937

CHAPTER 4

GAS METALARCWELDING

Introduction 110

Fundamentals of the Process 111

Equipment 123

Consumables 132

Shielding Gases 133

Applications 136

Special Applications 142

Inspection and Weld Quality 146

Troubleshooting 150

Safe Practices 152

Supplementary Reading List 154

Copyright by the American Welding Society IncSat Jul 0510:16:41 1997

PREPARED BVACOMMITTEE CONSISTINGOF:

D. B. Holliday, ChairmanWestinghouse Electric

S. R. CarterScott Paper Company

L. DeFreitasCollege of San Mateo

D. A. FinkLincoln Electric Company

R. W. FolkeningFMC Corporation

D. D. HodsonTweco Products, Incorporated

R. H. MannMiller Electric Manufacturing Company

WELDING HANDBOOKCOMMITTEE MEMBER:P. I. TempleDetroitEdison

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110 GAS METAL ARC WELDING

CHAPTER 4

GAS METAL ARCWELDINGINTRODUCTION

DEFINITION AND GENERAL BACKGROUNDGAS 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 shieldingfrom an externally supplied gas and without the application of pressure.

The basic concept of GMAW was introduced in the1920's, but it was not until 1948 that it was made commerciallyavailable. At first it was considered to be, fundamentally, a high-current density, small diameter, bare metalelectrode process using an inert gas for arc shielding. Itsprimary application was for welding aluminum. As a result,the term MIG (Metal Inert Gas)was used and is still a common reference for the process. Subsequent process developments included operation at low-current densities andpulsed direct current, application to a broader range ofmaterials, and the use of reactive gases (particularly C02)and gas mixtures. This latter development has led to theformal acceptance of the term gas metal arc welding(GMAW) for the process because both inert and reactivegases are used.

A variation of the GMAW process uses a tubular electrode wherein metallic powders make up the bulk of thecore materials (metal cored electrode). Such electrodes require a gas shield to protect the molten weld pool fromatmospheric contamination.

Metal cored, electrodes are considered a segment ofGMAW by the American Welding Society. Foreign welding associations may group metal cored electrodes withflux cored electrodes.

GMAW may be operated in semiautomatic, machine, orautomatic modes. All commercially important metals suchas carbon steel, high-strength low alloy steel, stainlesssteel, aluminum, copper, titanium, and nickel alloys canbe welded in all positions with this process by choosing


the appropriate shielding gas, electrode, and weldingvariables.

USES AND ADVANTAGESTHE USES OF the process are, of course, dictated b}l" itsadvantages, the most important of which are thefollowing:

(1) It is the only consumable electrode process that canbe 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 notfound in submerged arc welding.

(4) Deposition rates are significantly higher than thoseobtained with shielded metal arc welding.

(5) Welding speeds are higher than those with shieldedmetal arc welding because of the continuous electrodefeed and higher filler metal deposition rates.

(6) Because the wire feed is continuous, long welds canbe deposited without stops and starts.

(7) When spray transfer is used, deeper penetration ispossible than with shielded metal arc welding, which maypermit the use of smaller size fillet welds for equivalentstrengths.

(8) Minimal postweld cleaning is required due to theabsence of a heavy slag.

These advantages make the process particularly wellsuited to high production and automated welding applications. This has become increasingly evident with the advent of robotics, where GMAW has been the predominantprocess choice.

AWS WHB-2 90 • 0784265 0010894 7 •

LIMITATIONSAs WITHANYwelding process, there are certain limitationswhich restrict the use of gas metal arc welding. Some ofthese are the following:

(1) The welding equipment is more complex, morecostly, and less portable than that for SMAW.

(2) GMAW is more difficult to use in hard-to-reachplaces because the welding gun is larger than a shielded

GAS METAL ARC WELDING 111

metal arc welding holder, and the welding gun must beclose to the joint, between 3/8 and 3/4 in. (10 and19 mrn), to ensure that the weld metal is properly shielded.

(3) The welding arc must be protected against air draftsthat will disperse the shielding gas. This limits outdoor applications unless protective shields are placed around thewelding 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 OPERATIONTHE GMAW PROCESS incorporates the automatic feedingof a continuous, consumable electrode that is shielded byan externally supplied gas. The process is illustrated in Figure 4.1. After initial settings by the operator, the equipment provides for automatic self-regulation of the electri-

cal characteristics of the arc. Therefore, the only manualcontrols required by the welder for semiautomatic operation are the travel speed and direction, and gun positioning. Given proper equipment and settings, the arc lengthand the current (wire feed speed) are automaticallymaintained.

SOLIDELECTRODEWIRE

SHIELDING GAS INCURRENTCONDUCTOR

DIRECTIONOF TRAVEL

GAS NOZZLE

l

CONSUMABLE ELECTRODE (\ ~ GASEOUS__~"\1 ..."'1 ~/ SHIELD

ARC (~\~ ... .

~~.. ·~\~"'I~WELD

BASE METALMETAL

Figure 4.1-Gas Metal Arc Welding Process


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SHIELDING GASREGULATOR

WELDING GUN

WATERCIRCULATOR(OPTIONAL)

WORKPIECE

CD WORKLEAD

® WATERTO GUN® WATERFROMGUN

o GUNSWITCH CIRCUT® SHIELDING GASTO GUN

SHIELDINGGAS SUPPLY

'<::::============(110

® GABLE ASSEMBLY

G SHIELDING GASFROMCYLINDER

® WELDING CONTACTOR CONTROL

® POWER CABLE@ PRIMARY INPUTPOWER

Figure 4.2-Diagram of Gas Metal Arc Welding Equipment

Equipment required for GMAW is shown in Figure 4.2.The basic equipment components are the welding gun andcable assembly, electrode feed unit, power supply, andsource of shielding gas.

The gun guides the consumable electrode and conductsthe electrical current and shielding gas to the work, thusproviding the energy to establish and maintain the arc andmelt the electrode as well as the needed protection fromthe ambient atmosphere. Two combinations of electrodefeed units and power supplies are used to achieve the desirable self-regulation of arc length. Most commonly this regulation consists of a constant-potential (voltage) powersupply (characteristically providing an essentially flat voltampere curve) in conjunction with a constant-speed electrode feed unit. Alternatively, a constant-current powersupply provides a drooping volt-ampere curve, and theelectrode feed unit is arc-voltage controlled.

With the constant potential/constant wire feed combination, changes in the torch position cause a change in thewelding current that exactly matches the change in theelectrode stick-out (electrode extension), thus the arclength remains fixed. For example, an increased stick-outproduced by withdrawing the torch reduces the currentoutput from the power supply, thereby maintaining thesame resistance heating of the electrode.


In the alternative system, self-regulation results whenarc voltage fluctuations readjust the control circuits of thefeeder, which appropriately changes the wire feed speed.In some cases (welding aluminum, for example), it may bepreferable to deviate from these standard combinationsand couple a constant-current power source with a constant-speed electrode feed unit. This combination provides only a small degree of automatic self-regulation, andtherefore requires more operator skill in semiautomaticwelding. However, some users think this combination affords a range of control over the arc energy (current) thatmay be important in coping with the high thermal conductivity of aluminum base metals.

METAL TRANSFER MECHANISMSTHE CHARACTERISTICS OF the GMAW process are best described in terms of the three basic means by which metal istransferred from the electrode to the work:

(1) Short circuiting transfer(2) Globular transfer(3) Spray transfer

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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 TransferSHORT CIRCUITING ENCOMPASSES the lowest range of welding currents and electrode diameters associated withGMAW. 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 largeroot openings. Metal is transferred from the electrode tothe work only during a period when the electrode is incontact with the weld pool. No metal is transferred acrossthe arc gap.

The electrode contacts the molten weld pool in a rangeof 20 to over 200 times per second. The sequence of eventsin the transfer of metal and the corresponding current andvoltage are shown in Figure 4.3. As the wire touches theweld metal, the current increases [(A),(B), (C), (D)in Figure4.3]. The molten metal at the wire tip pinches off at D andE, initiating an arc as shown in (E) and (F). The rate ofcurrent increase must be high enough to heat the electrodeand promote metal transfer, yet low enough to minimizespatter caused by violent separation of the drop of metal.This rate of current increase is controlled by adjustment ofthe inductance in the power source.


The optimum inductance setting depends on both theelectrical resistance of the welding circuit and the meltingtemperature of the electrode. When the arc is established,the wire melts at the tip as the wire is fed forward towardsthe next short circuit at (H), Figure 4.3. The open circuitvoltage of the power source must be so low that the dropof molten metal at the wire tip cannot transfer until ittouches the base metal. The energy for arc maintenance ispardy provided by energy stored in the inductor during theperiod of short circuiting.

Even though metal transfer occurs only during short circuiting, shielding gas composition has a dramatic effect onthe molten metal surface tension. Changes in shielding gascomposition may dramatically affect the drop size and theduration of the short circuit. In addition, the type of gasinfluences the operating characteristics of the arc and thebase metal penetration. Carbon dioxide generally produces high spatter levels compared to inert gases, but C02also promotes deeper penetration. To achieve a good compromise between spatter and penetration, mixtures of C02and argon are often used when welding carbon and lowalloy steels. Additions ofhelium to argon increase penetration on nonferrous metals.

Globular TransferWITHAPOSITIVE electrode (DCEP), globular transfer takesplace when the current is relatively low, regardless of thetype of shielding gas. However, with carbon dioxide andhelium, this type of transfer takes place at all usable welding currents. Globular transfer is characterized by a dropsize with a diameter greater than that of the electrode. The

IIZERO

ZERO

TIME----

ARCING PERIOD

zo6z~

Figure 4.3-Schematic Representation of Short Circuiting Metal Transfer


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large drop is easily acted on by gravity, generally limitingsuccessful transfer to the flat position.

At average currents, only slightly higher than those usedin short circuiting transfer, globular axially-directed transfer can be achieved in a substantially inert gas shield. If thearc length is too short (low voltage), the enlarging dropmay short to the workpiece, become superheated, and disintegrate, producing considerable spatter. The arc musttherefore be long enough to ensure detachment of thedrop before it contacts the weld pool. However, a weldmade using the higher voltage is likely to be unacceptablebecause 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 directedglobular transfer when the welding current and voltage aresignificantly above the range for short circuiting transfer.The departure from axial transfer motion is governed byelectromagnetic forces, generated by the welding currentacting upon the molten tip, as shown in Figure 4.4. Themost important of these are the electromagnetic pinchforce (P) and anode reaction force (R).

The magnitude of the pinch force is a direct function ofwelding current and wire diameter, and is usually responsible for drop detachment. With C02 shielding, the welding

current is conducted through the molten drop and theelectrode tip is not enveloped by the arc plasma. Highspeed photography shows that the arc moves over the surface of the molten drop and workpiece, because force Rtends to support the drop. The molten drop grows until itdetaches by short circuiting (Figure 4.4B) or by gravity[Figure 4.4(A)],because R is never overcome by P alone. Asshown in Figure 4.4(A), it is possible for the drop to become detached and transfer to the weld pool without disruption. The most likely situation is shown in Figure 4.4(B), which shows the drop short circuiting the arc columnand exploding. Spatter can therefore be severe, which limits the use of C02 shielding for many commercialapplications.

Nevertheless, C02 remains the most commonly usedgas for welding mild steels. The reason for this is that thespatter problem can be reduced significantly by"burying"the arc. In so doing, the arc atmosphere becomes a mixtureof the gas and iron vapor, allowing the transfer to becomealmost spraylike. The arc forces are sufficient to maintain adepressed cavity which traps much of the spatter. Thistechnique requires higher welding current and results indeep penetration. However, unless the travel speed is carefully controlled, poor wetting action may result in excessive weld reinforcement..

GAS NOZZLE

R (Al

\......._-------.. -------)V(8)

Figure 4.4-Nonaxial Globular Transfer


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Spray Transfer

WITH ARGON-RICH SHIELDING it is possible to produce avery stable, spatter-free "axial spray" transfer mode as illustrated in Figure 4.5. This requires the use of direct current and a positive electrode (DeEP), and a current levelabove a critical value called the transition current. Belowthis 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 theform of very small drops that are formed and detached atthe rate of hundreds per second. They are accelerated axially across the arc gap. The relationship between transferrate and current is plotted in Figure 4.6.

The transition current, which is dependent on the liquidmetal surface tension, is inversely proportional to the electrode diameter and, to a smaller degree, to the electrodeextension. It varies with the filler metal melting temperature and the shielding gas composition. Typical transitioncurrents for some of the more common metals are shownin Table 4.1.

The spray transfer mode results in a highly directedstream of discrete drops that are accelerated by arc forcesto velocities which overcome the effects of gravity. Because ofthat, the process, under certain conditions, can beused in any position. Because the drops are smaller thanthe arc length, short circuits do not occur, and spatter isnegligible if not totally eliminated.

Another characteristic of the spray mode of transfer isthe "finger" penetration which it produces. Although thefinger can be deep, it is affected by magnetic fields, which

AXIAL SPRAY TRANSFER

Figure 4.5-AXial Spray Transfer

must be controlled to keep it located at the center of theweld penetration profile.

The spray-arc transfer mode can be used to weld almostany metal or alloy because of the inert characteristics ofthe argon shield. However, applying the process to thinsheets may be difficult because of the high currents neededto produce the spray arc. The resultant arc forces can cutthrough 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.

