ch 5 ( gas shield arc welding )
TRANSCRIPT
Chapter 5
Gas Metal Arc Welding*
Introduction
Definition and General Background
Gas metal arc welding (GMAW) is an arc welding process that uses an arc between a
continuous filler metal electrode and the weld pool. The process is used with shielding
from an externally supplied gas and without the application of pressure.
. As a result, the term, and the use of reactive gases (particularly CO2) and gas
mixtures.
GMAW may be operated in semiautomatic, machine, or automatic modes. All
commercially important metals such as carbon steel, high-strength low alloy steel,
stainless steel, aluminum, copper, titanium, and nickel alloys can be welded in all
positions with this process by choosing the. appropriate shielding gas, electrode, and
welding variables.
*Welding Handbook, Vol. 2, p. 109, 1991
Uses and Advantages
The uses of the process are, of course, directed by Its advantages, the most Important
of which are the following:
1. It is the only consumable electrode process that can be used to weld all
commercial metals and alloys.
2. GMAW overcomes the restriction of limited electrode length encountered with
shielded metal arc welding,
3. Welding can be done in all positions, a feature not found in submerged arc
welding.
4. Deposition rates are significantly higher than those obtained with shielded metal
arc welding.
5. Welding speeds are higher than those with shielded metal arc welding because
of the continuous electrode feed and higher filler metal deposition rates.
6. Because the wire feed is continuous, long welds can be deposited without stops
and starts.
7. When spray transfer is used, deeper penetration is possible than with shielded
metal arc welding, which may permit the use of smaller size fillet welds for
equivalent strengths.
8. Minimal post-weld cleaning is required due to the absence of a heavy slag.
These advantages make the process particularly well suited to high production and
automated welding applications. This has become increasingly evident with the advent
of robotics, where GMAW has been the predominant process choice.
Limitations
As with any welding process, there are certain limitations which restrict the use of gas
metal arc welding. Some of these are the following:
1. The welding equipment is more complex, more costly and less portable than that
for SMAW.
2. GMAW is more difficult to use in hard-to-reach places because the welding gun
is larger than a shielded metal arc welding holder, and the welding gun must be
close to the joint, between 3/8 and 3/4 in. (10 and 19 mm), to ensure that the
weld metal is properly shielded.
3. The welding arc must be protected against air drafts that will disperse the
shielding gas. This limits outdoor applications unless protective shields are
placed around the welding area.
4. Relatively high levels of radiated heat and arc intensity can result In operator
resistance to the process.
Fundamentals of the Process
Principles of Operation
The GMAW process incorporates the automatic feeding of a continuous, consumable
electrode that is shielded by an externally supplied gas. The process is illustrated in
Figure 1. After initial settings by the operator, the equipment provides for automatic self-
regulation of the electrical characteristics of the arc. Therefore the only manual controls
Q.How we set the machine ??????
required by the welder for semiautomatic operation are the travel speed and direction,
and gun positioning. Given proper equipment and settings, the arc length and the current
(wire feed speed) are automatically maintained.
What are the suitable positions for gun ?
Equipment required for GMAW is shown in Figure 2.
Most commonly this regulation consists of a constant-potential (voltage) power supply
(characteristically providing an essentially flat volt-ampere curve) in conjunction with a
constant-speed electrode feed unit. Alternatively, a constant-current power supply
provides a drooping volt-ampere curve, and the electrode feed Unit is arc-voltage
controlled.
1- With the constant potential/constant wire feed combination, changes in the torch
position cause a change in the welding current that exactly matches the change in the
electrode stick-out (electrode extension), thus the arc length remains fixed. For example,
an increased stick-out produced by withdrawing the torch reduces the current output
from the power supply, thereby maintaining the same resistance heating of the
electrode.
2- In the alternative system, self-regulation results when arc voltage fluctuations readjust
the control circuits of the feeder, which appropriately changes the wire feed speed. In
some cases (welding aluminum, for example), it may be preferable to deviate from these
standard combinations and couple a constant-current power source with a constant
speed electrode feed unit. This combination provides only a small degree of automatic
self-regulation, and therefore requires more operator skill in semiautomatic welding.
However, some users think this combination affords a range of control over the arc
energy (current) that may be important in coping with the high thermal conductivity of
aluminum base metals.
Metal Transfer Mechanisms
The characteristics of the GMAW process are best described in terms of the three basic
means by which metal is transferred from the electrode to the work:
1. Short circuiting transfer
2. Globular transfer
3. Spray transfer
The type of transfer is determined by a number of factors, the most influential of which
are the following:
1. Magnitude and type of welding current
2. Electrode diameter
3. Electrode composition
4. Electrode extension
5. Shielding gas
Short Circuiting Transfer
Short circuiting encompasses the lowest range of welding currents and electrode
diameters associated with GMAW. This type of transfer produces a small, fast-freezing
weld pool that is generally suited for joining thin sections, for out-of-position welding, and
for bridging large root openings. Metal is transferred from the electrode to the work only
during a period when the electrode is in contact with the weld pool. No metal is
transferred across the arc gap.
The electrode contacts the molten weld pool in a range of 20 to over 200 times per
second. The sequence of events in the transfer of metal and the corresponding current
and voltage are shown in Figure 3. As the wire touches the weld metal, the current
increases [(A),(B), (C), (D) in Figure 3]. The molten metal at the Wire tip pinches off at D
and E, initiating an arc as shown in (E) and (F). The rate of current increase must be
high enough to heat the electrode and promote metal transfer, yet low enough to
minimize spatter caused by violent separation of the drop of metal. This rate of current
increase is controlled by adjustment of the inductance in the power source.
The optimum inductance setting depends on both the electrical resistance of the welding
circuit and the melting temperature of the electrode. When the arc is established, the
wire melts at the tip as the wire is fed forward towards the next short circuit at (H),
Figure 3. The open circuit voltage of the power source must be so low that the drop of
molten metal at the wire tip cannot transfer until it touches the base metal. The energy
for arc maintenance is partly provided by energy stored in the inductor during the period
of short circuiting.
Even though metal transfer occurs only during short circuiting, shielding gas composition
has a dramatic effect on the molten metal surface tension. Changes in shielding gas
composition may dramatically affect the drop size and the duration of the short circuit. In
addition, the type of gas influences the operating characteristics of the arc and the base
metal penetration. Carbon dioxide generally produces high spatter levels compared to
inert gases, but CO2 also promotes deeper penetration. To achieve a good compromise
between spatter and penetration, mixtures of CO2 and argon are often used when
welding carbon and low alloy steels. Additions of helium to argon increase penetration
on nonferrous metals.
Globular Transfer
With a positive electrode (DCEP), globular transfer takes place when the current is
relatively low, regardless of the type of shielding gas. However, with carbon dioxide and
helium, this type of transfer takes place at all usable welding currents. Globular transfer
is characterized by a drop size with a diameter greater than that of the electrode. The
large drop is easily acted on by gravity, generally limiting successful transfer to the flat
position.
At average currents, only slightly higher than those used in short circuiting transfer,
globular axially-directed transfer can be achieved in a substantially inert gas shield. If the
arc length is too short (low voltage), the enlarging drop may short to the workpiece,
become superheated, and disintegrate, producing considerable spatter. The arc must
therefore be long enough to ensure detachment of the drop before it contacts the weld
pool. However, a weld made using the higher voltage is likely to be unacceptable
because of lack of fusion, insufficient penetration, and excessive reinforcement. This
greatly limits use of the globular transfer mode in production applications.
Carbon dioxide shielding results in randomly directed globular transfer when the welding
current and voltage are significantly above the range for short circuiting transfer. The
departure from axial transfer motion is governed by electromagnetic forces, generated
by the welding current acting upon the molten tip, as shown in Figure 4. The most
important of these are the electromagnetic pinch force (P) and anode reaction force (R).
The magnitude of the pinch force is a direct function of welding current and wire
diameter, and is usually responsible for drop detachment. With CO2 shielding, the
welding current is conducted through the molten drop and the electrode tip is not
enveloped by the arc plasma. High- speed photography shows that the arc moves over
the surface of the molten drop and workpiece, because force R tends to support the
drop. The molten drop grows until it detaches by short circuiting (Figure 4B) or by
gravity [Figure 4(A)], because R is never overcome by P alone. As shown in Figure
4(A), it is possible for the drop to be come detached and transfer to the weld pool
without disruption. The most likely situation is shown in Figure 4(B), which shows the
drop short circuiting the arc column and exploding. Spatter can therefore be severe,
which limits the use of CO2 shielding for many commercial applications.
Nevertheless, CO2 remains the most commonly used gas for welding mild steels. The
reason for this is that the spatter problem can be reduced significantly by "burying" the
arc. In so doing, the arc atmosphere becomes a mixture of the gas and iron vapor,
allowing the transfer to become almost spray-like. The arc forces are sufficient to
maintain a depressed cavity which traps much of the spatter. This technique requires
higher welding current and results in deep penetration. However, unless the travel speed
is carefully controlled, poor wetting action may result in excessive weld reinforcement.
Spray Transfer
With argon-rich shielding it is possible to produce a very stable, spatter-free "axial spray"
transfer mode as illustrated in Figure 5. This requires the use of direct current and a
positive electrode (DCEP), and a current level above a critical value called the transition
current. Below this current, transfer occurs in the globular mode described previously, at
the rate of a few drops per second. Above the transition current, the transfer occurs in
the form of very small drops that are formed and detached at the rate of hundreds per
second. They are accelerated axially across the arc gap. The relationship between
transfer rate and current is plotted in Figure 6.
The transition current, which is dependent on the liquid metal surface tension, is
inversely proportional to the electrode diameter and, to a smaller degree, to the
electrode extension. It varies with the filler metal melting temperature and the shielding
gas composition. Typical transition currents for some of the more common metals are
shown in Table 1.
