough air to render the work
area oxygen deficient. Dizziness or euphoria is usually the first sign of oxygen
deficiency. Precautions must always be taken to ensure adequate breathing air is
supplied.
6. a) ozone.
7. b) chlorinated degreasing solvents.
8.
GMAW is an arc welding process that uses an arc between a continuous filler
metal electrode and the weld pool. The process uses shielding gas from an
externally supplied source.
9. a) MIG welding.
10. e) all of the above
11.
FCAW uses an arc established between a continuously fed consumable electrode
and the work piece. Appropriate settings made by the operator maintain a
constant burn-off rate of the wire electrode. Depending upon the power source
and wire drive system used, the arc length is maintained automatically. Shielding
gas can be externally applied and/or obtained from a flux contained within the
hollow electrode core (depending upon the type of flux cored wire used).
12. c) the flux.
13.
For self-shielded electrodes, extensions range from 19 mm to 95 mm (34" to 334").
14. c) FCAW
15. a)
b)
c)
d)
e)
f)
power source
wire feeder
gun and cable assembly
spooled filler wire
shielding gas and
gas control equipment
16. c) by current-carrying capacity
17. b) current/voltage.
18. b) false
19. a) true
20. constant voltage
26
21. c) relatively flat volt-amp curve.
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22. b) constant voltage power source.
23. d) drooping
24. b) constant voltage machine
25. Any three (3) of the following advantages are correct.
a)
b)
c)
d)
e)
f)
lower skill requirements than SMAW
excellent for light gauge metals
good arc control when working with poor fit-ups
less heat input means less distortion than SMAW
high production rates
no slag to remove, little or no post weld clean-up
Any three (3) of the following disadvantages are correct.
a)
b)
c)
d)
e)
special equipment required resulting in higher initial cost
equipment requires more maintenance and down time
lack of fusion can be a problem
cannot be used outdoors or in drafty locations
high radiant energy requires protection for the welder and others in the
vicinity
f) wire feeder must be in close proximity to welding
26. d) high deposition rates.
27. Any three (3) of the following advantages are correct.
a) The deposition rate is up to 4 times greater than with SMAW.
b) There is a high operating factor, so it is easily mechanized.
c) The visible arc is easy to use.
d) The weld metal deposits are high quality.
Any three (3) of the following disadvantages are correct.
a) The equipment is more expensive to purchase than that needed for SMAW.
b) More smoke and fumes are generated, which require suitable exhaust
removal equipment.
c) The wire feeder must be in close proximity to the welding.
d) Equipment is more complex than for SMAW, so it requires more
maintenance.
28. b) metal
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Module Number 120103a
Version 8.0