300 ,..---T""""--..,.----,---.,----.---..,,7115 X 10-4

EEui~::::>...J

~

Sa..oc:::c

o

5

20

10

15

5

~M

10 .Sui:2:::)...J

~tu...Ja..oc:::C

1/16 in. (1.6 mm)MILD STEELELECTRODE, DCRP

ARGON-1% OXYGENSHIELDING GAS1/4 in. (6.4 mm)ARC LENGTH

200 300 400CURRENT,A

••,.IIIIII

TRANSITI0Y:CURRENT I

DROPVOLUME

200

100

~~W...Ja..~ca;'WLJ.enz

~u..ow!;(c:::

Figure 4.6-Variation in Volume and Transfer Rate of Drops With Welding Current (Steel Electrode)


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Table 4.1Globular-to-Spray Transition Currents for a Variety of Electrodes

Wire MinimumWire Electrode Electrode Diameter Spray Arc

Type in. mm Shielding gas Current, AMild Steel 0.030 0.8 98% argon - 2% oxygen 150Mild Steel 0.035 0.9 98% argon - 2% oxygen 165Mild Steel 0.045 1.1 98% argon - 2% oxygen 220Mild Steel 0.062 1.6 98% argon - 2% oxygen 275Stainless Steel 0.035 0.9 98% argon - 2% oxygen 170Stainless Steel 0.045 1.1 98% argon - 2% oxygen 225Stainless Steel 0.062 1.6 98% argon - 2% oxygen 285Aluminum 0.030 0.8 Argon 95Aluminum 0,045 1.1 Argon 135Aluminum 0,062 1.6 Argon 180Deoxidized Copper 0.035 0.9 Argon 180Deoxidized Copper 11.045 1.1 Argon 210Deoxidized Copper 0.062 1.6 Argon 310Silicon Bronze 0.035 0.9 Argon 165Silicon Bronze 0,045 1.1 Argon 205Silicon Bronze 0.062 1.6 Argon 270

Figure 4.7-Pulsed-Spray Arc Welding Current Characteristic

able features of spray transfer available for joining sheetmetals and welding thick metals in all positions.

Many variations of such power sources are available.The simplest provide a single frequency of pulsing (60 or120 pps) with independent control of the background andpulsing current levels. More sophisticated power sources,sometimes called synergic, automatically provide the optimum combination of background and pulse for any givensetting of wire feed speed.

THEFOLLOWING ARE some of the variables that affect weldpenetration, bead geometry and overall weld quality:

PROCESS VARIABLES

PULSE PEAK CURRENT SPRAY TRANSFER3

CURRENT RANGE-,-------PULSE 2 GLOBULAR

« TRANSITION TRANSFER

to-=' CURRENT CURRENTz

1 RANGEwcccc::J 1 2 3 4 5o BACKGROUND CURRENT

~U~~UTIME

The work thickness and welding position limitations ofspray arc transfer have been largely overcome with specially designed power supplies. These machines producecarefully controlled wave forms and frequencies that"pulse" the welding current. As shown in Figure 4.7, theyprovide two levels of current; one a constant, low background current which sustains the arc without providingenough energy to cause drops to form on the wire tip; theother a superimposed pulsing current with amplitudegreater than the transition current necessary for spraytransfer. During this pulse, one or more drops are formedand transferred. The frequency and amplitude of thepulses control the energylevel of the arc, and therefore therate at which the wire melts. By reducing the average arcenergy and the wire melting rate, pulsing makes the desir-


AWS WHB-2 90 II 0784265 0010900 9 II

As shown in Figures 4.8, 4.9, 4.10 and 4.11, when thediameter of the electrode is increased (while maintainingthe same electrode feed speed), a higher welding current isrequired. The relationship between the electrode feedspeed and the welding current is affected by the electrodechemical composition. This effect can be seen by comparing Figures 4.8, 4.9, 4.10 and 4.11, which are for carbon


trodes is shown in Figure 4.8. At the low-current levels foreach electrode size, the curve is nearly linear. However, athigher welding currents, particularly with small diameterelectrodes, 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

whereWFS = the electrode feed speed, in/min (mru/s)

a = a constant of proportionality for anode orcathode heating. Its magnitude is dependentupon polarity, composition, and other factors, in./(min.. A) [(mm/(s . A)]

b = constant of proportionality for electrical resistance heating, mirr! k 2 (s-l k 2)

L = the electrode extension or stick out, in. (mm)I = the welding current, A

(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 toconsistently produce welds of satisfactory quality. Thesevariables are not completely independent, and changingone generally requires changing one or more of the othersto produce the desired results. Considerable skill and experience are needed to select optimum settings for eachapplication. The optimum values are affected by (1) type ofbase metal, (2) electrode composition, (3) welding position, and (4) quality requirements. Thus, there is no singleset of parameters that gives optimum results in every case.

Welding CurrentWHENALLOTHER variables are held constant, the weldingamperage varies with the electrode feed speed or meltingrate in a nonlinear relation. As the electrode feed speed isvaried, the welding amperage will vary in a like manner if a .constant-voltage power source is used. This relationship ofwelding current to wire feed speed for carbon steel elec-

WFS = aI + bLP (4.1)

20800wI-::::>

700z~[J:;w 6000..(f)Wr 500u~

c:i 400wW0..(f)

3000wwu..w 200[J:;

§100

o 50

II

100 150 200 250 300 350 400 450

WELDING CURRENT, A (DCEP)

wI::::>z~

15 ffi0..

encr:wIii

10 ~owW0..enow

5 Ifwcr:§

Figure 4.8-Typical Welding Currents Versus Wire Feed Speeds for Carbon Steel Electrodes


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800

~ 700::>z~6000::w0..

13 500J:u~400ciw~300(J)

cw~200

w0::~ 100

20

w~::>z~

·150::w0..(J)0::

~10 ::2:

owwa..(J)

ow

SWu.wa:~

100 150 200 250 300 350 400

WELDING CURRENT. A (DCEP)

500+---+-----.11--+---+--1--+---+---'--+----+ 0

o

Figure 4.9-Welding Currents Versus Wire Feed Speed for ER4043 Aluminum Electrodes

steel, aluminum, stainless steel, and copper electrodes respectively. The different positions and slopes of the curvesare due to differences in the melting temperatures andelectrical resistivities of the metals. Electrode extensionalso affects the relationships.

With all other variables held constant, an increase inwelding current (electrode feed speed) will result in thefollowing:

(1) An increase in the depth and width of the weldpenetration

(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 advantagesof the spray mode of -metal transfer at average currents

800 20

~::>zs

15 a:wa.(J)a:wtu

10::2:owUJa..(J)

Cl5tll

u.wa:~

50 100 150 200 250 300 350 400

WELDING CURRENT. A (DeEP)

0+---+---1--;---+---1--;---+---1--++0o

~ 700::>z~600a:wa..13 500

is~ 400ow~300(J)

Clt:J 200u.w!!: 100s

Figure 4.10-Typical Welding Currents Versus Wire Feed Speeds for 300 Series Stainless SteelElectrodes


AWS WH8-2 90 .. 0784265 0010902 2 ..GAS METAL ARC WELDLNG 119

800

~ 700:::JZ

~ 6000::Wc,

t3 500J:U

~ 400ci 1-----w~ 300eno~ 200u.wa:§ 100

~ O~#~O.09'~II

200 300 400 500

WELDING CURRENT, A (DCEP)

20

w~::Jz

15~a:wn,

ena:

~10 :2

ciww0.eno

5 ~u.wa:§

Figure 4.11-Welding Currents Versus Wire Feed Speed for ECu Copper Electrodes

equal to or less than the globular-to-spray transitioncurrent.

Sincearc force and deposition rate are exponentially dependent on current, operation above the transition currentoften makes the arc forces uncontrollable in the verticaland overhead positions. By reducing the average currentwith pulsing, the arc forces and deposition rates can bothbe reduced, allowing welds to be made in all positions andin thin sections.

With solid wires, another advantage of pulsed powerwelding is that larger diameter wires [i.e., 1j16-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 arcthat is less sensitive to changes in electrode extension(stickout) and voltage compared to solid wires. Thus, theprocess is more tolerant of operator guidance fluctuations.Pulsed power also minimizes spatter from an operation already low in spatter generation.

PolarityTHE TERM polarity is used to describe the electrical connection of the welding gun with relation to the terminalsof a direct current power source. When the gun powerlead is connected to the positive terminal, the polarity isdesignated as direct current electrode positive (DCEP),arbitrarily called reverse polarity. When the gun is connectedto the negative terminal, the polarity is designated as directcurrent electrode negative (DCEN), originally called


straight polarity. The vast majority of GMAW applicationsuse direct current electrode positive (DCEP). This condition yields a stable are, smooth metal transfer, relativelylow spatter, good weld bead characteristics and greatestdepth of penetration for a wide range of welding currents.

Direct current electrode negative (DCEN) is seldomused because axial spray transfer is not possible withoutmodifications that have bad little commercial acceptance.DCEN has a distinct advantage of high melting rates thatcannot be exploited because the transfer is globular. Withsteels, the transfer can be improved by adding a minimumof 5 percent oxygen to the argon shield (requiring specialalloys to compensate for oxidation losses) or by treatingthe wire to make it thermionic (adding to the cost of thefiller metal). In both cases, the deposition rates drop, eliminating the only real advantage of changing polarity. However, because of the high deposition rate and reducedpenetration, DCEN has found some use in surfacingapplications.

Attempts to use alternating current with the GMAWprocess have generally been unsuccessful. The cyclic waveform creates arc instability due to the tendency of the arcto extinguish as the current passes through the zero point.Although special wire surface treatments have been developed to overcome this problem, the expense of applyingthem has made the technique uneconomical.

Arc Voltage (Arc Length)Arc voltage and arc length are terms that are often usedinterchangeably. It should be pointed out, however, thatthey are different even though they are related. WithGMAW, arc length is a critical variable that must be care-

AWS WHB-2 90 II 0784265 0010903 4 II

120 GAS MET A L ARC W E L DIN G

fully controlled. For example, in the spray-arc mode withargon shielding, an arc that is too short experiences momentary short circuits. They cause pressure fluctuationswhich pump air into the arc stream, producing porosity orembrittlement due to absorbed nitrogen. Should the arc betoo long, it tends to wander, affecting both the penetration and surface bead profiles. A long arc can also disruptthe gas shield. In the case of buried arcs with a carbondioxide shield, a long arc results in excessive spatter as wellas porosity; if the arc is too short, the electrode tip shortcircuits 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, theshield gas, the welding technique and, since it often is measured at the power supply, even the length of the weldingcable. Arc voltage is an approximate means of stating thephysical arc length (see Figure 4.12) in electrical terms, although the arc voltage also includes the voltage drop in theelectrode extension beyond the contact tube.

With all variables held constant, arc voltage is directlyrelated to arc length. Even though the arc length is thevariable of interest and the variable that should be controlled, the voltage is more easily monitored. Because ofthis, and the normal requirement that the arc voltage bespecified in the welding procedure, it is the term that ismore commonly used.

Arc voltage settings vary depending on the material,shielding gas, and transfer mode. Typical values are shownin Table 4.2. Trial runs are necessary to adjust the arc voltage to produce the most favorable arc characteristics and

weld bead appearance. Trials are essential because the optimum arc voltage is dependent upon a variety of factors,including metal thickness, the type of joint, welding position, electrode size, shielding gas composition, and thetype of weld. From any specific value of arc voltage, a voltage increase tends to flatten the weld bead and increase thewidth of the fusion zone. Excessively high voltage maycause porosity, spatter, and undercut. Reduction in voltage results in a narrower weld bead with a higher crownand deeper penetration. Excessively low voltage may causestubbing of the electrode.

Travel Speed

TRAVEL SPEED IS the linear rate at which the arc is movedalong the weld joint. With all other conditions held constant, weld penetration is a maximum at an intermediatetravel speed.

When the travel speed is decreased, the filler metal deposition per unit length increases. At very slow speeds thewelding arc impinges on the molten weld pool, rather thanthe base metal, thereby reducing the effective penetration.A wide weld bead is also a result.

As the travel speed is increased, the thermal energy perunit length of weld transmitted to the base metal from thearc is at first increased, because the arc acts more directlyon the base metal. With further increases in travel speed,less thermal energy per unit length of weld is imparted tothe base metal. Therefore, melting of the base metal firstincreases and then decreases with increasing travel speed.As travel speed is increased further, there is a tendencytoward undercutting along the edges of the weld bead because there is insufficient deposition of filler metal to fillthe path melted by the arc.

NOZZLE

Figure 4. 12-Gas Metal Arc WeldingTerminology

Electrode ExtensionTHE ELECTRODE EXTENSION is the distance between theend of the contact tube and the end of the electrode, asshown in Figure 4.12. An increase in the electrode extension results in an increase in its electrical resistance. Resistance heating in turn causes the electrode temperature torise, and results in a small increase in electrode meltingrate. Overall, the increased electrical resistance produces agreater voltage drop from the contact tube to the work.This is sensed by the power source, which compensates bydecreasing the current. That immediately reduces the electrode melting rate, which then lets the electrode shortenthe physical arc length. Thus, unless there is an increase inthe voltage at the welding machine, the filler metal will bedeposited as a narrow, high-crowned weld bead.

The desirable electrode extension is generally from 1/4to 1/2 in. (6 to 13 mm) for short circuiting transfer andfrom 1/2 to 1 in. (13 to 25 mm) for other types of metaltransfer.