The spray transfer mode results in a highly directed stream of discrete drops that are
accelerated by arc forces to velocities which overcome the effects of gravity. Because of
that, the process, under certain conditions, can be used in any position. Because the
drops are smaller than the arc length, short circuits do not occur, and spatter is
negligible if not totally eliminated.
Another characteristic of the spray mode of transfer is the "finger" penetration which it
produces. Although the finger can be deep, it is affected by magnetic fields, which must
be controlled to keep it located at the center of the weld penetration profile.
The spray-arc transfer mode can be used to weld almost any metal or alloy because of
the inert characteristics of the argon shield. However, applying the process to thin sheets
may be difficult because of the high currents needed to produce the spray arc. The
resultant arc forces can cut through relatively thin sheets instead of welding them. Also,
the characteristically high deposition rate may produce a weld pool too large to be
supported by surface tension in the vertical or overhead position.
The work thickness and welding position limitations of spray arc transfer have been
largely overcome with specially designed power supplies. These machines produce
carefully controlled wave forms and frequencies that "pulse" the welding current. As
shown in Figure 7, they provide two levels of current; one a constant, low background
current which sustains the arc without providing enough energy to cause drops to form
on the wire tip; the other a superimposed pulsing current with amplitude greater than the
transition current necessary for spray transfer. During this pulse, one or more drops are
formed and transferred. The frequency and amplitude of the pulses control the energy
level of the arc, and therefore the rate at which the wire melts. By reducing the average
arc energy and the wire melting rate, pulsing makes the desirable features of spray
transfer available for joining sheet metals and welding thick metals in all positions.
Many variations of such power sources are available. The simplest provide a single
frequency of pulsing (60 or 120 pps) with independent control of the background and
pulsing current levels. More sophisticated power sources, sometimes called synergic,
automatically provide the optimum combination of background and pulse for any given
setting of wire feed speed.
Process Variables
The following are some of the variables that affect weld penetration, bead geometry and
overall weld quality:
1. Welding current (electrode feed speed)
2. Polarity
3. Arc voltage (arc length)
4. Travel speed
5. Electrode extension
6. Electrode orientation (trail or lead angle)
7. Weld joint position
8. Electrode diameter
9. Shielding gas composition and flow rate
Knowledge and control of these variables is essential to consistently produce welds of
satisfactory quality. These variables are not completely independent, and changing one
generally requires changing one or more of the others to produce the desired results.
Considerable skill and experience are needed to select optimum settings for each
application. The optimum values are affected by:
1. Type of base metal.
2. Electrode composition.
3. Welding position.
4. Quality requirements.
Thus, there is no single set of parameters that gives optimum results in every case.
Welding Current
When all other variables are held constant, the welding amperage varies with the
electrode feed speed or melting rate in a nonlinear relation. As the electrode feed speed
is varied, the welding amperage will vary in a like manner if a constant-voltage power
source is used. This relationship of welding current to wire feed speed for carbon steel
electrodes is shown in Figure 8. At the low-current levels for each electrode size, the
curve is nearly linear. However, at higher welding currents, particularly with small
diameter electrodes, the curves become nonlinear, progressively increasing at a higher
rate as welding amperage increases. This is attributed to resistance heating of the
electrode extension beyond the contact tube. The curves can be approximately
represented by the equation
WFS = aI + bLI2
Where
WFS = the electrode feed speed, in/min (mm/s)
a = a constant of proportionality for anode or cathode heating. Its magnitude is
����� dependent upon polarity, composition, and other factors, in./(min. A) [(mm
/(s . A)]
b = constant of proportionality for electrical resistance heating, min-1 A-2 (s-1 A-2)
L = the electrode extension or stick out, in. (mm)
I = the welding current, A�
As shown in Figures 8, 9, 10 and 11, when the diameter of the electrode is increased
(while maintaining the same electrode feed speed), a higher welding current is required.
The relationship between the electrode feed speed and the welding current is affected
by the electrode chemical composition. This effect can be seen by comparing Figures 8,
9, 10 and 11, which are for carbon steel, aluminum, stainless steel, and copper
electrodes respectively. The different positions and slopes of the curves are due to
differences in the melting temperatures and electrical resistivities of the metals.
Electrode extension also affects the relationships.
With all other variables held constant, an increase in welding current (electrode feed
speed) will result in the following:
1. An increase in the depth and width of the weld penetration
2. An increase in the deposition rate
3. An increase in the size of the weld bead
Pulsed spray welding is a variation of the GMAW process in which the current is pulsed
to obtain the advantages of the spray mode of metal transfer at average currents equal
to or less than the globular-to-spray transition current.
Since arc force and deposition rate are exponentially dependent on current, operation
above the transition current often makes the arc forces uncontrollable in the vertical and
overhead positions. By reducing the average current with pulsing, the arc forces and
deposition rates can both be reduced, allowing welds to be made in all positions and in
thin sections.
With solid wires, another advantage of pulsed power welding is that larger diameter
wires [i.e., 1/16-in. (1.6 mm)] can be used. Although deposition rates are generally no
greater than those with smaller diameter wires, the advantage is in the lower cost per
unit of metal deposited. There is also an increase in deposition efficiency because of
reduced spatter loss.
With metal cored wires, pulsed power produces an arc that is less sensitive to changes
in electrode extension (stickout) and voltage compared to solid wires. Thus, the process
is more tolerant of operator guidance fluctuations. Pulsed power also minimizes spatter
from an operation already low in spatter generation.
Polarity
The term polarity is used to describe the electrical connection of the welding gun with
relation to the terminals of a direct current power source. When the gun power lead is
connected to the positive terminal, the polarity is designated as direct current electrode
positive (DCEP), arbitrarily called reverse polarity. When the gun is connected to the
negative terminal, the polarity is designated as direct current electrode negative (DCEN),
originally called straight polarity. The vast majority of GMAW applications use direct
current electrode positive (DCEP). This condition yields a stable arc, smooth metal
transfer, relatively low spatter, good weld bead characteristics and greatest depth of�
penetration for a wide range of welding currents.
Direct current electrode negative (DCEN) is seldom used because axial spray transfer is
not possible without modifications that have had little commercial acceptance. DCEN
has a distinct advantage of high melting rates that cannot be exploited because the
transfer is globular. With steels, the transfer can be improved by adding a minimum of 5
percent oxygen to the argon shield (requiring special alloys to compensate for oxidation
losses) or by treating the wire to make it thermionic (adding to the cost of the filler
metal). In both cases, the deposition rates drop, eliminating the only real advantage of
changing polarity. However, because of the high deposition rate and reduced
penetration, DCEN has found some use in surfacing applications.
Attempts to use alternating current with the GMAW process have generally been
unsuccessful. The cyclic wave form creates arc instability due to the tendency of the arc
to extinguish as the current passes through the zero point. Although special wire surface
treatments have been developed to overcome this problem, the expense of applying
them has made the technique uneconomical.
Arc Voltage (Arc Length)
Arc voltage and arc length are terms that are often used interchangeably. It should be
pointed out, however, that they are different even though they are related. With GMAW,
arc length is a critical variable that must be carefully controlled. For example, in the
spray-arc mode argon shielding, an arc that is too short experiences momentary short
circuits. They cause pressure fluctuations which pump air into the arc stream, producing
porosity embrittlement due to absorbed nitrogen. Should the arc too long, it tends to
wander, affecting both the penetration and surface bead profiles. A long arc can also
disrupt the gas shield. In the case of buried arcs with a carbon dioxide shield, a long arc
results in excessive spatter as porosity; if the arc is too short, the electrode tip short
circuits the weld pool, causing instability.
Arc length is the independent variable. Arc voltage depends on the arc length as well as
many other variables, such as the electrode composition and dimensions, shield gas, the
welding technique and, since it often is measured at the power supply, even the length
of the welding cable. Arc voltage is an approximate means of stating physical arc length
(see Figure 12) in electrical terms, though the arc voltage also includes the voltage drop
in electrode extension beyond the contact tube.
With all variables held constant, arc voltage is directly related to arc length. Even though
the arc length is variable of interest and the variable that should be trolled, the voltage is
more easily monitored. Because this, and the normal requirement that the arc voltage
specified in the welding procedure, it is the term that more commonly used.
Arc voltage settings vary depending on the material, shielding gas, and transfer mode.
Typical values are shown in Table 2. Trial runs are necessary to adjust the arc voltage
to produce the most favorable arc characteristics with weld bead appearance. Trials are
essential because the optimum arc voltage is dependent upon a variety of factors,
fluctuations including metal thickness, the type of joint, welding position, electrode size,
shielding gas composition, and the be type of weld. From any specific value of arc
voltage, a voltage increase tends to flatten the weld bead and increase the disrupt width
of the fusion zone. Excessively high voltage may carbon cause porosity, spatter, and
undercut. Reduction in volt well age results in a narrower weld bead with a higher crown
short and deeper penetration. Excessively low voltage may cause stubbing of the
electrode.
Travel Speed
Travel speed is the linear rate at which the arc is moved the along the weld joint. With all
other conditions held constant, weld penetration is a maximum at an intermediate the
travel speed.
When the travel speed is decreased, the filler metal deposition per unit length increases.
At very slow speeds the welding arc impinges on the molten weld pool, rather than the
base metal, thereby reducing the effective penetration. A wide weld bead is also a result.
As the travel speed is increased, the thermal energy per unit length of weld transmitted
to the base metal from the arc is at first increased, because the arc acts more directly
material, on the base metal. With further increases in travel speed, shown less thermal
energy per unit length of weld is imparted to volt- the base metal. Therefore, melting of
the base metal first and increases and then decreases with increasing travel speed. As
travel speed is increased further, there is a tendency toward undercutting along the
edges of the weld bead because there is insufficient deposition of filler metal to fill the
path melted by the arc.
