Fall -2004

InstructorDr. Mona A. El-Demellawy

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.