CONTACT TUBE.TO-WORKDISTANCE

J

ARC LENGTH

tELECTRODEEXTENSION

t

WORK PIECE

1NOZZLE TOWORK DISTANCE

CONTACT TUBE


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Table 4.2Typical Arc Voltages for Gas Metal Arc Welding of Various Metalsa

Sprayb Globular Transfer Short Circuiting Transfer1/16 in. (1.6 mm) Diameter Electrode Diameter Electrode

25% Ar- Ar-Oz Ar-Oz 75% Ar-Metal Argon Helium 75% He (1-5% Oz) CDz Argon (1-5% Oz) 25% COz COz

Aluminum 25 30 29 - - 19 - - -Magnesium 26 - 28 - - 16 - - -Carbon steel - - - 28 30 17 18 19 20Low alloy steel - - - 28 30 17 18 19 20Stainless steel 24 - - 26 - 18 19 21 -

Nickel 26 30 28 - - 22 - - -Nickel-copper alloy 26 30 28 - - 22 - - -Nickel-chromium-iron alloy 26 30 28 - - 22 - - -

Copper 30 36 33 - - 24 22 - -Copper-nickel alloy 28 32 30 - - 23 - - -Silicon bronze 28 32 30 28 - 23 - - -Aluminum bronze 28 32 30 - - 23 - - -Phosphor bronze 28 32 30 23 - 23 - - -a. Plus or minus approximately ten percent. The lower voltages are normally used on light material and atlow amperage; the higher voltages are used onheavy material at high amperage.

b. For the pulsed variation of spray transfer the arc voltage would be from 18-28 volts depending on the amperage range used.

Electrode OrientationAs WlTH ALL arc welding processes, the orientation of thewelding electrode with respect to the weld joint affects theweld bead shape and penetration. Electrode orientationaffects bead shape and penetration to a greater extent thanarc voltage or travel speed. The electrode orientation isdescribed in two ways: (1) by the relationship of the electrode axis with respect to the direction of travel (the travelangle), and (2) the angle between the electrode axis and theadjacent work surface (work angle). When the electrodepoints opposite from the direction of travel, the techniqueis called backhand welding with a drag angle. When theelectrode points in the direction of travel, the technique isforehand welding with a lead angle. The electrode orientation and its effect on the width and penetration of the weldare illustrated in Figures 4.13 (A), (B), and (C).

When the electrode is changed from the perpendicularto 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 dragangle of about 25 degrees from perpendicular. The dragtechnique also produces a more convex, narrower bead, amore stable are, and less spatter on the workpiece. For allpositions, the electrode travel angle normally used is a dragangle in the range of 5 to 15 degrees for good control andshielding of the molten weld pool.


For some materials, such as aluminum, a lead techniqueis preferred. This lead technique provides a "cleaning action" ahead of the molten weld metal, which promoteswetting and reduces base metal oxidation.

When producing fillet welds in the horizontal position,the electrode should be positioned about 45 degrees to thevertical member (work angle), as illustrated in Figure 4.14.

Weld Joint PositionMOST SPRAY TYPE GMAW is done in the flat or horizontalpositions, while at low-energy levels, pulsed and short circuiting GMAW can be used in all positions. Fillet weldsmade in the flat position with spray transfer are usuallymore uniform, less likely to have unequal legs and convexprofiles, 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 thevertical and overhead positions of welding, small diameterelectrodes are usually used, with either short circuitingmetal transfer or spray transfer with pulsed direct current.Electrode diameters of 0.045 in. (1.1 mm) and smaller arebest suited for out-of-position welding. The low-heat input allows the molten pool to freeze quickly. Downwardwelding progression is usually effective on sheet metal inthe vertical position.

When welding is done in the "flat" position, the inclination of the weld axis with respect to the horizontal planewill influence the weld bead shape, penetration, and travel

AWS WHB-2 90 II 0784265 0010905 8 II

122 GAS· METAL ARC WE L DIN G

/

P

(A) FOREHANDTECHNIQUE

DIRECTION OF WELDING

(B) TORCHPERPENDICULAR

\

Si

(C) BACKHANDTECHNIQUE

Figure 4. 13-Effect of Electrode Position and Welding Technique

speed. In flat position circumferential welding, the workrotates under the welding gun and inclination is obtainedby moving the welding gun in either direction from topdead 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 thatwould produce excessive reinforcement when the work isin the flat position. Also, when traveling downhill, speedscan usually be increased. At the same time, penetration islower, which is beneficial for welding sheet metal.

Downhill welding affects the weld contour and penetration, as shown in Figure 4.15(A). The weld puddle tends toflow toward the electrode and preheats the base metal,particularly at the surface. This produces an irregularlyshaped fusion zone, called a secondary wash. As the angleof declination increases, the middle surface of the weld isdepressed, penetration decreases, and the width of the


weld increases. For aluminum, this downhill technique isnot recommended due to loss of cleaning action and inadequate shielding.

Uphill welding affects the fusion zone contour and theweld surface, as illustrated in Figure 4.15(B). The force ofgravity causes the weld puddle to flow back and lag behindthe electrode. The edges of the weld lose metal, whichflows to the center. As the angle of inclination increases,reinforcement and penetration increase, and the width ofthe weld decreases. The effects are exactly the opposite ofthose produced by downhill welding. When higher welding currents are used, the maximum usable angle decreases.

Electrode SizeTHE ELECTRODE SIZE (diameter) influences the weld beadconfiguration. A larger electrode requires higher minimumcurrent than a smaller electrode for the same metal transfer

AWS WHB-2 90 II 0784265 0010906 T IIGAS M ETA L ARC W E L DIN G 123

ing are usually done with smaller diameter electrodes andlower currents.

Figure 4.14-NormaJ Work Angle for Filet Welds

Shielding GasTHECHARACTERISTICS OF the various gasesand their effecton weld quality and arc characteristics are discussed in detail in the consumables section of this chapter.

Figure 4. 15-Effect of Work Inclination on WeldBead Shape

characteristics. Higher currents in turn produce additionalelectrode melting and larger, more fluid weld deposits.Higher currents also result in higher deposition rates andgreater penetration. However, vertical and overhead weld-

EQUIPMENT

(A) DOWNHill (B) UPHill

THE GMAW PROCESS can be used either semiautomatically or automatically. The basic equipment for anyGMAW 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)

Typicalsemiautomatic and mechanized components areillustrated in Figures 4.2 and 4.16.

WELDING GUNSDIFFERENT TYPES OF welding guns have been designed toprovide maximum efficiency regardless of the application,ranging from heavy-duty guns for high current, high-production work, to lightweight guns for low current, out-ofposition welding.

Water or air cooling and curved or straight nozzles areavailable for both heavy-duty and lightweight guns. An aircooled gun is generally heavier than a water-cooled gun atthe same rated amperage and duty cycle, because the aircooled gun requires more mass to overcome its less efficient cooling.


The following are basic components of arc weldingguns:

(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 4.17.The contact tube, usually made of copper or a copper

alloy, transfers welding current to the electrode and directsthe electrode towards the work. The contact tube is connected electrically to the welding power supply by thepower cable. The inner surface of the contact tube shouldbe smooth so the electrode will feed easily through thistube and also make good electrical contact. The instruction booklet supplied with every gun will list the correctsize contact tube for each electrode size and material.

Generally, the hole in the contacttube should be 0.005to 0.010 in. (0.13 to 0.25 mm) larger than the wire beingused, although larger hole sizes may be required for aluminum. The contact tube must be held firmly in the torchand must be centered in the gas shielding nozzle. The positioning of the contact tube in relation to the end of thenozzle may be a variable depending on the mode of transfer being used. For short-circuiting transfer, the tube isusually flush or extended beyond the nozzle, while for

AWS WHB-2 90 II 0784265 0010907 1 II124 GAS METAL ARC WELDING

SIDE BEAM

WORKPIECE

SIDE BEAM CARRIAGE

POWERSUPPLY

SHIELDING GAS SUPPLYAND REGULATOR

CD PRIMARY INPUT POWER (j) COOLING WATER OUT

CD WORK LEAD ® SHIELDING GAS INPUT TO WELDING CONTROL

CD POWER CABLE ® COOLING WATER INPUT TO WELDING CONTROL

CD SHIELDING GAS INPUT @ 115 V Be INPUT TO WELDING CONTROL

CD COOLING WATER INPUT @ 115 V Be INPUT TO CARRIAGE CONTROL

® ELECTRODE FEED UNIT INPUT @ INPUT TO CARRIAGE DRIVE MOTOR

Figure 4. 16-Mechanized Gas Metal Arc Welding Installation

spray are it is recessed approximately 1/8 in. During welding, it should be checked periodically and replaced if thehole has become elongated due to excessive wear or if itbecomes clogged with spatter. Using a worn or cloggedtip can result in poor electrical contact and erratic arccharacteristics.

The nozzle directs an even-flowing column of shieldinggas into the welding zone. An even flow is extremely important to assure adequate protection of the molten weldmetal from atmospheric contamination. Different sizenozzles are available and should be chosen according to

the application, i.e., larger nozzles for high-current workwhere the weld puddle is large, and smaller nozzles for lowcurrent and short circuiting welding. For spot weldingapplications the nozzles are made with ports that allow thegas to escape when the nozzle is pressed onto theworkpiece.

The electrode conduit and its liner are connected to abracket adjacent to the feed rolls on the electrode feedmotor. The conduit supports, protects, and directs theelectrode from the feed rolls to the gun and contact tube.Uninterrupted electrode feeding is necessary to insure


AWS WHB-2 90 II 0784265 0010908 3 IIGAS MET A L ARC W E L DIN G 125

POWER CABLE!RETURN WATER

CONTROLSWITCH

SHIELDING GAS PATH

INPUT WATER LINE

WATER CHA:M:B~E~R~~~=~::::;:j~!~~~§"J---~CONTACT TIP

GAS NOZZLE POWER BLOCK

WIRE CONDUIT

ELECTRODE

Figure 4.17-Cross-Sectional View of Typical Gas Metal Arc Welding Gun

good arc stability. Buckling or kinking of the electrodemust be prevented. The electrode will tend to jam anywhere between the drive rolls and the contact tube if notproperly supported.

The liner may be an integral part of the conduit or supplied separately. In either case, the liner material and innerdiameter are important. Liners require periodic maintenance to assure they are clean and in good condition toassure consistant feeding of the wire.

A helical steel liner is recommended when using hardelectrode materials such as steel and copper. Nylon linersshould be used for soft electrode materials such as aluminum and magnesium.

Care must be taken not to crimp or excessivelybend theconduit even though its outer surface is usually steel-supported. The instruction manual supplied with each unitwill generally list the recommended conduits and liners foreach electrode size and material.

The remaining accessories bring the shielding gas, cooling water, and welding power to the gun. These hoses andcables may be connected directly to the source of thesefacilities or to the welding control. Trailing gas shields areavailable and may be required to protect the weld poolduring high-speed welding.

The basic gun, Figure 4.18, is connected to an electrodefeed unit that pushes the electrode from a remote locationthrough the conduit. Other designs are also available including a unit with a small electrode feed mechanism builtinto the gun, Figure 4.19. This gun will pull the electrodefrom the source, where an additional drive may also belocated to simultaneously push the electrode into the conduit (i.e., a "push-pull" system). This type of gun is alsouseful for feeding small diameter or soft electrodes (e.g.aluminum), where pushing might cause the electrode tobuckle. Another variation is the "spool-on-gun" type illustrated in Figure 4.20 in which the electrode feed mechanism and the electrode source are self-contained.

ELECTRODE FEED UNITTHEELECTRODE FEED unit (wire feeder) consists of an electric motor, drive rolls, and accessories for maintainingelectrode 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. Itpushes the electrode through the gun to the work. Itshould have a control circuit that varies the motor speedover a broad range.

Constant-speed wire feeders are normally used in combinationwith constant-potential power sources. They may

Figure 4.18-Commercially Available Gas MetalArc Welding Gun Figure 4.19-Pull-Type GMAW Gun


AWS WHB-2 90 II 0784265 0010909 5 II126 GAS MET A L ARC W E L DIN G

Figure 4.20-Exploded View of a Spool-on-GunType Torch

be used with constant-current power supplies if a slowelectrode "run-in" circuit is added.

When a constant-current power source is used, an automatic voltage sensing control is necessary. This control detects changes in the arc voltage and adjusts the wire feedspeed to maintain a constant arc length. This combinationof variable speed wire feeder and constant-current powersource is limited to larger diameter wires [over 1/16 in.(1.6 mm)], where the feed speeds are lower. At high wirefeed 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 itthrough the welding gun. Wire feed units may use a tworoll or four-roll arrangement. A typical four-roll wire feeding unit is shown in Figure 4.21. The drive roll pressureadjustment allows for variable force to be applied to thewire, depending on its characteristics (e.g. solid or cored,hard or soft). The inlet and outlet guides provide forproper alignment of the wire to the drive rolls and supportthe wire to prevent buckling.

The type of feed rolls generally used with solid wires isshown in Figure 4,22A, where a grooved roll is combinedwith a flat backup roll. A V-groove is used for solid hardwires such as carbon and stainless steels, and aU-groove isused for soft wires such as aluminum.

Serrated or knurled feed rolls with a knurled back uproll, as shown in Figure 4.22B, are generally used withcored wires. The knurled design allows for maximum driveforce to be transmitted to the wire with a minimum ofdrive roll pressure. These types of rolls are not. recommended for softer wire, such as aluminum, because therolls tend to cause a flaking of the wire which can eventually clog the gun or liner.