Electrode Extension
The electrode extension is the distance between the end of the contact tube and the end
of the electrode, as shown in Figure 12. An increase in the electrode extension results in
an increase in its electrical resistance. Resistance heating in turn causes the electrode
temperature to rise, and results in a small increase in electrode melting rate. Overall, the
increased electrical resistance produces a greater voltage drop from the contact tube to
the work. This is sensed by the power source, which compensates by decreasing the
current. That immediately reduces the electrode melting rate, which then lets the
electrode shorten the physical arc length. Thus, unless there is an increase in the
voltage at the welding machine, the filler metal will be deposited as a narrow, high-
crowned weld bead.
The desirable electrode extension is generally from to in. (6 to 13 mm) for short� �
circuiting transfer and from to 1 in. (13 to 25 mm) for other types of metal transfer.�
Electrode Orientation
As with all arc welding processes, the orientation of the welding electrode with respect to
the weld joint affects the weld bead shape and penetration. Electrode orientation affects
bead shape and penetration to a greater extent than arc voltage or travel speed. The
electrode orientation is described in two ways: �
1. By the relationship of the electrode axis with respect to the direction of travel (the
travel angle).
2. The angle between the electrode axis and the adjacent work surface (work
angle).
When the electrode points opposite from the direction of travel, the technique is called
backhand welding with a drag angle. When the electrode points in the direction of travel,
the technique is forehand welding with a lead angle. The electrode orientation and its
effect on the width and penetration of the weld are illustrated in Figures 13 (A), (B), and
(C).
When the electrode is changed from the perpendicular to a lead angle technique with all
other conditions unchanged, the penetration decreases and the weld bead becomes
wider and flatter. Maximum penetration is obtained in the flat position with the drag
technique, at a drag angle of about 25 degrees from perpendicular. The drag technique
also produces a more convex, narrower bead, a more stable arc, and fewer spatters on
the workpiece. For all positions, the electrode travel angle normally used is a drag. For
good control shielding of the molten weld pool, angle in the range of 5 to 15 degrees.
For some materials, such as aluminum, a lead technique is preferred. This lead
technique provides a "cleaning action" ahead of the molten weld metal, which promotes
wetting and reduces base metal oxidation. When producing fillet welds in the horizontal
position, the electrode should be positioned about 45 degrees to the vertical member
(work angle), as illustrated in Figure 14.
Weld Joint Position
Most spray type GMAW is done in the flat or horizontal positions, while at low-energy
levels, pulsed and short circuiting GMAW can be used in all positions. Fillet welds made
in the flat position with spray transfer are usually more uniform, less likely to have
unequal legs and convex profiles, and are less susceptible to undercutting than similar
fillet welds made in the horizontal position.
To overcome the pull of gravity on the weld metal in the vertical and overhead positions
of welding, small diameter electrodes are usually used, with either short circuiting metal
transfer or spray transfer with pulsed direct current. Electrode diameters of 0.045 in. (1.1
mm) and smaller are best suited for out-of-position welding. The low-heat input allows
the molten pool to freeze quickly. Downward welding progression is usually effective on
sheet metal in the vertical position.
When welding is done in the "flat" position, the inclination of the weld axis with respect to
the horizontal plane will influence the weld bead shape, penetration, and travel speed. In
flat position circumferential welding, the work rotates under the welding gun and
inclination is obtained by moving the welding gun in either direction from top dead
center.
By positioning linear joints with the weld axis at 15 degrees to the horizontal and welding
downhill, weld reinforcement can be decreased under welding conditions that would
produce excessive reinforcement when the work is in the flat position. Also, when
traveling downhill, speeds can usually be increased. At the same time, penetration is
lower, which is beneficial for welding sheet metal.
Downhill welding affects the weld contour and penetration, as shown in Figure 15(A).
The weld puddle tends to flow toward the electrode and preheats the base metal,
particularly at the surface. This produces an irregularly shaped fusion zone, called a
secondary wash. As the angle of declination increases, the middle surface of the weld is
depressed, penetration decreases, and the width of the weld increases. For aluminum,
this downhill technique is not recommended due to loss of cleaning action and
inadequate shielding.
Uphill welding affects the fusion zone contour and the weld surface, as illustrated in
Figure 15(B). The force of gravity causes the weld puddle to flow back and lag behind
the electrode. The edges of the weld lose metal, which flows to the center. As the angle
of inclination increases, reinforcement and penetration increase, and the width of the
weld decreases. The effects are exactly the opposite of those produced by downhill
welding. When higher welding currents are used, the maximum usable angle decreases.
Electrode Size
The electrode size (diameter) influences the weld bead configuration. A larger electrode
requires higher minimum current than a smaller electrode for the same metal transfer
characteristics. Higher currents in turn produce additional electrode melting and larger,
more fluid weld deposits. Higher currents also result in higher deposition rates and
greater penetration. However, vertical and overhead welding are usually done with
smaller diameter electrodes and lower currents.
Shielding Gas
The characteristics of the various gases and their effect on weld quality and arc
characteristics are discussed in detail in the consumables section of this chapter.
Equipment
The GMAW process can be used either semi-automatically or automatically. The basic
equipment for any GMAW installation consists of the following:
1. Welding gun (air or water cooled)
2. Electrode feed unit
3. Welding control
4. Welding power supply
5. A regulated supply of shielding gas
6. A source of electrode
7. Interconnecting cables and hoses
8. Water circulation system (for water-cooled torches)
Typical semiautomatic and mechanized components are illustrated in Figures 2 and 16.
Welding Guns
Different types of welding guns have been designed to provide maximum efficiency
regardless of the application, ranging from heavy-duty guns for high current, high-
production work, to lightweight guns for low current, out-of-position welding.
Water or air cooling and curved or straight nozzles are available for both heavy-duty and
lightweight guns. An air- cooled gun is generally heavier than a water-cooled gun at the
same rated amperage and duty cycle, because the air cooled gun requires more mass to
overcome its less efficient cooling. The following are basic components of arc welding
guns:
1. Contact tube (or tip)
2. Gas shield nozzle.
3. Electrode conduit and liner
4. Gas hose
5. Water hose
6. Power cable
7. Control switch
These components are illustrated In Figure 17.
The contact tube usually made of copper or a copper alloy, transfers welding current to
the electrode and directs the electrode towards the work. The contact tube is connected
electrically to the welding power supply by the power cable. The inner surface of the
contact tube should be smooth so the electrode will feed easily through this tube and
also make good electrical contact. The Instruction booklet supplied with every gun will
list the correct size contact tube for each electrode size and material.
Generally, the hole in the contact tube should be 0.005 to 0.010 in. (0.13 to 0.25 mm)
larger than the wire being used, although larger hole sizes may be required for
aluminum. The contact tube must be held firmly in the torch and must be centered in the
gas shielding nozzle. The positioning of the contact tube in relation to the end of the
nozzle may be a variable depending on the mode of transfer being used. For short-
circuiting transfer, the tube is usually flush or extended beyond the nozzle, while for
spray arc it is recessed approximately ⅛ in. During welding, it should be checked
periodically and replaced if the hole has become elongated due to excessive wear or if it
becomes clogged with spatter. Using a worn or clogged tip can result in poor electrical
contact and erratic arc characteristics.
The nozzle directs an even-flowing column of shielding gas into the welding zone. An
even flow is extremely important to assure adequate protection of the molten weld metal
from atmospheric contamination. Different size nozzles are available and should be
chosen according to the application, i.e., larger nozzles for high-current work where the
weld puddle is large, and smaller nozzles for low current and short circuiting welding. For
spot welding applications the nozzles are made with ports that allow the gas to escape
when the nozzle is pressed onto the workpiece.
The electrode conduit and its liner are connected to a bracket adjacent to the feed rolls
on the electrode feed motor. The conduit supports, protects, and directs the electrode
from the feed rolls to the gun and contact tube. Uninterrupted electrode feeding is
necessary to insure good arc stability. Buckling or kinking of the electrode must be
prevented. The electrode will tend to jam any-where between the drive rolls and the
contact tube if not properly supported.
The liner may be an integral part of the conduit or supplied separately. In either case, the
liner material and inner diameter are important. Liners require periodic maintenance to
assure they are clean and in good condition to assure consistent feeding of the wire.
A helical steel liner is recommended when using hard electrode materials such as steel
and copper. Nylon liners should be used for soft electrode materials such as aluminum
and magnesium.
Care must be taken not to crimp or excessively bend the conduit even though its outer
surface is usually steel-supported. The instruction manual supplied with each Unit will
generally list the recommended conduits and liners for each electrode size and material.
The remaining accessories bring the shielding gas, cooling water, and welding power to
the gun. These hoses and cables may be connected directly to the source of these
facilities or to the welding control. Trailing gas shields are available and may be required
to protect the weld pool during high-speed welding.
Electrode Feed Unit
The electrode feed unit (wire feeder) consists of an electric motor, drive rolls, and
accessories for maintaining electrode alignment and pressure. These units can be
integrated with the speed control or located remotely from it. The electrode feed motor is
usually a direct current type. It pushes the electrode through the gun to the work. It
should have a control circuit that varies the motor speed over a broad range.
Constant-speed wire feeders are normally used in combination with constant-potential
power sources. They may be used with constant-current power supplies if a slow
electrode "run-in" circuit is added.
When a constant-current power source is used an automatic voltage sensing control is
necessary. This control detects changes m the arc voltage and adjusts the wire feed
speed to maintain a constant arc length. This combination of variable speed wire feeder
and constant-current power source is limited to larger diameter wires [over 1/16 in. (1.6
mm)], where the feed speeds are lower. At high wire feed speeds the adjustments to
'motor speed cannot normally be made quickly enough to maintain arc stability.