Figure 4.21-Typical 4 Drive Roll Wire FeedingUnit

Welding ControlTHEWELDING CONTROL and electrode feed motor for semiautomatic operation are availablein one integrated package.The main function of the welding control is to regulate thespeed of the electrode feed motor, usually through the use ofan electronic governor. By increasing the wire feed speed theoperator increases the welding current. Decreases in wirefeed speed result in lower welding currents. The control also

Figure 4.22A-Ground Feed Roll with FlatBackup Used to Feed Solid Wires

AWS WHB-2 90 II 0784265 0010910 1 II

regulates the starting and stopping of the electrode feedthrough a signal received from the gun switch.

Also available are electrode feed control features thatpermit the use of a "touch-start" (the electrode feed is initiated when the electrode touches the work), or a "slowrun-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 conjunctionwith constant-current type power supplies, and are particularly useful for gas metal arc welding of aluminum.

Normally, shielding gas, cooling water, and weldingpower 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 ofsolenoid valves. The control may also sequence the starting and stopping of gas flow, and may energize the powersource contactor, The control may allow some gas to flowbefore welding starts (prepurge) and after welding stops(postpurge) to protect the molten weld puddle. The control is usually independently powered by 115 V ac.

Power SourceTHE WELDING POWER source delivers electrical power tothe electrode and workpiece to produce the arc. For thevast majority of GMAW applications, direct current withelectrode positive (DCEP) is used; therefore, the positivelead is connected to the gun and the negative lead to theworkpiece. The major types of direct current powersources are engine-driven-generators (rotating) and transformer-rectifiers (static). Inverters are included in the staticcategory. The transformer-rectifier type is usually preferred for in-shop fabrication where a source of either 230V or 460 V is available. The transformer-rectifier type re-

Figure 4.22B-Knurled Feed Rolls GenerallyUsed with Cored Wires



sponds faster than the engine-driven-generator type whenthe arc conditions change. The engine-driven generator isused when there is no other available source of electricalenergy, e.g., remote locations.

Both types of power source can be designed and built toprovide either constant current or constant potential.Early applications of the GMAW process used constantcurrent power sources (often referred to as a droopers).Droopers maintain a relatively fixed current level duringwelding, regardless of variations in arc length, as illustratedin Figure 4.23. These machines are characterized by highopen circuit voltages and limited short circuit currentlevels. Since they supply a virtually constant current output, the arc will maintain a fixed length only if the contacttube-to-work distance remains constant, with a constantelectrode feed rate.

In practice, since this distance will vary, the arc will thentend to either "burn back" to the contact tube or "stub"into the workpiece. This can be avoided by using a voltagecontrolled electrode feed system. When the voltage (arclength) increases or decreases, the motor speeds up orslows down to hold the arc length constant. The electrodefeed 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 shortcircuiting transfer makes control by voltage regulationimpractical.

As GMAW applications increased, it was found that aconstant-potential (CP) power source provided improvedoperation. Used in conjunction with a constant-speed wirefeeder, it maintains a nearly constant voltage during thewelding operation. The volt-ampere curve of this typepower source is illustrated in Figure 4.24. The CP systemcompensates for variations in the contact-tip-to-workpiece distance, which occur during normal welding operations, by instantaneously increasing or decreasing thewelding current to compensate for the changes in stickoutdue to the changes in gun-to-work distance.

The arc length is established by adjusting the weldingvoltage at the power source. Once this is set, no otherchanges during welding are required. The wire feed speed,which also becomes the current control, is preset by thewelder or welding operator prior to welding. It can be adjusted over a considerable range before stubbing to theworkpiece or burning back into the contact tube occurs.Welders and welding operators easily learn to adjust thewire feed and voltage controls with only minimuminstruction.

The self-correction mechanism of a constant-potentialpower source is illustrated in Figure 4.25. As the contacttip-to-work distance increases, the arc voltage and arclength would tend to increase. However, the welding current decreases with this slight increase in voltage, thuscompensating for the increase in stickout. Conversely, ifthe distance is shortened, the lower voltage would be accompanied by an increase in current to compensate for theshorter stickout.

AWS WHB-2 90 II 0784265 0010911 3 II

128 GAS M ETA L ARC W E L DIN G

CONSTANT CURRENT POWER SOURCE

>uj(9

~J-----------------R.l:..Jo>

AA

CURRENT,A

Figure 4.23-Volt-Amper Relationship for a Constant-Current (ee) Power Supply

The self-correcting arc length feature of the CP powersource is important in producing stable welding conditions, but there are additional variables that contribute tooptimum welding performance, particularly for short circuiting transfer.

In addition to the control of the output voltage, somedegree 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 betweenthe electrode and the workpiece. Arc voltage is lower thanthe voltage measured directly at the power source becauseof voltage drops at connections and along the length of thewelding cable. As previously mentioned, arc voltage is di-

CONSTANT VOLTAGE POWER SOURCE

Figure 4.24-Volt-Ampere Relationship for a Constant-Potential (ep) Power Supply

-OPERATING POINT

CURRENT, A

T II II II II II-AA-....!-----·I

l :

t

I

AV


STABLECONDITION

RE-ESTABLISHEDSTABLECONDITION

GUN

1/\'

W

NOZZLE

J__L

tl~_

GUN

NOZZLE

1/4 in. (6.4 mm)32310400 in./min. (170 mm/s)

ARC LENGTH, LARC VOLTAGE, VARC CURRENT, AELECTRODE FEED SPEED

1/4 in. (6.4 mm)32280400 in./min (170 mm/s)

Figure 4.25-Automatic Regulation of Arc Length in the GMAW Process

rectly related to arc length; therefore, an increase or a decrease in the output voltage at the power source will resultin a like change in the arc length.

Slope. The static volt-ampere characteristics (staticoutput) of a CP power source is illustrated in Figure 4.24.The slope of the output is the algebraic slope of the voltampere curve and is customarily given as the voltage dropper 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 thetotal slope of the arc welding system. Anything that addsresistance to the welding system (i.e., power cables, poorconnections, loose terminals, dirty contacts, etc.) adds tothe slope. Therefore, slope is best measured at the arc in agiven welding system. Two operating points are needed tocalculate the slope of a constant-potential type weldingsystem, as shown in Figure 4.26. It is not safe to use theopen circuit voltage as one of the points, because of asharp voltage drop with some machines at low currents.This is shown in Figure 4.24. Two stable arc conditionsshould be chosen at currents that envelope the range likelyto 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 flowswhen the electrode is shorted to the workpiece. InGMAW, the separation of molten drops of metal from theelectrode is controlled by an electrical phenomenon calledthe electromagnetic pinch effect. Pinch is the magnetic"squeezing" force on a conductor produced by the currentflowing through it. For short circuiting transfer, the effectis illustrated in Figure 4.27.

The short circuit current (and therefore the pinch effectforce) is a function of the slope of the volt-ampere curve ofthe power source, as illustrated in Figure 4.28. The operating voltage and the amperage of the two power supplies areidentical, but the short circuit current of curve A is lessthan that of curve B. Curve A has the steeper slope, or agreater voltage drop per 100 amperes, as compared tocurve B thus, a lower short circuit current and a lowerpinch effect.

In short circuiting transfer the amount of short circuitcurrent 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 slopeis present in the power supply circuit, the short circuit current will rise rapidly to a high level. The pinch effect willalso be high, and the molten drop will separate violentlyfrom the wire. The excessive pinch effect will abruptly


AWS WHB-2 90 II 0784265 0010913 7 II


CURRENT,A10 V

100A

OPEN CIRCUITVOLTAGE =48 V

38 V, 100A

CURRENT,A

6 V 38 V- 28 VSLOPE =~ = 100 A

>ule~ t------L-----~~:...J

§?

Figure 4.28-Effect of Changing Slope

Table 4.3Typical Short Circuit Currents Required forMetal Transfer in the Short Circuiting Mode

300320175195

Short Circuit~ Current,Amperes (dcrp)

0.80.90.80.9

mm

ElectrodeDiameter

in.0.0300.0350.0300.035

trade will carry the full current, but the pinch effect maybe too low to separate the drop and reestablish the arc.Under these conditions, the electrode will either pile up onthe workpiece or freeze to the puddle. When the shortcircuit current is at an acceptable value, the parting of themolten drop from the electrode is smooth with very littlespatter. Typical short circuit currents required for metaltransfer with the best arc stability are shown in Table 4.3.

Many constant-potential power sources are equippedwith a slope adjustment. They may be stepped or continuously adjustable to provide desirable levels of short circuitcurrent for the application involved. Some have a fixedslope which has been preset for the most common weldingconditions.

Inductance. When the electrode shorts to the work,the current increases rapidly to a higher level. The circuitcharacteristic affecting the time rate of this increase in current is inductance, usually measured in henrys. The effectof inductance is illustrated by the curves plotted in Figure4.29. Curve A is an example of a current-time curve imme-

Carbon steelCarbon steelAluminumAluminum

Electrode Material

-~_~~-.... PINCH EFFECT FORCE, P

ELECTRODE

Figure 4.27-lIIustration of Pinch Effect DuringShort Circuiting Transfer

Figure 4.26-Calculation of the Slope for aPower Supply

CURRENT (A)

j

squeeze the metal aside, clear the short circuit, and createexcessive spatter.

When the short circuit current available from the powersource is limited to a low value by a steep slope, the elec-


AWS WHB-2 90 II 0784265 0010914 9 II------- -

GAS M ETA L ARC WELD I N G 131

IZwa:a:~ulSoa:U «Ix: 1-"o ZJ: W(f) a:

a:~o

EXCESSIVE CURRENT,HIGH SPATIER

DESIRED CURRENTFOR GOOD STABILITYAND LOW SPATIER

TIME, s

Figure 4.29-Change in Rate of Current Rise Due to Added Inductance

diately after a short circuit when some inductance is in thecircuit. Curve B illustrates the path the current would havetaken if there were no inductance in the circuit.

The maximum amount of pinch effect is determined bythe final short circuit current level. The instantaneouspinch 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 appliedrapidly 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-ontime makes the puddle more fluid and results in a flatter,smoother weld bead.

In spray transfer, the addition of some inductance to thepower source will produce a softer arc start without affecting the steady-state welding conditions. Power source adjustments required for minimum spatter conditions varywith the electrode material and diameter. As a general rule,higher short circuit currents and higher inductance areneeded for larger diameter electrodes.

Power sources are available with fixed, stepped, or continuously adjustable inductance levels.

Shielding Gas RegulatorsA SYSTEM IS required to provide constant shielding gasflow rate at atmospheric pressure during welding. A gasregulator reduces the source gas pressure to a constantworking pressure regardless of variations at the source.Regulators may be single or dual stage and may have abuilt-in flowmeter. Dual stage regulators deliver gas at a


more consistant pressure than single stage regulators whenthe 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 areused to obtain the correct proportions when two or moregas or liquid sources are used. The size and type of the gasstorage source should be determined by the user, based onthe volume of gas consumed per month.

Electrode SourceTHE GMAW PROCESS uses a continuously fed electrodethat is consumed at relatively high speeds. The electrodesource must, therefore, provide a large volume of wire thatcan readily be fed to the gun to provide maximum processefficiency. This source usually takes the form of a spool orcoil that holds approximately 10 to 60 pounds (,45 to 27kg) of wire, wound to allow free feeding without kinks ortangles. Larger spools of up to 250 pounds (114 kg) are alsoavailable, and wire can be provided in drums or reels of750 to 1000 pounds (340 to 450 kg). For spool-on-gunequipment small spools [1 to 2 pounds (,45 to .9 kg)] areused. The applicable AWS or military electrode specification defines standard packaging requirements. Normally,special requirements can be agreed to by the user and thesupplier.

The electrode source may be located in close proximityto the wire feeder, or it can be positioned some distanceaway and fed through special dispensing equipment. Normally, the electrode source should be as close as possibleto the gun to minimize feeding problems, yet far enoughaway to give flexibility and accessability to the welder.

AWS WHB-2 90 II 0784265 0010915 0 II


CONSUMABLES

Table 4.4Specifications for Various GMAW Electrodes

metal properties and to have acceptable operatingcharacteristics.

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 metalmechanical properties. The addition of appropriate deoxidizers in the right quantity is essential to the production ofsound welds. Deoxidizers most commonly used in steelelectrodes are manganese, silicon, and aluminum. Titanium and silicon are the principal deoxidizers used innickel alloy electrodes. Copper alloy electrodes may be deoxidized with titanium, silicon, or phosphorus.

The electrodes used for GMAW are quite small in diameter compared to those used for submerged arc or fluxcored 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 sizesare smalland the currents comparatively high, GMAW wire feedrates are high. The rates range from approximately 100 to800 in.jmin. (40 to 340 mm/s) for most metals exceptmagnesium, where rates up to 1400 in.yrnin. (590 mrny's)may be required.

For such wire speeds, electrodes are provided as long,continuous strands of suitably tempered wire that can befed smoothly and continuously through the weldingequipment. 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 porosityin 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

IN ADDITION TO equipment components, such as contacttips and conduit liners that wear out and have to be replaced, the process consumables in GMAW are electrodesand shielding gases.The chemical composition of the electrode, the base metal, and the shielding gas determine theweld metal chemical composition. This weld metal composition in turn largely determines the chemical and mechanical properties of the weldment. The following arefactors that influence the selection of the shielding gas andthe 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

ELECTRODESTHE ELECfRODES (FILLER metals) for gas metal arc weldingare covered by various AWS filler metal specifications.Other standards writing societies also publish filler metalspecifications for specific applications. For example, theAerospace Materials Specifications are written by SAE,and are intended for Aerospace applications. The AWSspecifications, designated as AS.XX standards, and a listing of GMAW electrode specifications are shown in Table4.4. They define requirements for sizes and tolerances,packaging, chemical composition, and sometimes mechanical properties. The AWS also publishes Filler MetalComparison Charts in which manufacturer's may show theirtrade name for each of the filler metal classifications.

Generally, for joining applications, the composition ofthe electrode (filler metal) is similar to that of the basemetal. The filler metal composition may be altered slightlyto compensate for losses that occur in the welding arc, orto provide for deoxidation of the weld pool. In some cases,this involves very little modification from the base metalcomposition. In certain applications, however, obtainingsatisfactory welding characteristics and weld metal properties requires an electrode with a different chemical composition from that of the base metal. For example, the mostsatisfactory electrode for GMAW welding manganesebronze, a copper-zinc alloy, is either aluminum bronze or acopper-manganese-nickel-aluminum alloy electrode.

Electrodes that are most suitable for welding the higherstrength aluminum and steel alloys are often different incomposition from the base metals on which they are to beused. This is because aluminum alloy compositions such as6061 are unsuitable as weld filler metals. Accordingly,electrode alloys are designed to produce the desired weld


Base Material TypeCarbon SteelLow Alloy SteelAluminum AlloysCopper AlloysMagnesiumNickel Alloys300 Series Stainless Steel400 Series Stainless SteelTitanium

AWSSpecification

A5.18A5.28A5.10A5.7A5.19A5.14A5.9A5.9A5.16

AWS WH~-2 9_D. 0784265 0010916 2 •

surface to preclude the collection of contaminants inseams or laps.

In addition to joining, the GMAW process is widelyused for surfacing where an overlayed weld deposit mayprovide desirable wear or corrosion resistance or otherproperties. Overlays are normally applied to carbon ormanganese steels and must be carefully engineered andevaluated to assure satisfactory results. During surfacing,the weld metal dilution with the base metal becomes an

SHIELDING GASES

GENERALTHE PRIMARY FUNCTION of the shielding gas is to excludethe atmosphere from contact with the molten weld metal.This is necessary because most metals, when heated totheir melting point in air, exhibit a strong tendency to formoxides and, to a lesser extent, nitrides. Oxygen will alsoreact with carbon in molten steel to form carbon monoxide and carbon dioxide. These varied reaction productsmay result in weld deficiencies, such as trapped slag, porosity, and weld metal ernbrittlement. Reaction productsare easily formed in the atmosphere unless precautions aretaken to exclude nitrogen and oxygen.

In addition to providing a protective environment, theshielding gas and flow rate also have a pronounced effecton 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 GMAWare shown in Table4.5. Most of these are mixtures of inert gases which mayalso contain small quantities of oxygen or C02. The use ofnitrogen in welding copper is an exception. Table 4.6 listsgases used for short circuiting transfer GMAW.

THE INERT SHIELDING GASES-ARGONAND HELIUMARGON AND HELIUM are inert gases. These gases and mixtures of the two are used to weld nonferrous metals andstainless, 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 timesthat of air. The heavier argon is most effective at shielding



important consideration; it is a function of arc characteristics and technique. With GMAW, dilution rates from 10 to50 percent can be expected depending on the transfermode. Multiple layers are normally required, therefore, toobtain suitable deposit chemistry at the surface. Mostweld metal overlays are deposited automatically to precisely control dilution, bead width, bead thickness, andoverlaps by placing each bead against the preceding bead.

the arc and blanketing the weld area in the flat position.Helium requires approximately two to three times higherflow rates than argon to provide equal protection.

Helium has a higher thermal conductivity than argonand produces an arc plasma in which the arc energy is moreuniformly distributed. The argon arc plasma, on the otherhand, is characterized by a high-energy inner core and anouter zone of less energy. This difference strongly affectsthe weld bead profile. A welding arc shielded by heliumproduces a deep, broad, parabolic weld bead. An arcshielded by argon produces a bead profile characterized bya "finger" type penetration. Typical bead profiles for argon, helium, argon-helium mixtures and carbon dioxideare illustrated in Figure 4.30.

Helium has a higher ionization potential than argon, andconsequently, a higher arc voltage when other variables areheld constant. Helium can also present problems in arcinitiation. Arcs shielded only by helium do not exhibit trueaxial spray transfer at any current level. The result is thathelium-shielded arcs produce more spatter and haverougher bead surfaces than argon-shielded arcs. Argonshielding (including mixtures with as low as 80 percent argon) will produce axial spray transfer when the current isabove the transition current.

MIXTURES OF ARGON AND HELIUMPURE ARGON SHIELDING is used in many applications forwelding nonferrous materials. The use of pure helium isgenerally restricted to more specialized areas because anarc 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. Theresult, illustrated in Figure 4.30, is an improved weld beadprofile plus the desirable axial spray metal transfer characteristic of argon.

In short circuiting transfer, argon-helium mixtures offrom 60 to 90 percent helium are used to obtain higherheat input into the base metal for better fusion characteristics. For some metals, such as the stainless and low alloysteels, helium additions are chosen instead of C02 addi-

AWS WHB-2 90 II 0784265 0010917 4 II


Table 4.5GMAW Shielding Gases for Spray Transfer

Shielding Gas Thickness Advontages

Table 4.6GMAW Shielding Gases for Short Circuiting Transfer

Advantages

No effect on corrosion resistance; small heat-affected zone;no undercutting; minimum distortion.

Minimum reactivity; excellent toughness; excellent arcstability, wetting characteristics, and bead contour; littlespatter.

Fair toughness; excellent arc stability, wetting characteristics,and bead contour; little spatter.

Argon satisfactory on sheet metal; argon-helium preferredbase material.

Best metal transfer and arc stability; least spatter.Higher heat input than straight argon; improved fusion

characteristics with 5XXX series AI-Mg alloys.Highest heat input; minimizes porosity

Minimizes undercutting; provides good toughness.

Excellent cleaning action.Improves arc stability; produces a more fluid and controllable weld

puddle; good coalescence and bead contour; minimizesundercutting; permits higher speeds than pure argon.

High-speed mechanized welding; low-cost manual welding.

Improves arc stability; produces a more fluid and controllable weldpuddle, good coalescence and bead contour; minimizesundercutting on heavier stainless steels.

Provides better arc stability, coalescence, and welding speed than1 percent oxygen mixture for thinner stainless steel materials.

Provides good wetting; decreases fluidity ofweld metal.

Higher heat inputs of 50& 75 percent helium mixtures offset highheat dissipation of heavier gages.

Good arc stability; minimum weld contamination; inert gas backingis required to prevent air contamination on back of weld area.

Over 1/8 in. (3.2 mm)

Over 3 in. (76 mm)

oto 1 in. (0 to 25 mm)1 to 3 in. (25 to 76 mm)

Up to 1/8 in. (3.2 mm)

Thickness

75% argon less than 1/8 in. (3.2 mml High welding speeds without burn-through; minimum+25%C02 distrotion and spatter.75% argon More than 1/8 in. (3.2 mm) Minimum spatter; clean weld appearance; good puddle control+25% C02 in vertical and overhead positions.Argon with Deeper penetration; faster welding speeds.5-10% C0290% helium +7.5%argon + 2.5% C0260-70% helium+25-35% argon+4.5% C0275% argon+25% C02Argon & argon+ helium

Shielding Gas

100% Argon35% argon-65% helium25% argon-75% helium100% Argon95% Argon+3.5% oxygen

90% Argon+8/10% carbon dioxide98% Argon-2% oxygen99% Argon-1 %oxygen

Argon-helium100% Argon

98% Argon-2% oxygen100% Argon

MetalAluminum

Nickel, copper,and theiralloys

MagnesiumSteel carbon

Steel low-alloy

Metol

Steel stainless

Titanium

Carbon steel

low alloy steel

Aluminum, coppermagnesium, nickel,and their alloys

Stainless steel


AWS ~H8-2 90 "D184265 0010918 ~ •


ARGON ARGON-HELIUM HELIUM

Figure 4.30-Bead Contour and Penetration Patterns for Various Shielding Gases

tions because C02 may adversely affect the mechanicalproperties of the deposit.

Mixtures of argon and 50 to 75 percent helium increasethe arc voltage (for the same arc length) over that in pureargon. These gasesare used for welding aluminum, rnagnesium, and copper because the higher heat input (from thehigher voltage) reduces the effect of the high thermal conductivity of these base metals.

OXYGEN AND C02 ADDITIONS TO ARGONAND HELIUMPURE 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 argonof from 1 to 5 percent oxygen or from 3 to 25 percent C02produce a noticeable improvement in arc stability andfreedom from undercut by eliminating the arc wandercaused by cathode sputtering.

The optimum amount of oxygen or C02 to be added tothe inert gas is a function of the work surface condition(presence of mill scale or oxides), the joint geometry, thewelding position or technique, and the base metal composition. Generally, 2 percent oxygen or 8 to 10 percent C02is considered a good compromise to cover a broad range ofthese variables.

Carbon dioxide additions to argon may also enhance theweld bead appearance byproducing a more readily defined"pear-shaped" profile, as illustrated in Figure 4.31. Addingbetween 1 and 9 percent oxygen to the gas improves the

ARGON-0 2 ARGON-C0 2

Figure 4.31-Relative Effect of Oxygen Versus Carbon Dioxide Additions to the Argon Shield


AWS WHB-2 90 II 0784265 0010919 8 II


fluidity of the weld pool, penetration, and the arc stability.Oxygen also lowers the transition current. The tendencyto undercut is reduced, but greater oxidation of the weldmetal occurs, with a noticeable loss of silicon andmanganese.

Argon-carbon dioxide mixtures are used on carbon andlow alloy steels, and to a lesser extent on stainless steels.Additions of carbon dioxide up to 25 percent raise theminimum transition current, increase spatter loss, deepenpenetration, and decrease arc stability. Argon - C02 mixtures are primarily used in short circuiting transfer applications, but are also usable in spray transfer and pulse arcwelding.

A mixture of argon with 5 percent C02 has been usedextensively for pulsed arc welding with solid carbon steelwires. Mixtures of argon, helium, and C02 are favored forpulsed arc welding with solid stainless steel wires.

MULTIPLE SHIELDING GAS MIXTURES

Argon-Oxygen-Carbon DioxideGAS MIXTURES OF argon with up to 20 percent carbon dioxide and 3 to 5 percent oxygen are versatile. They provideadequate shielding and desirable arc characteristics forspray, short circuiting, and pulse mode welding. Mixtureswith 10 to 20 percent carbon dioxide are not in commonuse in the United States but are popular in Europe.

Argon-Helium-Carbon DioxideMIXTURES OF ARGON, helium, and carbon dioxide are usedwith short circuiting and pulse arc welding of carbon, lowalloy, and stainless steels. Mixtures in which argon is theprimary constituent are used for pulse arc welding, andthose in which helium is the primary constituent are usedfor short circuiting arc welding.

APPLICATIONS

GMAW CAN BE used on a wide variety of metals and configurations. Its successful application is dependent onproper 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


Argon-Helium-Carbon Dioxide-Oxygen

THIS MIXTURE, commonly referred to as quad mix, is popular for high-deposition GMAW using the high-currentdensity metal transfer type arc. This mixture will givegoodmechanical properties and operability throughout a widerange of deposition rates. Its major application is weldinglow alloy, high-tensile base materials, but it has been usedon mild steel for high-production welding. Weldecononmics are an important consideration in using thisgas for mild steel welding.

CARBON DIOXIDE

CARBON DIOXIDE (C02) is a reactive gas widely used in itspure form for gas metal arc welding of carbon and lowalloy steels. It is the only reactive gas suitable for use aloneas a shield in the GMAW process. Higher welding speed,greater joint penetration, and lower cost are general characteristics which have encouraged extensive use of C02shielding gas.

With a C02 shield, the metal transfer mode is eithershort circuiting or globular. Axial spray transfer requiresan argon shield and cannot be achieved with a C02 shield.With globular transfer, the arc is quite harsh and producesa high level of spatter. This requires that C02 welding conditions be set to provide a very short "buried are" (the tipof the electrode is actually below the surface of the work),in order to minimize spatter.

In overall comparison to the argon-rich shielded are, theC02 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 theburied arc. Very sound weld deposits are achieved, but mechanical properties may be adversely affected due to theoxidizing nature of the arc.

ELECTRODE SELECTIONIN THE ENGINEERING of weldments, the objective is to select filler metals that will produce a weld deposit with twobasic characteristics:

(1) A deposit that either closely matches the mechanicaland physical properties of the base metal or provides someenhancement to the base material, such as corrosion orwear resistance

(2) A sound weld deposit, free from discontinuities

AWS WHB-2 90 II 0784265 0010920 4 II

In the first case, a weld deposit, even one with composition nearly identical to the base metal, will possess uniquemetallurgical characteristics. This is dependent on factorssuch as the energy input and weld bead configuration. Thesecond characteristic is generally achieved through use of aformulated filler metal electrode, e.g., one containing deoxidizers that produce a relatively defect-free deposit.

CompositionTHE ELECTRODE MUST meet certain demands of the process regarding arc stability, metal transfer behavior, andsolidification characteristics. It must also provide a welddeposit 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 suchas corrosion, heat-treatment response, wear resistance,and color match. All such considerations, however, aresecondary to the metallurgical compatibility of the basemetal and the filler metal.