The feed motor is connected to a drive roll assembly. These drive rolls in turn transmit
the force to the electrode, pulling it from the electrode source and pushing it through the
welding gun. Wire feed units may use a roll or four-roll arrangement. The drive roll
pressure adjustment allows for variable force to be applied to the wire, depending on its
characteristics (e.g. solid or cored, hard or soft). The inlet and outlet guides provide for
proper alignment of the wire to the drive rolls and support the wire to prevent buckling.
Welding Control
The welding control and electrode feed motor for semi- automatic operation are available
in one integrated package. The main function of the welding control is to regulate the
speed of the electrode feed motor usually through the use of an electrode governor. By
increasing the wire feed speed the operator increases the welding current. Decreases in
wire assembly feed speed result in lower welding currents. The control also electrode,�
regulates the starting and stopping of the electrode feed through a signal received from
the gun switch.
Also available are electrode feed control features that permit the use of a "touch-start"
(the electrode feed is initiated when the electrode touches the work), or a "slow run-in"
(the initial feed rate is reduced until the arc is initiated and then increases to that
required for welding). These two features are employed primarily in conjunction with
constant-current type power supplies, and are particularly useful for gas metal arc
welding of aluminum.
Normally, shielding gas, cooling water, and welding power are also delivered to the gun
through the control, requiring direct connection of the control to these facilities and the
power supply. Gas and water flow are regulated to coincide with the weld start and stop
by use of solenoid valves. The control may also sequence the starting and stopping of
gas flow, and may energize the power source contactor. The control may allow some
gas to flow before welding starts (pre-purge) and after welding stops (post-purge) to
protect the molten weld puddle. The control is usually independently powered by 115 V
ac.
Power Source
The welding power source delivers electrical power to the electrode and workpiece to
produce the arc. For the vast majority of GMAW applications, direct current with
electrode positive (DCEP) is used; therefore, the positive lead is connected to the gun
and the negative lead to the workpiece. The major types of direct current power sources
are engine-driven-generators (rotating) and transformer-rectifiers (static). Inverters are
included in the static category. The transformer-rectifier type is usually preferred for in-
shop fabrication where a source of either 230 V or 460 V is available. The transformer-
rectifier type responds faster than the engine-driven-generator type when the arc
conditions change. The engine-driven generator is used when there is no other available
source of electrical energy, e.g., remote locations.
Both types of power source can be designed and built to provide either constant current
or constant potential. Early applications of the GMAW process used constant-current
power sources (often referred to as droopers). Droopers maintain a relatively fixed
current level during welding, regardless of variations in arc length, as illustrated in
Figure 18. These machines are characterized by high open circuit voltages and limited
short circuit current levels. Since they supply a virtually constant current output, the arc
will maintain a fixed length only if the contact tube-to-work distance remains constant,
with a constant electrode feed rate.
In practice, since this distance will vary, the arc will then tend to either "burn back" to the
contact tube or "stub" into the workpiece. This can be avoided by using a voltage-
controlled electrode feed system. When the voltage (arc length) increases or decreases,
the motor speeds up or slows down to hold the arc length constant. The electrode feed
rate is changed automatically by the control system. This type of power supply is
generally used for spray transfer welding since the limited duration of the arc in short
circuiting transfer makes control by voltage regulation impractical.
As GMAW applications increased, it was found that a constant-potential (CP) power
source provided improved operation. Used in conjunction with a constant-speed wire
feeder, it maintains a nearly constant voltage during the welding operation. The volt-
ampere curve of this type power source is illustrated in Figure 19. The CP system
compensates for variations in the contact-tip-to-work-piece distance, which occur during
normal welding operations, by instantaneously increasing or decreasing the welding
current to compensate for the changes in stickout due to the changes in gun-to-work
distance.
The arc length is established by adjusting the welding voltage at the power source. Once
this is set, no other changes during welding are required. The wire feed speed, which
also becomes the current control, is preset by the welder or welding operator prior to
welding. It can be adjusted over a considerable range before stubbing to the workpiece
or burning back into the contact tube occurs. Welders and welding operators easily learn
to adjust the wire feed and voltage controls with only minimum instruction.
The self-correction mechanism of a constant-potential power source is illustrated in
Figure 20. As the contact tip-to-work distance increases, the arc voltage and arc length
would tend to increase. However, the welding current decreases with this slight increase
in voltage, thus compensating for the increase in stickout. Conversely, if the distance is
shortened, the lower voltage would be accompanied by an increase in current to
compensate for the shorter stickout.
The self-.correcting arc length feature of the CP power source is important in producing
stable welding conditions, but there are additional variables that contribute to optimum
welding performance, particularly for short circuiting transfer. In addition to the control of
the output voltage, some degree of slope and inductance control may be desirable. The
welder or welding operator should understand the effect of these variables on the
welding arc and its stability.
Voltage. Arc voltage is the electrical potential between the electrode and the workpiece.
Arc voltage is lower than the voltage measured directly at the power source because of
voltage drops at connections and along the length of the welding cable. As previously
mentioned, arc voltage is directly related to arc length; therefore, an increase or a
decrease in the output voltage at the power source will result in a like change in the arc
length.
Slope. The static volt-ampere characteristics (static output) of a CP power source are
illustrated in Figure 19. The slope of the output is the algebraic slope of the volt ampere
curve and is customarily given as the voltage drop per 100 amperes of current rise.
The slope of the power source, as specified by the manufacturer, is measured at its
output terminals and is not the total slope of the arc welding system. Anything that adds
resistance to the welding system (i.e., power cables, poor connections, loose terminals,
dirty contacts, etc.) adds to the slope. Therefore, slope is best measured at the arc in a
given welding system. Two operating points are needed to calculate the slope of a
constant-potential type welding system, as shown in Figure 21. It is not safe to use the
open circuit voltage as one of the points, because of a sharp voltage drop with some
machines at low currents. This is shown in Figure 19. Two stable arc conditions should
be chosen at currents that envelope the range likely to be used.
Slope has a major function in the short-circuiting transfer mode of GMAW in that it
controls the magnitude of the short circuit current, which is the amperage that flows
when the electrode is shorted to the workpiece. In GMAW, the separation of molten
drops of metal from the electrode is controlled by an electrical phenomenon called the
electromagnetic pinch effect. Pinch is the magnetic "squeezing" force on a conductor
produced by the current flowing through it. For short circuiting transfer, the effect is
illustrated in Figure 22.
The short circuit current (and therefore the pinch effect force) is a function of the slope of
the volt-ampere curve of the power source, as illustrated in Figure 23. The operating
voltage and the amperage of the two power supplies are identical, but the short circuit
current of curve A is less than that of curve B. Curve A has the steeper slope or a
greater voltage drop per 100 amperes, as compared to curve B thus, a lower short circuit
current and a lower pinch effect.
In short circuiting transfer the amount of short circuit current is important since the
resultant pinch effect determines the way a molten drop detaches from the electrode.
This in turn affects the arc stability. When little or no slope is present in the power supply
circuit, the short circuit current will rise rapidly to a high level. The pinch effect will also
be high, and the molten drop will separate violently from the wire. The excessive pinch
effect will abruptly squeeze the metal aside, clear the short circuit, and create excessive
spatter.
When the short circuit current available from the power source is limited to a low value
by a steep slope, the electrode will carry the full current, but the pinch effect may be too
low to separate the drop and reestablish the arc. Under these conditions, the electrode
will either pile up on the workpiece .or freeze to the puddle. When .the short circuit
current is at an acceptable value, the parting of the molten drop from the electrode is
smooth with very little spatter. Typical short circuit currents required for metal transfer
with the best arc stability are shown in Table 3.
Many constant-potential power sources are equipped with a slope adjustment. They may
be stepped or continuously adjustable to provide desirable levels of short circuit current
for the application involved. Some have a fixed slope which has been preset for the most
common welding conditions.
Inductance, The current increases rapidly to a higher level when the electrode shorts to
the work,. The circuit characteristic affecting the time rate of this increase in current is
inductance, usually measured in henrys. The effect of inductance is illustrated by the
curves plotted in Figure 24. Curve A is an example of a current-time curve immediately
after a short circuit when some inductance is in the circuit. Curve B illustrates the path
the current would have taken if there were no inductance in the circuit.
The maximum amount of pinch effect is determined by the final short circuit current level.
The instantaneous pinch effect is controlled by the instantaneous current, and therefore
the shape of the current-time curve is significant. The inductance in the circuit controls
the rate of current rise. Without inductance the pinch effect is applied rapidly and the
molten drop will be violently "squeezed" off the electrode and cause excessive spatter.
Higher inductance results in a decrease in the short circuits per second and an increase
in the "arc-on" time. Increased arc-on time makes the puddle more fluid and results in a
flatter smoother weld bead.
In spray transfer, the addition of some inductance to the power source will produce a
softer arc start without affecting the steady-state welding conditions. Power source
adjustments required for minimum spatter conditions vary with the electrode material
and diameter. As a general rule, higher short circuit currents and higher inductance are
needed for larger diameter electrodes. Power sources are available with fixed, stepped,
or continuously adjustable inductance levels.
Shielding Gas Regulators
A system is required to provide constant shielding gas flow rate at atmospheric pressure
during welding. A gas regulator reduces the source gas pressure to a constant working
pressure regardless of variations at the source. Regulators may be single or dual stage
and may have a built-in flowmeter. Dual stage regulators deliver gas at a more
consistant pressure than single stage regulators when the source pressure varies.
The shielding gas source can be a high-pressure cylinder, a liquid-filled cylinder, or a
bulk-liquid system. Gas mixtures are available in single cylinders. Mixing devices are
used to obtain the correct proportions when two or more gas or liquid sources are used.