American Welding Society specifications have been established for filler metals in common usage. Table 4.7 provides a basie guide to selecting appropriate filler metaltypes for the listed base metals, along with each applicableAWS filler metal specification.

Tubular WiresBOTH SOLID AND tubular wires are used with GMAW. Thetubular wires have a powdered metallic core which includes small amounts of arc stabilizing compounds. Thesewires have good arc stability and deposition efficienciessimilar to a solid wire. This tubular approach permits themanufacture of low-slag, high-efficiency metallic electrodes in compositions which would be difficult to manufacture as a solid wire.

SHIELDING GAS SELECTIONAs NOTED INearlier sections, the shielding gas used for thegas metal arc process can be inert (argon or helium), reactive (C02), or a mixture of the two types. Additions ofoxygen and sometimes hydrogen can be made to achieveother 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 ofmetal transfer that will be used. For spray arc transfer, Table 4.5 lists the more commonly used shielding gases forvarious materials. Table 4.6 lists those gases used with the


GAS MET A L ARC WELD I N G 137

short circuiting mode of transfer. These tables do not listall the special gas combinations that are available.

SETTING PROCESS VARIABLESTHESELECTION OF the process parameters (amperage, voltage, travel speed, gas flow rate, electrode extension, etc.)requires some trial and error to determine an acceptableset of conditions. This is made more difficult because ofthe interdependence of several of the variables. Typicalranges of seven variables have been established and arelisted in Tables 4.8 through 4.13 for various base metals.

SELECTION OF JOINT DESIGNTYPICAL WELD JOINT designs and dimensions for theGMAW process, as used in the welding of steel, are shownin Figure 4.32. The dimensions indicated will generallyproduce 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 tominimize problems with incomplete fusion.

The deep penetration characteristics of spray transferGMAW may permit the use of smaller included angles.This reduces the amount of filler metal required and laborhours to fabricate weldments.

EQUIPMENT SELECTIONWHEN SELECTING EQUIPMENT, the buyer must considerapplication requirements, range of power output, staticand dynamic characteristics, and wire feed speeds. If a major part of the production involves small diameter aluminum wire, for example, the fabricator should consider apush-pull type of wire feeder. If out-of-position welding iscontemplated, the user should look into pulsed powerwelding machines. For the welding of thin gage stainlesssteel, a power supply with adjustable slope and inductancemay be considered.

When new equipment is to be purchased, some consideration should be given to the versatility of the equipmentand to standardization. Selection of equipment for singlepurpose or high-volume production can generally be basedupon the requirements of that particular application only.However, if a multitude of jobs will be performed (as injob shop operation), many of which may be unknown atthe time of selection, versatility is very important.

Other equipment already in use at the facility should beconsidered. Standardizing certain components and complementing existing equipment will minimize inventory requirements and provide maximum efficiency of the overalloperation. Details of equipment components are providedin earlier sections of this chapter.

AWS WHB-2 90 • 0784265 0010921 6 •138 GAS METAL ARC WELDING

Table 4.7Recommended Electrodes for GMAW

Base MaterialAWS ElectrodeSpecification

Type Classification Electrode Classification (Use latest edition)

Aluminum 1100 ER4043and 3003, 3004 ER5356

)aluminum 5052, 5454 ER5554, ER5556, A5.1Oalloys or ER5183(ASTM Standards 5083,5086, ER5556 or ER5356

5456Volume 2.02) 6061,6063 ER4043 or ER5356

Magnesium AZ10A ERAZ61A, ERAZ92Aalloys AZ31B, Al61A,(ASTM Standards Al80A ERAl61A, ERAZ92AVolume 2.02) ZElDA ERAZ61A, ERAZ92A

lK21A ERAl92A A5.19AZ63A, Al81 A,

AZ91C EREZ33AAZ92A, AMlDOA EREZ33AHK31A, HM21A,

HM31A EREZ33ALA141A EREZ33A

Copper Commercially pure ERCu

)and Brass ERCuSi-A, ERCuSn-Acopper Cu-Ni alloys ERCuNialloys Manganese bronze ERCuAI-A2 A5.7(ASTM Standards Aluminum bronze ERCuAI-A2Volue 2.01) Bronze ERCuSn-A

Nickel Commercially pure ERNi

)and nickel Ni-Cu alloys ERNiCu-7 A5.14alloys Ni-Cr-Fe alloys ERNiCrFe-5(ASTM StandardsVolume 2.04)

Titanium Commercially pure ERTi-l,-2,-3,-4

)and titanium Ti-6 AL-4V ERTi-6AI-4Valloys Ti-0.15Pd ERTi-0.2Pd A5.16(ASTM Standards Ti-5AI-2 5Sn ERTi-5AI-2.5SnVolume 2.04) Ti-13V-l1 Cr-3AL ERTi-13V-ll Cr-3AL

Austenitic Type 201 ER308stainless steels Types 301, 302,(ASTM Standards 304 & 308 ER308Volume 1.04) Type 304L ER308L

Type 310 ER310 A5.9Type 316 ER316Type 321 ER321Type 347 ER347

Carbon Hot and cold rolled E70S-3, or E7DS-l )steels plain carbon steels E70S-2, E70S-4 A5.18E70S-5, E70S-6


AWS_ WHB-S ~~O .~07l!J':L2p5__00109?2_8 •GAS MET A L ARC W E L DIN G 139

Table 4.8Typical Conditions for Gas Metal Arc Welding of Carbon and Low Alloy Steel in the Flat Position

Material Wire Current Wire FeedThickness Diameter Voltage1 Speed Gas Flow

in. mm Type of Weld in. mm amps volts IPM mmjs Shielding Gas2 CFH LPM.062 1.6 Butt3 .035 0.9 95 18 150 64 Ar 75%, C02 -25% 25 12

.125 3.2 Butt3 .035 0.9 140 20 250 106 Ar 75%, C02 25% 25 12

.187 4.7 Butt3 .035 0.9 150 20 265 112 Ar 75%, C02 25% 25 12

.250 6.4 Butt3 .035 0.9 150 21 265 112 Ar 75%, C02 25% 25 12

.250 6.4 Butt4 .045 1.1 200 22 250 106 Ar 75%, C02 -25% 25 12

1. Direct current electrode positive.2. Welding grade C02 may also be used.3. Root opening of .03 in. (0.8 mm).4. Root opening of .062 in. (1.6 mm).

Table 4.9Typical Conditions for Gas Metal Arc Welding of Aluminum in the Flat Position

Material Wire Current Wire FeedThickness Diameter Voltage* Speed Shielding Gas Flow

in. mm Type of Weld in. mm amps volts IPM mmjs Gas CFH LPM.062 1.6 Butt .030 0.8 90 18 365 155 Argon 30 14

.125 3.2 Butt .030 0.8 125 20 440 186 Argon 30 14

.187 4.8 Butt .045 1.1 160 23 275 116 Argon 35 16

.250 6.4 Butt .045 1.1 205 24 335 142 Argon 35 16

.375 9.5 Butt .063 1.6 240 26 215 91 Argon 40 19

* Direct current electrode positive.

Table 4.10Typical Conditions for Gas Metal Arc Welding of Austenitic Stainless Steel Using a Spray Arc in

the Flat Position

Material Wire Current Wire FeedThickness Diameter Voltage1 Speed Gas Flow

in. mm Type ofWeld in. mm amps volts IPM mmjs Shielding Gas CFH LPM

.125 3.2 Butt Joint withBacking .062 1.6 225 24 130 55 Ar 98%, 022% 30 14

.250(1) 6.4 V-Butt Joint6011 Inc. Angle .062 1.6 275 26 175 74 Ar 98%,022% 35 16

.375(1) 9.5 V-Butt Joint6011 Inc. Angle .062 1.6 300 28 240 102 Ar 98%, 022% 35 16

1. Direct current electrode positive.2. Two passes required.


AWS WHB-2 90 .. 0784265 0010923 T II


Table 4.11Typical Conditions for Gas Metal Arc Welding of Austenitic Stainless Steel Using a Short

Circuiting Arc

Material Wire Current Wire FeedThickness Diameter Voltage* Speed Gas Flow

in. mm Type of Weld in. mm amps volts IPM mm/s Shielding Gas CFH LPM.062 1.6 Butt Joint .030 0.8 85 21 185 78 He 90%, Ar 7.5% 30 14

C022.5%

.093 2.4 Butt Joint ,030 0,8 105 23 230 97 He 90%, Ar 7.5% 30 14C022.5%

.125 3.2 Butt Joint .030 0,8 125 24 280 118 He 90%, Ar 7.5% 30 14C022.5%


Table 4.12Typical Conditions for Gas Metal Arc Welding of Copper Alloys in the Flat Position

Material Wire Current Wiro FeedThickness Diameter Voltage* Speed Shielding Gas Flow

in. mm Type ofWeld in. mm amps volts IPM mm/s Gas CFH LPM.125 3.2 Butt .035 175 23 430 182 Argon 25 12

.187 4.8 Butt .045 210 25 240 101 Argon 30 14

.250 6.4 Butt, Spaced .062 365 26 240 101 Argon 35 16


Table 4.13Typical Variable Settings for Gas Metal Arc Welding of Magnesium

MaterialThickness Wire Diameter Current Voltage* Wire Feed Speed Argon Flow

in. mm Type of Weld in. mm amps volts IPM mm/s CFH LPM.062 1.6 Square Groove ,062 1.6 70 16 160 68 50 24

or Fillet

.090 2.3 Square Groove ,062 1.6 105 17 245 104 50 24or Fillet

.125 3.2 Square Groove ,062 1.6 125 18 290 123 50 24or Fillet

.250 6.4 Square Groove .062 1.6 265 25 600 254 60 28or Fillet

.375 9.5 Square Groove .094 2.4 335 26 370 157 60 28or Fillet



AWS WHB-2 90 II 0784265 0010924 1 IIGAS METAL ARC WELDING 141

SQUARE GROOVE JOINTS WELDED FROM BOTH SIDES

Tl -II-R Tl -If-RTr---t~ T~

R MIN = T, T = 3/16 MAX USQUARE GROOVE JOINTS WELDED FROMONE SIDE WITH BACKING

Tl --II-R_RTc=lO

oTI2 MAX qQlI. 16 MAX

600?'. aMIN

~Ol1/16 TO 1/8

10 0 TO 15°

JOINT RECOMMENDED FORHORIZONTALPOSITION

DOUBE V-GROOVE JOINTS WELDED FROM BOTH SIDES

60°

\-M 1N7oarMAX

....1~1/16MAX

'\450

/' ,\45°." ,;,OU~ QIil 45' ~(jIf

R~!:HL 5° TO 10° 118 TO 1143/16 MIN.:.J

R = 118 MIN JOINT RECOMMENDED FORHORIZONTALPOSITION

SINGLE V-GROOVE JOINTS WELDED FROMONE SIDE WITH BACKING

FLARE V-GROOVEJOINTS WELDEDFROMONESIDE

T":1.- -II- R

Tc:=::J~

UP TO 118118 TO 114

-H-R -.11.. -II- Rc:::::J~:J L~0R MAX = T, T = 1/16 MAX

SQUARE GROOVE JOINTS WELDEDFROM ONE SIDE

60° 600 )-"~1-1I8 MAX

'\MIN/lIB MAX\'MIN:1 60~ -, 118 to 114

DQ!J"OIil MIN~lR-JI- Tf jkW ~00TJ15O

1116 MAX R H-I3/16 MIN~ t- JOINT RECOMMENDED FOR

R =1116 MAX HORIZONTALPOSITION

SINGLE V·GROOVE JOINTS WELDED FROMONE OR BOTH SIDES

450 45° If "..........-t- 45° 45°

MINt'Y MINI'c:j] r: R ~ I MIN rM~

DQ!t:::::=£~D~~R-ll- If Lf I Lf IR-.jl- R+jl-

R = 1/8 MAX, f = 1/16 MAX 118 MIN ~

SINGLE BEVEL-GROOVE JOINTS WELDEDFROM ONE OR BOTH SIDES

(AIALL DIMENSIONS IN INCHES EXCEPT ANGLES

Figure 4.32-Typical Weld Joint Designs and Dimensions for the Gas Metal Arc Welding Process


AWS WHB-2 90 II 0784265 0010925 3 II


5° TO 10°JOINT RECOMMENDEDFORHORIZONTAL POSITION

mf

45~MIN~ri-11-? ~R

,\5pr ,~5°145~nlQ~::Lt~ MIN~~1 L !.--? L RR--l1- f II- 5° TO 100 1-1/16 TO

R = 3/32 MAX R-!I- 3/16f = 1/16 TO 3/16 JOINT RECOMMENDEDFORr = 1/4 HORIZONTAL POSITION

SINGLE U·GROOVE JOINTS WELDED FROMONE OR BOTH SIDES

R = 3/32 MAXf = 1/16 TO 3/16r = 1/4

DOUBLEU-GROOVEJOINTS WELDED FROMBOTH SIDES

ts50 ...t.. ~ frt" _ fP"'t ~of r 7 35°~---ltiro'16 7ld 11 ItT I T If -JI-R R = 3/32 MAX R...jl- +Jr-R

f ~ 1/16 TO 3/16r ~ 1/2

SINGLE J·GROOVE JOINTS WELDED FROMONE OR BOTH SIDES

POSITIONS

ALLALL

POSITIONS

ALLALL

R

1/8 MIN1/4 MIN

45° min35° MIN

ANGLE a

SINGLE BEVEL·GROOVE JOINTS WELDED FROMONE SIDE WITH BACKING

DOUBLE BEVEL·GROOVE JOINTS WELDED FROMBOTH SIDES

1 °A A~ I

S6?~ANGLE a R I- R

45° MIN 1/4 MIN35° MIN 3/8 MIN

~ rihf

1~

~g~~nf = 1/16 MAX, R = 1/8 MAX JI-R

(6) ALL DIMENSIONS IN INCHES EXCEPT ANGLES

Figure 4.32-Typical Weld Joint Designs and Dimensions for the Gas Metal Arc Welding Process

SPECIAL APPLICATIONS

SPOT WELDINGGAS METAL ARC spot welding is a variation of continuousGMAW wherein two pieces of sheet metal are fused togetherby penetrating entirely through one piece into the other. Theprocess has been used for joining light-gage materials, up toapproximately 3/16 in. (5 rnm) thick, in the production ofautomobile bodies, appliances,and electrical enclosures. Nojoint preparation is required other than cleaning of the overlap areas. Heavier sections can also be spot welded with thistechnique 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 aresistance spot weld is shown in Figure 4.33. Resistancespot welds are made through resistance heating and electrode pressure which melts the two components at theirinterface and fuses them together. In the gas metal arc spotweld, the arc penetrates through the top member and fusesthe bottom component into its weld puddle. One big advantage of the gas metal arc spot weld is that access to onlyone side of the joint is necessary.