The size and type of the gas storage source should be determined by the user, based on
the volume of gas consumed per month.
Electrode Source
The GMAW process uses a continuously fed electrode that is consumed at relatively
high speeds. The electrode source must, therefore, provide a large volume of wire that
can readily be fed to the gun to provide maximum process efficiency. This source usually
take the form of a spool or coil that. hold approximately 10 to 60 pounds (0.45 to 27 kg)
of Wire, wound to allow free feeding without kinks or tangles. Larger spools of up to 25.0
pounds (114 kg) are also available, and wire can be provided m drums or reels of 750 to
1000 pounds (340 to 450 kg). For spool-on-gun equipment small spools [1 to 2 pounds
(0.45 to .9 kg)] are used. The applicable AWS or military electrode specification defines
standard packaging requirements. Normally, special requirements can be agreed to by
the user and the supplier.
The electrode source may be located in close proximity to the wire feeder, or it can be
positioned some distance away and fed through special dispensing equipment.
Normally, the electrode source should be as close as possible to the gun to minimize
feeding problems, yet far enough away to give flexibility and accessibility to the welder.
Consumables
In addition to equipment components, such as contact tips and conduit liners that wear
out and have to be replaced, the process consumables in GMAW are electrodes and
shielding gases. The chemical composition of the electrode, the base metal, and the
shielding gas determine the weld metal chemical composition. This weld metal
composition in turn largely determines the chemical and mechanical properties of the
weldment. The following are factors that influence the selection of the shielding gas and
the welding electrode:
1. Base metal
2. Required weld metal mechanical properties
3. Base metal condition and cleanliness
4. Type of service or applicable specification requirement
5. Welding position
6. Intended mode of metal transfer
Electrodes
The electrodes (filler metals) for gas metal arc. Welding are covered by various AWS
filler metal specifications. Other standards writing societies also publish filler metal
specifications for specific applications. For. example, the Aerospace Materials
Specifications are written by SAE, and are intended for Aerospace applications. The
AWS specifications, designated as A5.XX standards, and a listing of GMAW, electrode
specifications are shown in Table 4. They define requirements for sizes and tolerances,
packaging, chemical composition, and sometimes mechanical properties. The AWS also
publishes, Filler Metal Comparison Charts in which manufacturer s may show their trade
name for each of the filler .metal classifications.
Generally, for joining applications, the composition of the electrode (filler metal) is similar
to that of the base metal. The filler metal composition may be altered slightly to
compensate for losses that occur in the welding arc, or to provide for deoxidation of the
weld pool. In some cases, this involves very little modification from the base metal
composition. In certain applications, however, obtaining satisfactory welding
characteristics and weld metal properties requires an electrode with a different chemical
composition from that of the base metal. For example, the most satisfactory electrode for
GMAW welding manganese bronze, a copper-zinc alloy, is either aluminum bronze or a
copper-manganese-nickel-aluminum alloy electrode.
Electrodes that are most suitable for welding the higher strength aluminum and steel
alloys are often different in composition from the base metals on which they are to be
used. This is because aluminum alloy compositions such as 6061 are unsuitable as weld
filler metals. Accordingly, electrode alloys are designed to produce the desired weld
metal properties and to have acceptable operating characteristics.
Whatever other modifications are made in the composition of electrodes, deoxidizers or
other scavenging elements are generally added. This is done to minimize porosity in the
weld or to assure satisfactory weld metal mechanical properties. The addition of
appropriate deoxidizers in the right quantity is essential to the production of sound
welds. Deoxidizers most commonly used in steel electrodes are manganese, silicon, and
aluminum. Titanium and silicon are the principal deoxidizers used in nickel alloy
electrodes. Copper alloy electrodes may be deoxidized with titanium, silicon, or
phosphorus.
The electrodes used for GMAW are quite small m diameter compared .to those used for
submerged arc or flex cored arc welding. Wire diameters of 0.035 to 0.062 in. (0.9 to 1.6
mm) are common. However, electrode diameters as small as 0.020 in (0.5 mm) and as
large as 1/8 in (3.2 mm) may be used. Because the electrode sizes are small and the
currents comparatively high, GMAW wire feed rates are high. The rates range from
approximately 100 to 800 in./min. (40 to 340 mm/s) for most metals except magnesium,
where rates up to 1400 in./min. (590 mm/s) may be required.
For such wire speeds, electrodes are provided as long, continuous strands of suitably
tempered wire that can be fed smoothly and continuously through the welding
equipment. The wires are normally wound on conveniently sized spools or in coils.
The electrodes have high surface-to-volume ratios because of their relatively small size.
Any drawing compounds or lubricants worked into the surface of the electrode may
adversely affect the weld metal properties. These foreign materials may result in weld
metal porosity in aluminum and steel alloys, and weld metal or heat-affected zone
cracking in high-strength steels. Consequently, the electrodes should be manufactured
with a high-quality surface to preclude the collection of contaminants in seams or laps.
In addition to joining, the GMAW process is widely used for surfacing where an
overlayed weld deposit may provide desirable wear or corrosion resistance or other
properties. Overlays are normally applied to carbon or manganese steels and must be
carefully engineered and evaluated to assure satisfactory results. During surfacing, the
weld metal dilution with the base metal becomes an important consideration; it is a
function of arc characteristics and technique. With GMAW, dilution rates from 10 to 50
percent can be expected depending on the transfer mode. Multiple layers are normally
required, therefore, to obtain suitable deposit chemistry at the surface. Most weld metal
overlays are deposited automatically to precisely control dilution, bead width, bead
thickness, and overlaps by placing each bead against the preceding bead.
Shielding Gases
General
The primary function of the shielding gas is to exclude the atmosphere from contact with
the molten weld metal. This is necessary because most metals, when heated to their
melting point in air, exhibit a strong tendency to form oxides and, to a lesser extent,
nitrides. Oxygen will also react with carbon in molten steel to form carbon monoxide and
carbon dioxide. These varied reaction products may result m weld deficiencies, such as
trapped slag, porosity, .and weld metal embrittlement. Reaction products are easily
formed m, the atmosphere unless precautions are taken to .exclude nitrogen and oxygen
In addition to providing a protective environment, the shielding gas and flow rate also
have a pronounced effect on the following:
1. Arc characteristics
2. Mode of metal transfer
3. Penetration and weld bead profile
4. Speed of welding
5. Undercutting tendency
6. Cleaning action
7. Weld metal mechanical properties
The principal gases used in GMAW are shown in Table 5. Most of these are mixtures of
inert gases which may also contain small quantities of oxygen or CO2. The use
of .nitrogen in welding copper is an exception. Table 6 listed gases used for short
circuiting transfer GMAW.
The Inert Shielding Gases-Argon and Helium
Argon and helium are inert gases. These gases and mixtures of the two are used to weld
nonferrous metals and stainless, carbon, and low alloy steels. The physical differences
between argon and helium are density, thermal conductivity, and arc characteristics.
Argon is approximately 1.4 times more dense than air, while the density of helium is
approximately 0.14 times that of air. The heavier argon is most effective at shielding the
arc and blanketing the weld area in the flat position. Helium requires approximately two
to three times higher flow rates than argon to provide equal protection.
Helium has a higher thermal conductivity than argon and produces arc plasma in which
the arc energy is more uniformly distributed. The argon arc plasma, on the other hand, is
characterized by a high-energy inner core and an outer zone of less energy. This
difference strongly affects the weld bead profile. A welding arc shielded by helium
produces a deep, broad, parabolic weld bead. An arc shielded by argon produces a
bead profile characterized by a "finger" type penetration. Typical bead profiles for argon,
helium, argon-helium mixtures and carbon dioxide are illustrated in Figure 25.
Helium has a higher ionization potential than argon, and consequently, a higher arc
voltage when other variables are held constant. Helium can also present problems in arc
initiation. Arcs shielded only by helium do not exhibit true axial spray transfer at any
current level. The result is that helium-shielded arcs produce more spatters and have
rougher bead surfaces than argon-shielded arcs. Argon shielding (including mixtures
with as low as 80 percent argon) will produce axial spray transfer when the current is
above the transition current.
Mixtures of Argon and Helium
Pure argon shielding is used in many applications for welding nonferrous materials. The
use of pure helium is generally restricted to more specialized areas because an arc in�
helium has limited arc stability. However, the desirable weld profile characteristics (deep,
broad, and parabolic) obtained with the helium arc are quite often the objective in using
an argon-helium shielding gas mixture. The result, illustrated in Figure 25, is an
improved weld bead profile plus the desirable axial spray metal transfer characteristic of
argon.
In short circuiting transfer, argon-helium mixtures of from 60 to 90 percent helium are
used to obtain higher heat input into the base metal for better fusion characteristics. For
some metals, such as the stainless and low alloy steels, helium additions are chosen
instead of CO2 addition because CO2 may adversely affect the mechanical properties of
the deposit.
Mixtures of argon and 50 to 75 percent helium increase the arc voltage (for the same arc
length) over that in pure argon. These gases are used for welding aluminum,
magnesium, and copper because the higher heat input (from the higher voltage) reduces
the effect of the high thermal conductivity of these base metals.
Oxygen and Co2 Additions to Argon and Helium
Pure argon and, to a lesser extent, helium, produce excellent results in welding
nonferrous metals. However, pure argon shielding on ferrous alloys causes an erratic
arc and a tendency for undercut to occur. Additions to argon of from 1 to 5 percent
oxygen or from 3 to 25 percent CO2 produce a noticeable improvement in arc stability
and freedom from undercut by eliminating the arc wander caused by cathode sputtering.
The optimum amount of oxygen or CO2 to be added to the inert gas is a function of the
work surface condition (presence of mill scale or oxides), the joint geometry, the welding
position or technique, and the base metal composition. Generally, 2 percent oxygen or 8
to 10 percent CO2 is considered a good compromise to cover a broad range of these
variables.
Carbon dioxide additions to argon may also enhance the weld bead appearance by
producing a more readily defined "pear-shaped" profile, as illustrated in Figure 26.
Adding between 1 and 9 percent oxygen to the gas improves the fluidity of the weld pool,
penetration, and the arc stability. Oxygen also lowers the transition current. The
tendency to undercut is reduced, but greater oxidation of the weld metal occurs, with a
noticeable loss of silicon an manganese.�
Argon-carbon dioxide mixtures are use on carbon and low alloy steels, and to a lesser
extent on stainless steels. Addition of carbon dioxide up to 25 percent raise the minimum
transition current, Increase spatter loss, deepen penetration, and decrease arc stability.
Argon - CO2 mixtures are primarily used m short circuiting transfer applications, but are
also usable m spray transfer and pulse arc welding.
A mixture of argon with 5 percent CO2 has been used extensively for pulsed arc welding
with solid carbon steel wires. Mixtures of argon, helium, and CO2 are favored for pulsed
arc welding with solid stainless steel wires.
Multiple Shielding Gas Mixtures
Argon-Oxygen-Carbon Dioxide
Gas mixtures of argon with up to 20 percent carbon dioxide and 3 to 5 percent oxygen
are versatile. They provide adequate shielding and desirable arc characteristics for
spray, short circuiting, and pulse mode welding. Mixtures with 10 to 20 percent carbon
dioxide are not in common use in the United States but are popular in Europe.
Argon-Helium-Carbon Dioxide
Mixtures of argon, helium, and carbon dioxide are used with short circuiting and pulse
arc welding of carbon, low alloy, and stainless steels. Mixtures in which argon is the
primary constituent are used for pulse arc welding, and those in which helium is the
primary constituent are used for short circuiting arc welding,
Argon-Helium-Carbon Dioxide-Oxygen
This mixture commonly referred as quad mix, is popular for high-deposition GMAW�
using high current density. This mixture will give good mechanical properties and
operability throughout a wide range of deposition rate. Its major application is welding
low alloy, high tensile base materials, but it has been used on mild steel for high-
production welding. Weld economics are an important consideration in using this gas for
mild steel welding.
Carbon Dioxide
Carbon dioxide (CO2) is a reactive gas widely used in its pure form for gas metal arc
welding of carbon and low alloy steels. It is the only reactive gas suitable for use alone
as a shield in the GMAW process. Higher welding speed, greater joint penetration, and
lower cost are general characteristics which have encouraged extensive use of CO2
shielding gas. �
With a CO2 shield, the metal transfer mode is either short circuiting or globular. Axial
spray transfer requires an argon shield and cannot be achieved with a CO2 shield. With
globular transfer, the arc is quite harsh and produces a high level of spatter. This
requires that CO2 welding condition be set to provide a very short "buried arc" (the tip of
the electrode is actually below the surface of the work), in order to minimize spatter.
In overall comparison to the argon-rich shielded arc, the CO2 shielded arc produces a
weld bead of excellent penetration with a rougher surface profile and much less
"washing" action at the sides of the weld bead, due to the buried arc. Very sound weld
deposits are achieved, but mechanical properties may be adversely affected due to the
oxidizing nature of the arc.
Applications
GMAW can be used on a wide variety of metals and configurations. Its successful
application is dependent on proper selection of the following:
1. Electrode - composition, diameter, and packaging
2. Shielding gas and flow rate
3. Process variables, including amperage, voltage, travel speed, and mode of
transfer
4. Joint design
5. Equipment, including power source, gun, and wire feeder
Electrode Selection
In the engineering of weldments, the objective is to select filler metals that will produce a
weld deposit with two characteristics:
1. A deposit that either closely matches the mechanical and physical properties of
the base metal or provides some enhancement to the base material, such as
corrosion or wear resistance
2. A sound weld deposit, free from discontinuities
In the first case, a weld deposit, even one with composition nearly identical to the base
metal, will possess unique metallurgical characteristics. This is dependent on factors
such as the energy input and weld bead configuration. The second characteristic is
generally achieved through use of a formulated filler metal electrode, e.g., one
containing deoxidizers that produce a relatively defect-free deposit.
Composition
The electrode must meet certain demands of the process regarding arc stability, metal
transfer behavior, and solidification characteristics. It must also provide a weld deposit
that is compatible with one or more of the following base metal characteristics:
1. Chemistry
2. Strength
3. Ductility
4. Toughness
Consideration should be given to other properties such as corrosion, heat-treatment
response, wear resistance, and color match. All such considerations, however, are
secondary to the metallurgical compatibility of the base metal and the filler metal.
American Welding Society specifications have been established for filler metals in
common usage. Table 7 provides a basic guide to selecting appropriate filler metal types
for the listed base metals, along with each applicable AWS filler metal specification.
Tubular Wires
Both solid and tubular Wires are used with GMAW. The tubular wires have a powdered
metallic core which includes small amounts of and stabilizing compounds. These wires
have good arc stability and deposition efficiencies similar to a solid Wire. This tubular
approach permits the manufacture of low-slag, high-efficiency metallic electrodes in
compositions which would be difficult to manufacture as a solid Wire.
Shielding Gas Selection
As noted in earlier sections, the shielding gas used for the gas metal arc process can be
inert (argon or helium), reactive (CO2), or a mixture of the two types. Additions of oxygen
and sometimes hydrogen can be made to achieve other desired arc characteristics and
weld bead geometries. The selection of the best shielding gas is based on consideration
of the material to be welded and the type of metal transfer that will be used. For spray
arc transfer, Table 5 lists the more commonly used shielding gases for various
materials. Table 6 lists those gases used with the short circuiting mode of transfer.
These tables do not list all the special gas combinations that are available.
Setting Process Variables
The selection of the process parameters (amperage, voltage, travel speed, gas flow rate,
electrode extension, etc.) requires some trial and error to determine an acceptable set of
conditions. This is made more difficult because of the interdependence of several of the
variables. Typical ranges of seven variables have been established and are listed in
Tables 8, 9, 10, 11, 12, 13 for various base metals.
Selection of Joint Design
Typical weld joint designs and dimensions for the GMAW process, as used in the
welding of steel, are shown in Figure 27, Figure 27c. The dimensions indicated will
generally produce complete joint penetration and acceptable reinforcement with suitable
welding procedures. Similar joint configurations may be used on other metals, although
the more thermally conductive types (e.g. aluminum and copper) should have larger
groove angles to minimize problems with incomplete fusion.
The deep penetration characteristics of spray transfer GMAW may permit the use of
smaller included angles. This reduces the amount of filler metal required and labor hours
to fabricate weldments.
Equipment Selection
When selecting equipment, the buyer must consider application requirements, range of
power output, static and dynamic characteristics, and wire feed speeds. If a major part of
the production involves small diameter aluminum wire, for example, the fabricator should
consider a push-pull type of wire feeder. If out-of-position welding is contemplated, the
user should look into pulsed power welding machines. For the welding of thin gage
stainless steel a power supply with adjustable slope and inductance may be considered.
When new equipment is to be purchased, some consideration should be given to the
versatility of the equipment and to standardization. Selection of equipment for single-
purpose or high-volume production can generally be based upon the requirements of
that particular application only. However, if a multitude of jobs will be performed (as in
job shop operation), many of which may be unknown at the time of selection, versatility
is very important.
Other equipment already in use at the facility should be considered. Standardizing
certain components and complementing existing equipment will minimize inventory
requirements and provide maximum efficiency of the overall operation.
Special Applications
Spot Welding
Gas metal arc spot welding is a variation of continuous GMAW wherein two pieces of
sheet metal are fused together by penetrating entirely through one piece into the other.
The process has been used for joining light-gage materials, up to approximately 3/16 in.
(5 mm) thick, in the production of automobile bodies, appliances, and electrical
enclosures. No. joint preparation is required other than cleaning of the overlap areas.
Heavier sections can also be spot welded with this technique by drilling or punching a
hole in the upper piece, through which the arc is directed for joining to the underlying
piece. This is called plug welding.
A comparison between a gas metal arc spot weld and a resistance spot weld is shown in
Figure 28. Resistance spot welds are made through resistance heating and electrode
pressure which melts the two components at their interface and fuses them together. In
the gas metal arc spot weld, the arc penetrates through the top member and fuses the
bottom component into its weld puddle. One big advantage of the gas metal arc spot
weld is that access to only one side of the joint is necessary.
The spot weld variation does require some modifications to conventional GMAW
equipment. Special nozzles are used which have ports to allow the shield gas to escape
as the torch is pressed to the work. Timers and wire feed speed controls are also
necessary, to provide regulation of the actual welding time and a current decay period to
fill the weld crater, leaving a desirable reinforcement contour.
Joint Design
Gas metal arc spot welding may be used to weld lap joints in carbon steel, aluminum,
magnesium, stainless steel, and copper-bearing alloys. Metals of the same or different
thicknesses may be welded together, but the thinner sheet should always be the top
member when different thickness are welded. Gas metal arc spot welding is normally
restricted to the fiat position. By modifying the nozzle design, it may be adapted to spot
weld lap-fillet, fillet, and corner joints in the horizontal position.
Equipment Operation
The spot welding GMAW gun is placed in position, pressing the workpieces together.
The gun's trigger is depressed to initiate the arc. The arc timer IS started by a device
that senses flow of welding current. The arc is maintained by the continuously-fed
consumable electrode until it melts through the top sheet and fuses Into the bottom
sheet with out gun travel. The time cycle is set to maintain an arc until the melt-�
through and fusing sequence is complete, i.e., until a spot weld has been formed. The
electrode continues to feed during the arc cycle and should produce a reinforcement on
the upper surface of the top sheet.