The spot weld variation does require some modifications to conventional GMAW equipment. Special nozzlesare used which have ports to allow the shield gas to escapeas the torch is pressed to the work. Timers and wire feedspeed controls are also necessary, to provide regulation ofthe actual welding time and a current decay period to fillthe weld crater, leaving a desirable reinforcement contour.

Joint DesignGAS METAL ARC spot welding may be used to weld lapjoints in carbon steel, aluminum, magnesium, stainlesssteel, and copper-bearing alloys. Metals of the same or different thicknesses may be welded together, but the thinnersheet should always be the top member when differentthickness are welded. Gas metal arc spot welding is normally restricted to the flat position. By modifying the nozzle design, it may be adapted to spotweld lap-fillet, fillet,and corner joints in the horizontal position.

Equipment OperationTHE SPOT WELDING GMAW gun is placed in position,pressing the workpieces together. The gun's trigger is de-


AWS WHB-2 90 .. 0784265 0010926 5 ..

SOLIDIFIED

IELD NUGG""r----L+-----,~ ~

:LLL ~ ~RESISTANCE GMASPOT WELD SPOT WELD

Figure 4.33-Comparison of Gas Metal Arc andResistance Spot Welds

pressed to initiate the arc. The arc timer is started by adevice that senses flow of welding current. The arc is maintained by the continuously-fed consumable electrode untilit melts through the top sheet and fuses into the bottomsheet without gun travel. The time cycle is set to maintainan arc until the melt-through and fusing sequence is complete, i.e., until a spot weld has been formed. The electrodecontinues to feed during the arc cycle and should producea reinforcement on the upper surface of the top sheet.

Effect of Process Variables on WeldCharacteristicsTHE WELD DIAMETER at the interface and the reinforcement are the two characteristics of a GMAW spot weldwhich determine whether the weld will satisfy the intended service. Three major process variables - weld current, voltage, and arc time - affect one or both of thesecharacteristics.

Current. Current has the greatest effect on penetration.Penetration is increased by using higher currents (with corresponding increase in wire feed speed). Increased penetration will generally result in a larger weld diameter at theinterface.

Arc Voltage. Arc voltage has the greatest effect on thespot 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. Weldsmade with arc voltages that are too low show a depressionin the center of the reinforcement. Arc voltages that aretoo high create heavy spatter conditions.

Weld Time. Welding conditions should be selectedthat produce a suitable weld within a time of 20 to 100cycles of 60 Hz current (0.3 to 1.7 seconds) to join basemetal up to 0.125 in. (3.2 mm) thick. Arc time up to 300



cycles (5 seconds) may be necessary on thicker materials toachieve adequate strength. The penetration, weld diameter, and reinforcement height generally increase with increased weld time.

As with conventional GMAW, the parameters for spotwelding are very interdependent. Changing one usually requires changing one or more of the others. Some trial anderror is needed to find a set or sets of conditions for aparticular application. "Starting" parameters for gas metalarc spot welding of carbon steel are shown in Table 4.14.

NARROW GROOVE WELDINGNARROW GROOVE WELDING is a multipass technique forjoining heavy section materials where the weld joint has anearly square butt configuration with a minimal groovewidth [approximately 1/2 in. (13 mm)]. A typical narrowgroove joint configuration is shown in Figure 4.34. Thetechnique is used with many of the conventional weldingprocesses, including GMAW,and is an efficient method.ofjoining heavy section carbon and low alloy steels, wahminimal distortion.

Using GMAW to weld joints in the narrow groove configuration requires special precautions to assure that thetip of the electrode is positioned accurately for proper fusion into the sidewalls. Numerous wire feeding methodsfor accomplishing this have been devised and successfullyused in production environment. Examples of some ofthese are shown in Figure 4.35.

Two wires with controlled cast and two contact tubesare used in tandem, as shown in Figure 4.35(A). The arcsare directed toward each sidewall, producing a series ofoverlapping fillet welds.

The same effect can be achieved with one wire by meansof a weaving technique, which involves oscillating the arcacross the groove in the course of welding. This oscillationcan be created mechanically by moving the contact tubeacross the groove (Figure4.35(B),but, because of the smallcontact tube-to-sidewall distance, this technique is notpractical and is seldom used.

Another mechanical technique uses a contact tube bentto an angle of about 15 degrees - Figure 4.35(C). Alongwith a forward motion during welding, the contact tubetwists to the right and left, which gives the arc a weavingmotion.

A more sophisticated technique is illustrated in Figure4.34(0). During feeding, this electrode is formed into awaved shape by the bending action of a "flapper plate" andfeed rollers as they rotate. The wire is continuously deformed plastically into this waved shape, as the feed rollerspress it against the bending plate. The electrode is almoststraightened 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. Thistechnique produces an oscillating arc even in a very narrowgroove, with the contact tube remaining centered in thejoint.

AWS WHB-2 90 II 0784265 0010927 7 II


The twist electrode technique, Figure 4.35(E), is anothermeans that has been developed to improve sidewall penetration without moving the contact tube. The twist electrode consists of two intertwined wires which, when fedinto the groove, generate arcs from the tips of the twowires. Due to the twist, the arcs describe a continuous rotational movement which increases penetration into thesidewall without any special weaving device.

Because these arc oscillation techniques often requirespecial feeding equipment, an alternate method has beendeveloped in which a larger diameter electrode [e.g..093 to.125 in. (2.4 to 3.2 mm)] is fed directly into the center ofthe 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 contacttube and the work, and standard welding equipment canbe used. It does, however, have a more limited thicknesspotential 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 ofsome typical values is shown in Table 4.15. For the narrowgroove application, however, the quality of the results issensitive to slight changes in these parameters, voltage being particularly important. An excessive arc voltage (arclength) 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 anddamage the contact tube. For this reason, pulsing powersupplies have become widely used in this application. Theycan maintain a stable spray arc at low arc voltages.

Various shielding gases have been used with the narrowgap technique, as with conventional GMAW. A gas con-

sisting of argon with 20 to 25 percent C02 has seen thewidest application because it provides a good combinationof arc characteristics, bead profile, and sidewall penetration. Delivering the shielding gas to the weld area is a challenge in the narrow groove configuration, and numerousnozzle designs have been developed.

-l r- 3/ 8 TO 5/8 in.

1 TO12 in.

Figure 4.34-Typical Joint Configuration forNarrow Gap Welding

Table 4.14Variable Settings for GMAW Spot Welding of Carbon Steel in the Flat Position (C02 Shielding Gas

- 1/4 in. (6.4 mm) Diameter Nugget)

Electrode Size ThicknessArc Spot

Current Voltage*Timein. mm Gauge in. mm s A V

0.030 0.8 24 0.022 0.56 1 90 2422 0.032 0.81 1.2 120 2720 0.037 0.94 1.2 120 27

0.035 0.9 18 0.039 0.99 1 190 2716 0.059 1.50 2 190 2814 0.072 1.83 5 190 28

0.045 1.2 14 0.072 1.83 1.5 300 3012 0.110 2.79 3.5 300 3011 0.124 3.15 4.2 300 30

0.063 1.6 11 1/8 3.15 1 490 325/32 4.0 1.5 490 32



AWS WHB-2 90 II 0784265 0010928 9 IIGAS MET A L ARC W E L DIN G 145

(A) (B) (C) (0) (E)

Figure 4-35-Typical Wire Feeding Techniques for Narrow Gap Gas Metal Arc Welding

Table 4.15Typical Welding Conditions for the GMAW-NG Process

Travel Speed,Technique, Groove

Weld Width, Current Voltage GasPosition in. mm Amp Volts' in·/min. mm/s Shield

NGW-I 0.375 9.5 260-270 25-26 40 17 Ar-C02horiz.

NGW-I 0.4-0.5 10-12 220-240 24-282 13 6 Ar-C02horiz.

NGW·I 0.375 9.5 280-300 292 9 4 Ar-C02flat

NGW-II 0.5 12.5 450 30-37.5 15 6 Ar-C02flat

NGW-II 0.47-0.55 12-14 450-550 38-42 20 8 Ar-C02flat

1. Direct current electrode positive.2. Pulse power at 120 pulses per second.


AWS WHB-2 90 II 0784265 0010929 0 II


INSPECTION AND WELD QUALITY

INTRODUCTIONWELD QUALITY CONTROL procedures for GMAW jointsare quite similar to those used for other processes. Depending upon the applicable specifications, inspectionprocedures should provide for determining the adequacyof 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 tonondestructive examination methods such as visual, liquidpenetrant, magnetic particle, radiographic, and ultrasonicinspection. Destructive testing (tensile, shear, fatigue, impact, bend, fracture, peel, cross section, or hardness tests)is usually confined to engineering development, weldingprocedure qualification, and welder and welding operatorperformance qualification tests.

weld metal deposited by gas tungsten arc welding. Itshould be noted here, however, that oxygen in percentagesof up to 5 percent and more can be added to the shieldinggas without adversely affecting weld quality.

CleanlinessBASE METAL CLEANLINESS when using GMAW is more critical than with SMAW or submerged arc welding (SAW).The fluxing compounds present in SMAW and SAWscavenge and cleanse the molten weld deposit of oxides andgas-forming compounds. Such fluxing slagsare not presentin GMAW. This places a premium on doing a thorough jobof preweld and interpass cleaning. This is particularly truefor aluminum, where elaborate procedures for chemicalcleaning or mechanical removal of metallic oxides, orboth, are applied.

WELD DISCONTINUITIESSOME OF THE more common weld discontinuities that mayoccur with the GMAW process are listed in the followingparagraphs.

UndercuttingTHE FOLLOWING ARE possible causes of undercutting andtheir corrective actions (see Figure 4.36):

Reduce wire feed speed.

Use slower travel speed.Reduce the voltage.

Corrective Actions

Increase dwell at edge ofmolten weld puddle.Change gun angle so arcforce can aid in metalplacement.

Possible Causes

(1) Travel speed too high(2) Welding voltage too

high(3) Excessive welding

current(4) Insufficient dwell

(5) Gun angle

Incomplete FusionTHE REDUCED HEAT input common to the short circuitingmode of GMAW results in low penetration into the basemetal. This is desirable on thin gauge materials and for outof-position welding. However, an improper welding technique may result in incomplete fusion, especially in rootareas or along groove faces.

POTENTIAL PROBLEMS

Hydrogen EmbrittlementAN AWARENESS OF the potential problems of hydrogen embrittlement is important, even though it is less likely tooccur with GMAW,since no hygroscopic flux or coating isused. However, other hydrogen sources must be considered. For example, shielding gas must be sufficiently low inmoisture content. This should be well controlled by thegas supplier, but may need to be checked. Oil, grease, anddrawing compounds on the electrode or the base metalmay become potential sources for hydrogen pick-up in theweld metal. Electrode manufacturers are aware of the needfor cleanliness and normally take special care to provide aclean electrode. Contaminants may be introduced duringhandling in the user's facility. Users who are aware of suchpossibilities take steps to avoid serious problems, particularly in welding hardenable steels. The same awareness isnecessary in welding aluminum, except that the potentialproblem is porosity caused by the relatively low solubilityof hydrogen in solidified aluminum, rather than hydrogenembrittlement.

Oxygen and Nitrogen ContaminationOXYGEN AND NITROGEN are potentially greater problemsthan hydrogen in the GMAW process. If the shielding gasis not completely inert or adequately protective, these elements may be readily absorbed from the atmosphere. Bothoxides and nitrides can reduce weld metal notch toughness. Weld metal deposited by GMAW is not as tough as


AWS WH8-2 90 II 0784265 0010930 7 II


Figure 4.36-Undercutting at the Toe of theWeld

PorosityTHEFOLLOWING ARE possible causes of porosity and theircorrective actions:

Figure 4.37-Porosity Resulting from InadequateShielding Gas Coverage.