Effect of Process Variables on Weld Characteristics
The weld diameter at the interface and the reinforcement are the two characteristics of a
GMAW spot weld which determine whether the weld will satisfy the intended service.
Three major process variables - weld current voltage and arc time - affect one or both of
these characteristics
Current: Current has the greatest effect on penetration. Penetrations increased by using
higher currents (with corresponding increase in wire feed speed). Increased penetration
will generally result In a larger weld diameter at the interface.
Arc Voltage. Arc voltage has the greatest effect on the spot weld shape. In general, with
current being held constant, an increase in the arc voltage will increase the diameter of
the fusion zone. However, it also causes a slight decrease in the reinforcement height
and penetration. Welds made with arc voltages that are too low show a depression in the
center of the reinforcement. Arc voltages that are too high create heavy spatter
conditions.
Weld Time. Welding conditions should be selected that produce a suitable weld within a
time of 20 to 100 cycles of 60 Hz current (0.3 to 1.7 seconds) to join base metal up to
0.125 in. (3.2 mm) thick. Arc time up to 300 cycles (5 seconds) may be necessary on
thicker materials to achieve adequate strength. The penetration, weld diameter, and
reinforcement height generally increase with increased weld time.
As with conventional GMAW, the parameters for spot welding are very interdependent.
Changing one usually requires changing one or more of the others. Some trial and error
is needed to find a set or sets of conditions for a particular application. "Starting"
parameters for gas metal arc spot welding of carbon steel are shown in Table 14.
Narrow Groove Welding
Narrow groove welding is a multipass technique for joining heavy section materials
where the weld joint has a nearly square butt configuration with a minimal groove width
[approximately 1/2 in. (13 mm)]. A typical narrow groove joint configuration is shown in
Figure 29. The technique is used with many of the conventional welding processes,
including GMAW, and is an efficient method of joining heavy section carbon and low
alloy steels, with minimal distortion.
Using GMAW to weld joints in the narrow groove configuration requires special
precautions to assure that the tip of the electrode is positioned accurately for proper
fusion into the sidewalls. Numerous wire feeding methods for accomplishing this have
been devised and successfully used in production environment. Examples of some of
these are shown in Figure 30.
Two wires with controlled cast and two contact tubes are used in tandem, as shown in
Figure 30(A). The arcs are directed toward each sidewall, producing a series of
overlapping fillet welds.
The same effect can be achieved with one wire by means of a weaving technique, which
involves oscillating the arc across the groove In the ,course of welding. This oscillation
can be created mechanically by moving the contact tube across the groove Figure
30(B), but, because of the small contact tube-to-sidewall distance, this technique is not
practical and is seldom used.
Another mechanical technique uses a contact tube bent to an angle of about 15 degrees
Figure 30(C). Along with a forward motion during welding, the contact tube twists to the
right and left, which gives the arc a weaving motion.
A more sophisticated technique is illustrated in Figure 30(D). During feeding, this
electrode is formed into a waved shape by the bending action of a "flapper plate" and
feed rollers as they rotate. The wire is continuously decrease formed plastically into this
waved shape, as the feed rollers press it against the bending plate. The electrode is
almost straightened while going through the contact tube and tip, but recovers its
waviness after passing through the tip. Continuous consumption of the waved electrode
oscillates the arc from one side of the groove to the other. This technique produces an
oscillating arc even in a very narrow groove, with the contact tube remaining centered in
the joint.
The twist electrode technique, Figure 30(E), is another means that has been developed
to improve sidewall penetration without moving the contact tube. The twist electrode
consists of two intertwined wires which, when fed into the groove, generate arcs from the
tips of the two wires. Due to the twist, the arcs describe a continuous rotational
movement which increases penetration into the sidewall without any special weaving
device.
Because these arc oscillation techniques often require special feeding equipment, an
alternate method has been developed in which a larger diameter electrode [e.g. .093 to
125 in. (2.4 to 3.2 mm)] is fed directly into the center of the groove from a contact tip
situated above the plate surface. With this technique, the wire placement is still critical,
but there is less chance of arcing between the contact tube and the work, and standard
welding equipment can be used. It does, however, have a more limited thickness
potential and is normally restricted to the flat position.
The parameters for narrow groove welding are very similar to those used for
conventional GMAW. A summary of some typical values is shown in Table 15. For the
narrow groove application, however, the quality of the results is sensitive to slight
changes in these parameters, voltage being particularly important. An excessive arc
voltage (arc length) can cause undercut of the sidewall, resulting in oxide entrapment or
lack of fusion in subsequent passes. High voltage may cause the arc to climb the
sidewall and damage the contact tube. For this reason, pulsing power supplies have
become widely used in this application. They can maintain a stable spray arc at low arc
voltages.
Various shielding gases have been used with the narrow gap technique, as with
conventional GMAW. A gas consisting of argon with 20 to 25 percent CO2 has seen the
widest application because it provides a good combination of arc characteristics, bead
profile, and sidewall penetration. Delivering the shielding gas to the weld area is a
challenge in the narrow groove configuration, and numerous nozzle designs have been
developed.
Inspection and Weld Quality
Introduction
Weld quality control procedures for GMAW joints are quite similar to those used for other
processes. Depending upon the applicable specifications, inspection procedures should
provide for determining the adequacy of welder and welding operator performance,
qualification of a satisfactory welding procedure, and making a complete examination of
the final weld product.
Weld inspection on the assembled product is limited to nondestructive examination
methods such as visual liquid penetrate, magnetic particle, radiographic, and ultrasonic
inspection. Destructive testing (tensile, shear, fatigue, Impact, bend, fracture, peel,
cross-section, or hardness tests) is usually confined to engineering development,
welding procedure qualification, and welder and welding operator performance
qualification tests.
Potential Problems
Hydrogen Embrittlement
An awareness of the potential problems of hydrogen embrittlement is important, even
though it is less likely to occur with GMAW, since no hygroscopic flux or coating is used.
However, other hydrogen sources must be considered. For example, shielding gas must
be sufficiently low in moisture content. This should be well controlled by the gas supplier,
but may need to be checked oil, grease, and drawing compounds on the electrode or the
base metal may become potential sources for hydrogen pick-up in the weld metal.
Electrode manufacturers are aware of the need for cleanliness and normally take special
care to provide a clean electrode. Contaminants may be introduced during handling in
the user's facility. Users who are aware of such possibilities take steps to avoid serious
problems, particularly in welding hardenable steels. The same awareness is necessary
in welding aluminum, except that the potential problem is porosity caused by the
relatively low solubility of hydrogen in solidified aluminum, rather than hydrogen
embrittlement.
Oxygen and Nitrogen Contamination
Oxygen and nitrogen are potentially greater problems than hydrogen in the GMAW
process. If the shielding gas is not completely inert or adequately protective, these
elements may be readily absorbed from the atmosphere. Both oxides and nitrides can
reduce weld metal notch toughness. Weld metal deposited by GMAW is not as tough as
weld metal deposited by gas tungsten arc welding. It should be noted here, however,
that oxygen m percentages of up to 5 percent and more can be added. to the shielding
gas without adversely affecting weld quality.
Cleanliness
Base metal cleanliness when using GMAW is more critical than with SMAW or
submerged arc welding (SAW). The fluxing compounds present in a SMAW and SAW
scavenge and cleanse the molten weld deposit of oxides and gas-forming compounds.
Such fluxing slages are not present in GMAW. This places a premium on doing a
through job of pre-weld and interpass cleaning. This is particularly true for aluminum,
where elaborate procedures for chemical cleaning or mechanical removal of metallic
oxides, or both, are applied.
Incomplete Fusion
The reduced heat input common to the short circuiting mode of GMAW results in low
penetration into the base metal. This is desirable on thin gauge materials and for out- of-
position welding. However, an improper welding technique may result in incomplete
fusion especially in root areas or along groove faces.
Weld Discontinuities
Some of the more common weld discontinuities that may occur with the GMAW process
are listed in the following paragraphs.
Undercutting
The following are possible. causes of undercutting and their corrective actions:
Possible Causes Corrective Action
1. Travel speed too high
2. Welding voltage too high
3. Excessive welding current
4. Insufficient dwell
1. Use slower travel speed.
2. Reduce the voltage.
3. Reduce wire feed speed.
4. Increase dwell at edge of molten
weld puddle.
5. Gun angle 5. Change gun angle so arc force can
aid in metal placement.
Porosity
The following are possible causes of porosity and their corrective actions:
Possible Causes Corrective Action
1. Inadequate shielding gas coverage
2. Gas contamination
3. Electrode contamination
4. Work-piece contamination
5. Arc voltage too high
6. Excess contact tube-to- work
distance
1. Optimize the gas flow. Increase gas
flow to displace all air from the weld
zone. Decrease excessive gas flow
to avoid turbulence and the
entrapment of air in the weld zone.
Eliminate any leaks in the gas line.
Eliminate drafts (from fans, open
doors, etc.) blowing into the
welding arc. Eliminate frozen
(clogged) regulators in CO2 welding
by using heaters. Reduce travel
speed. Reduce nozzle- to-work
distance. Hold gun at end of weld
until molten metal solidifies.
2. Use welding grade shielding gas.
3. Use only clean and dry electrode.
4. Remove all grease, oil, moisture,
rust, paint, and dirt from work
surface before welding. Use more
highly deoxidizing electrode.
5. Reduce voltage.
6. Reduce stick-out.
Incomplete Fusion
The following are possible causes of incomplete fusion and their corrective actions:
Possible Causes Corrective Action
1. Weld zone surfaces not free of film
or excessive oxides
2. Insufficient heat input
3. Too large a weld puddle
4. Improper weld technique
5. Improper joint design
6. Excessive travel speed
1. Clean all groove faces and weld
zone surfaces of any mill scale
impurities prior to welding.
2. Increase the wire feed speed and
the arc voltage. Reduce electrode
extension.
3. Minimize excessive weaving to
produce a more controllable weld
puddle. Increase the travel speed.
4. When using a weaving technique,
dwell momentarily on the side walls
of the groove. Provide improved
access at root of joints. Keep
electrode directed at the leading
edge of the puddle.
5. Use angle groove large enough to
allow access to bottom of the
groove and sidewalls with proper
electrode extension, or use a "J" or
"V" groove.
6. Reduce travel speed.
Incomplete Joint Penetration
The following are possible causes of incomplete joint penetration and their corrective
actions:
Possible Causes Corrective Actions
1. Improper joint preparation.
2. Improper weld technique
3. Inadequate welding current
1. Joint design must provide proper
access to the bottom of the groove
while maintaining proper electrode
extension. Reduce excessively
large root face. Increase the root
gap in butt joints, and increase
depth of back gouge.
2. Maintain electrode angle normal to
work surface to achieve maximum
penetration. Keep arc on leading
edge of the puddle.
3. Increase the wire feed speed
(welding current).
Excessive Melt-Through
The following are possible causes of excessive melt-through and their corrective actions:
Possible Causes Corrective Actions
1. Excessive heat input
2. Improper joint penetration
1. Reduce wire feed speed (welding
current) and the voltage. Increase
the travel speed.
2. Reduce root opening. Increase root
face dimension.
Weld Metal Cracks
The following are all possible causes of weld metal cracks and their corrective actions:
Possible Causes Corrective Actions
1. Improper Joint design 1. Maintain proper groove dimensions
to allow deposition of adequate
2. Too high a weld depth-to-width
ratio
3. Too small a weld bead (particularly
fillet and root beads)
4. Heat input too high, causing
excessive shrinkage and distortion
5. Hot-shortness
6. High restraint of the joint members
7. Rapid cooling in the crater at the
end of the joint
filler metal or weld cross section to
overcome restraint conditions.
2. Either increase arc voltage or
decrease the current or both to
widen the weld bead or decrease
the penetration.
3. Decrease travel speed to increase
cross section of deposit.
4. Reduce either current or voltage, or
both. Increase travel speed.
5. Use electrode with higher
manganese content (use shorter
arc length to minimize loss of
manganese across the arc). Adjust
the groove angle to allow adequate
percentage of filler meta addition.
Adjust pass sequence to reduce
restraint on weld during cooling.
Change to another filler metal
providing desired characteristics.
6. Use preheat to reduce magnitude
of residual stresses. Adjust welding
sequence to reduce restraint
conditions.
7. Eliminate craters by Back-stepping
technique.
Heat Affected Zone Cracks
Cracking in the heat-affected zone is almost always associated with hardenable steels.
Possible Causes Corrective Actions
1. Hardening in the heat-affected
zone
2. Residual stresses too high
3. Hydrogen embrittlement
1. Preheat to retard cooling rate.
2. Use stress relief heat treatment.
3. Use clean electrode and dry
shielding gas. Remove
contaminants from the base metal.
Hold weld at elevated temperatures
for several hours before cooling
(temperature and time required to
diffuse hydrogen are dependent on
base metal type).
Troubleshooting
Trouble shooting of any process requires a thorough knowledge of the equipment and
the function of the various components, the materials involved, and the process itself. It
is a more complicated task with gas metal arc than with manual processes such as
SMAW and GTAW because of the complexity of the equipment, the number of variables
and the inter-relationship of these variables.
For convenience, problems can be placed in one of the following three categories:
electrical, mechanical, and process. Tables 16, 17, 18 indicate some of the problems
that are likely to be encountered, what the causes might be, and possible remedies.
These are problems that occur during the welding operation or prevent the making of the
weld as opposed to those that are discovered as a result of inspecting the final product.
Safe Practices
Introduction
Safety in welding, cutting, and allied processes is covered in ANSI Z49.1, Safety in
Welding and Cutting, and ANSI Z49.2. Personnel should be familiar with the safe
practices discussed in these documents. In addition, there are other potential hazard
areas In arc welding and cutting (including fumes, gases, radiant energy, noise, handling
of cylinders and regulators, and electric shock) that warrant consideration.
Safe Handling of Gas Cylinders and Regulators
Compressed gas cylinders should be handled carefully and should be adequately
secured when stored or .in use. Knocks, falls, or rough handling may damage valves,
and fuse plugs, and cause leakage or an accident. Valve protecting caps, when
supplied, should be kept in place (hand tight) unless a regulator is attached to the
cylinder.
The following should be observed when setting up and using cylinders of shielding gas:
1. Properly secure the cylinder.
2. Before connecting a regulator to the cylinder valve, the valve should momentarily
be slightly opened and closed immediately ("cracking") to clear the valve of dust
or dirt that otherwise might enter the regulator. The valve operator should stand
to one side of the regulator gauges, never in front of them.
3. After the regulator is attached, the pressure adjusting screw should be released
by turning it counter-clock-wise. The cylinder valve should then be opened slowly
to prevent a rapid surge of high-pressure gas into the regulator. The adjusting
screw should then be turned clockwise until the proper pressure is obtained.
4. The source of the gas supply (i.e., the cylinder valve) should be shut off if it is to
be left unattended, and the adjusting screw should be backed off.
Gases
The major toxic gases associated with GMAW welding are ozone, nitrogen dioxide, and
carbon monoxide. Phosgene gas could also be present as a .result of thermal or
ultraviolet decomposition of chlorinated hydrocarbon cleaning agents located in the
vicinity of welding operations. Two such solvents are trichloroethylene and
perchlorethylene. Degreasing or other cleaning operations involving chlorinated
hydrocarbons should be located so that vapors from these operations cannot be reached
by radiation from the welding arc.
Ozone
The ultraviolet light emitted by the GMAW arc acts on the oxygen in the surrounding
atmosphere to produce ozone, the amount of which will depend upon the intensity and
the wave length of the ultraviolet energy, the humidity, the amount of screening afforded
by any welding fumes, and other factors: The ozone concentration will generally
increase with an increase in welding current, with the use of argon as the shielding gas,
and when welding highly reflective meals. If the ozone cannot. be reduced to a safe level
by ventilation or process Variations, It will be necessary to supply fresh air to the welder
either with an air supplied respirator or by other means.
Nitrogen Dioxide
Some test results show that high concentrations of nitrogen dioxide are found only within
6 in. (150 mm) of the arc. With normal natural ventilation, these concentrations are
quickly reduced to safe levels in the welder's breathing ozone, so long as the welder's
head is kept out of the plume proof fumes (and thus out of the plume of welding-
generated gases). Nitrogen dioxide is not thought to be a hazard in GMAW.
Carbon Monoxide
Carbon dioxide shielding used with the GMAW process will be dissociated by the heat of
the arc to form carbon monoxide. Only a small amount of carbon monoxide is created by
the welding process, although relatively high concentrations are formed temporarily in
the plume of fumes. However, the hot carbon monoxide oxidizes to carbon dioxide so
that the concentrations of carbon monoxide become insignificant at distances of more
than 3 or 4 in. (75 or 100 mm) from the welding plume.
Under normal welding conditions, there should be no hazard from this source. When
welders must work over the welding arc, or with natural ventilation moving the plume of
fumes towards their breathing zone, or where welding is performed in a confined space,
ventilation adequate to deflect the plume or remove the fumes and gases should be
provided (see ANSI Z49.1, Safety in Welding and Cutting).
Metal Fumes
The welding fumes generated by GMAW can be controlled by general ventilation, local
exhaust ventilation, or by respiratory protective equipment as described in ANSI Z49.1.
The method of ventilation required to keep the level of toxic substances within the
welder's breathing zone below threshold concentrations IS directly dependent upon a
number of factors. Among these are the material being welded, the size of the work
area, and the degree of confinement or obstruction to normal air movement where the
welding is being done. Each operation should be evaluated on an individual basis in
order to determine what will be required.
Acceptable exposure levels to substances associated with welding, and designated as
time-weighted average threshold limit values (TLV) and ceiling values, have been
established by the American Conference of Governmental Industrial Hygienists (ACGIH)
and by the Occupational Safety and Health Administration (OSHA). Compliance with
these acceptable levels of exposure can be checked by sampling the atmosphere under
the welder's helmet or in the immediate vicinity of the welder s breathing zone.�
Sampling should be in accordance ANSI/AWS F1 1, Method for Sampling Airborne
Particulates Generated by Welding and Allied Processes.
Radiant Energy
The total radiant energy produced by the GMAW process can be higher than that
produced by the SMAW process, 0 because of its higher arc energy, significantly lower
welding fume and the more exposed arc. Generally, the highest ultraviolet radiant
energy intensities are produced when using an argon shielding gas and when welding
on aluminum.
The suggested filter glass shades for GMAW, as presented in ANSI Z49.1 as a guide,
are shown in Table 19. To select the best shade for an application, first select a very
dark shade. If it is difficult to see the operation properly, select successively lighter
shades until the operation is sufficiently visible for good control. However, do not go
below the lowest recommended number, where given. Dark leather or wool clothing (to
reduce reflection which could cause ultraviolet bums to the face and neck underneath
the helmet) is recommended for GMAW. The greater intensity of the ultraviolet radiation
can cause rapid disintegration of cotton clothing.
Noise-Hearing Protection
Personnel should be protected against exposure to noise generated in welding and
cutting processes in accordance with paragraph 1910.95 "Occupational Noise Exposure"
of the Occupational Safety and Health Administration, U.S. Department of Labor.
Electric Shock
Line voltages to power supplies and auxiliary equipment used in GMAW range from 110
to 575 volts. Welders and service personnel should exercise caution not to come in
contact with these voltages.