Incomplete FusionTHEFOLLOWING ARE possible causes of incomplete fusionand their corrective actions:

Possible Causes

(1) Inadequate shielding gascoverage (see Figure4.37)

(2) Gas contamination

(3) Electrodecontamination

(4) Workpiececontamination

(5) Arc voltage too high(6) Excess contact tube-to

work distance


Corrective Actions

Optimize the gas flow.Increase gas flow to displace all air from theweld zone. Decrease excessivegas flow to avoidturbulence and the entrapment of air in theweld zone. Eliminateany leaks in the gas line.Eliminate drafts (fromfans, open doors, etc.)blowing into the welding arc. Eliminate frozen(clogged) regulators inC02 welding by usingheaters. Reduce travelspeed. Reduce nozzleto-work distance. Holdgun at end of weld untilmolten metal solidifies.Use welding gradeshielding gas.Use only clean and dryelectrode.Remove all grease, oil,moisture, rust, paint,and dirt from work surface before welding. Usemore highly deoxidizingelectrode.Reduce voltage.Reduce stick-out.

Possible Causes

(1) Weld zone surfaces notfree of film or excessiveoxides

(2) Insufficient heat input

(3) Too large a weld puddle

(4) Improper weldtechnique

(5) Improper joint design(see Figure 4.38)

Corrective Actions

Clean all groove facesand weld zone surfacesof any mill scale impurities prior to welding.Increase the wire feedspeed and the arc voltage. Reduce electrodeextension.Minimize excessiveweaving to produce amore controllable weldpuddle. Increase thetravel speed.When using a weavingtechnique, dwell momentarily on the sidewalls of the groove. Provide improved access atroot of joints. Keepelectrode directed at theleading edge of thepuddle.Use angle groove largeenough to allow accessto bottom of the groove

AWS WHB-2 90 .. 0784265 0010931 9 ..

Incomplete Joint PenetrationTHE FOLLOWING ARE possible causes of incomplete jointpenetration and their corrective actions:

Figure 4.39-lncomplete Penetration

Excessive Melt-ThroughTHE FOLLOWING ARE possible causes of excessive meltthrough and their corrective actions:

Corrective Actions

Joint design must provide proper access to thebottom of the groovewhile maintainingproper electrode extension. Reduce excessivelylarge root face. Increasethe root gap in buttjoints, and increasedepth of back gouge.Maintain electrode angle normal to work surface to achieve maximum penetration. Keeparc on leading edge ofthe puddle.Increase the wire feedspeed (welding current).

(6) Excessive travel speed

Possible Causes

(1) Improper jointpreparation

(2) Improper weldtechnique

(3) Inadequate welding current (see Figure 4.39)


and sidewalls withproper electrode extension, or use a "J" or "U"groove.Reduce travel speed.

Weld Metal CracksTHE FOLLOWING ARE all possible causes of weld metalcracks and their corrective actions:

Figure 4.38-lncomplete Fusion Due to NarrowGroove Preparation

Possible Causes

(1) Excessive heat input

(2) Improper jointpenetration

Possible Causes

(1) Improper joint design

Corrective Actions

Reduce wire feed speed(welding current) andthe voltage. Increase thetravel speed.Reduce root opening.Increase root facedimension.

Corrective Actions

Maintain proper groovedimensions to allow deposition of adequatefiller metal or weld crosssection to overcome restraint conditions.


AWS WHB-2 90 II 0784265 0010932 0 II


Heat-Affected Zone CracksCRACKING INTHE heat-affected zone is almost alwaysassociated with hardenable steels.

(2) Too high a weld depthto-width ratio (see Figure 4.40)

(3) Too small a weld bead(particularly fillet androot beads)

(4) Heat input too high,causing excessiveshrinkage and distortion

(5) Hot-shortness

(6) High restraint of thejoint members

Either increase arc voltage or decrease the current or both to widenthe weld bead or decrease the penetration.Decrease travel speed toincrease cross section ofdeposit.Reduce either current orvoltage, or both. Increase travel speed.Use electrode withhigher manganese content (use shorter arclength to minimize lossof manganese across thearc). Adjust the grooveangle to allow adequatepercentage of fillermetal addition. Adjustpass sequence to reducerestraint on weld duringcooling. Change to another filler metal providing desired characteristics.Use preheat to reducemagnitude of residualstresses. Adjust weldingsequence to reduce restraint conditions.

(7) Rapid cooling in the crater at the end of the joint(see Figure 4.41)

Possible Causes

(1) Hardening in the heataffected zone

(2) Residual stresses toohigh

(3) Hydrogenembrittlement

Eliminate craters bybackstepping technique.

Corrective Actions

Preheat to retard cooling rate.Use stress relief heattreatment.Use clean electrode anddry shielding gas. Remove contaminantsfrom the base metal.Hold weld at elevatedtemperatures for severalhours before cooling(temperature and timerequired to diffuse hydrogen are dependenton base metal type).

Figure 4.40-Too High a Weld Depth-to-WidthRatio


Figure 4.41-Weld Metal Cracking in Crater atthe End of a Weld

AWS WHB-2 90 .. 0784265 0010933 2 ..


TROUBLESHOOTINGTROUBLE SHOOTING OF any process requires a thoroughknowledge of the equipment and the function of the various components, the materials involved, and the processitself. It is a more complicated task with gas metal arc thanwith manual processes such as SMAWand GTAWbecauseof 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 thefollowing three categories: electrical, mechanical, andprocess.

Tables 4.16 through 4.18 indicate some of the problemsthat are likely to be encountered, what the causes mightbe, and possible remedies. These are problems that occurduring the welding operation or prevent the making of theweld as opposed to those that are discovered as a result ofinspecting the final product. This latter type are covered inthe "Inspection and Weld Quality" section of this chapter.

Table 4.16Troubleshooting Electrical Problems Encountered in Gas Metal Arc Welding

Problem

Difficult arc starting

Irregular wire feedand burnback

Welding cables overheating

No wire feed speed control

Unstable arcElectrode won't feed

Wire feeds but no gas flows

Electrode wire feeds but is not energized (noarc)

Porosity in weld


Possible Cause

Wrong polarityPoor work lead connectionPower circuit fluctuationsPolarity wrongCables are too small ortoo long

Cable connections looseBroken or loose wires in control circuitBad P. C. board in governorCable connections are looseControl circuit fuse blownFuse blown in power sourceDefective gun trigger switch orbroken wire

leadsDrive motor burned outFailure of gas valve solenoidLOOSIl or broken wires to gas valve solenoidPoor workpiece connection

Loose cable connectionsPrimary contactor coil or points defectiveContactor control leads brokenLoose orbroken wires to gas solenoid valve

Remedy

Check polarity; reverse leads if necessary.Secure work lead connectionCheck line voltageCheck polarity; reverse leads ifnecessaryCheck current carrying requirements - replace

or shorten if necessaryTightenCheck and repair ifnecessaryReplace P.C. boardTighten connectionsReplace fuseReplace fuseCheck connections; replace switch

Check and replaceReplaceCheck and repair if necessaryTighten if loose; clean work of paint, rust,

etc.TightenRepair orreplaceRepair or replaceRepair or replace

AWS WH8-~ 90 I! P?84265 0010934 4 ..

GAS MET A L ARC WE L DIN G 151

Table 4.17Troubleshooting Mechanical Problems Encountered in Gas Metal Arc Welding

Problem Possible Cause Remedy

Leaks in gas supply lines (including the gun)

Restricted shield gas flow

Excess or insufficient drive roll pressureWire drive rolls misaligned orwornLiner or contact tube plugged

Gas cylinder is emptyGas cylinder valve closedFlow meter not adjustedRestriction in gas line or nozzle

Failed gas valve solenoidGas cylinder valve closedInsufficient shielding gas flow

Porosity in the weld bead

Electrode wire stops feeding while welding

Heavily oxidized weld deposit

Electrode wire wrapsaround drive roll (obirdnestingo)

Wire feeds but no gas flows

Irregular wire feed and burnback Insufficient drive roll pressure AdjustContact tube plugged or worn Clean or replaceKinked electrode wire Cut out, replace spoolCoiled gun cable Straighten cables, hang the wire feederConduit liner dirty orworn Clean or replaceConduit too long Shorten or use push-pull drive system

Excessive feed roll pressure AdjustIncorrect conduit liner or contact tip Match liner and contact tip to electrode sizeMisaligned drive rolls orwire guides Check and align properlyRestriction in gun or gun cable Remove restriction

Air/water leaks in gun and cables Check for leaks and repair orreplace asnecessary

Check and clean nozzle

AdjustRealign and/or replaceClean or replace

Replace and purge lines before weldingOpen cylinder valveAdjust to give flow specified in the procedureCheck and clean

Repair or replaceTurn valve onCheck for restrictions in gas line or nozzle

and correctCheck for leaks (especially at connections]

and correct

Wire feed motor operatesbut wire does not feed

Welding gun overheats

Insufficient drive roll pressureIncorrect wire feed rollsExcessive pressure on wire spool brakeRestriction in the conduit

liner or gunIncorrect liner orcontact tube

Pinched or clogged coolant lineLow coolant level in pump reservoirWater pump not functioning correctly

AdjustMatch feed rolls to wire size and typeDecrease brake pressureCheck liner and contact tip.Clean and/or replaceCheck and replace with correct size

Check and correctCheck and add coolant as necessaryCheck and repair orreplace


AWS WHB-2 90 II 0784265 0010935 6 II


Table 4.18Troubleshooting Process Problems Encountered in Gas Metal Arc Welding

Problem Possible Cause Remedy

Clean to remove scale, rust, etc.Reduce.Replace gas cylinderKeep wire protected while using. Clean wire

before it enters feeder.

Reduce speedIncrease voltageReset to reduce slope

Reduce voltageIncrease slope setting

Adjust or replace with longer oneReduce flow

Reduce amperage or change to highercapacity gun

Weld joint area dirty Clean to remove scale, rust, etc.

Improper gun angle Use approximately 150 lead ortrail angleExcessive nozzle to work distance distance Reduce. Should be approximately 1/2 to

3/4"Protect weld area from draftsCenter contact tube

Air draftsContact tube not centered in the gas nozzle

distance

Dirty base materialExcessive wire feed speedMoisture in the shielding gasContaminated electrode

Excessive wire feed speedArc voltage too lowExcessive slope set on power source (for

short circuiting transfer)

Excessive arc voltageInsufficient slope set on power source (for

short circuiting transfer)Contact tube recessed too far in nozzleExcessive gas flow rate

Excessive amperage for gun

Unstable arc

Heavily oxidized weld deposit

Electrode wire stubs intothe workpiece

Excessive spatter

Porosity in the weld bead

Welding gun overheats

SAFE PRACTICES

INTRODUCTIONSAFETY IN WELDING, cutting, and allied processes is covered in ANSI Z49.1, Safety in Welding and Cutting,l andANSI Z49.2, Fire Prevention in the Use of Welding andCutting Prccesees.: and Chapter 16 Volume 1 of the Welding Handbook, 8th Edition.

Personnel should be familiar with the safe practices discussed in these documents.

In addition, there are other potential hazard areas in arcwelding and cutting (including fumes, gases, radiant energy, noise, handling of cylinders and regulators, and electric shock) that warrant consideration. Those areas whichmay be associated with the GMAW process are briefly discussed in this chapter.

1. ANSI Z49.1 is available from the American Welding Society, 550N.W, Lejeune Road, Miami, Florida 33135.2. ANSI Z49.2 isavailable from theAmerican National Standards Institute,1430 Broadway, NewYork, NY 10018.

SAFE HANDLING OF GAS CYLINDERS ANDREGULATORSCOMPRESSED GAS CYLINDERS should be handled carefullyand should be adequately secured when stored or in use.Knocks, falls, or rough handling may damage cylinders,valves, and fuse plugs, and cause leakage or an accide~t.

Valve protecting caps, when supplied, should be kept In

place (hand tight) unless a regulator is attached to the cylinder. (See CGA Pamphlet P-1, Safe Handling of Compressed Gas Cylinders)3.

The following should be observed when setting up andusing 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 andclosed immediately ("cracking") to clear the valve of dust

3. CGA P-l isavailable from theCompressed Gas Association, Inc.,500Fifth Avenue, NewYork, NY 10036.


AWS WHB-2 90 II 0784265 0010936 8 II

or dirt that otherwise might enter the regulator. The valveoperator 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-clockwise. The cylinder valve should then be opened slowly toprevent a rapid surge of high-pressure gas into the regulator. The adjusting screw should then be turned clockwiseuntil 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 theadjusting screw should be backed off.

GASESTHE MAJOR TOXIC gases associated with GMAW weldingare ozone, nitrogen dioxide, and carbon monoxide. Phosgene gas could also be present as a result of thermal orultraviolet decomposition of chlorinated hydrocarboncleaning agents located in the vicinity of welding operations. Two such solvents are trichlorethylene and perchlorethylene. Degreasing or other cleaning operations involving chlorinated hydrocarbons should be located sothat vapors from these operations cannot be reached byradiation from the welding arc.

OzoneTHE ULTRAVIOLET LIGHT emitted by the GMAW arc actson the oxygen in the surrounding atmosphere to produceozone, 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 weldingfumes, and other factors. The ozone concentration willgenerally increase with an increase in welding current, withthe use of argon as the shielding gas, and when weldinghighly reflective metals. If the ozone cannot be reduced toa safe level by ventilation or process variations, it will benecessary to supply fresh air to the welder either with an airsupplied respirator or by other means.

Nitrogen DioxideSOME TEST RESULTS show that high concentrations of nitrogen dioxide are found only within 6 in. (150 mm) of thearc. With normal natural ventilation, these concentrationsare quickly reduced to safe levels in the welder's breathingzone, so long as the welder's head is kept out of the plumeof fumes (and thus out of the plume of welding-generatedgases). Nitrogen dioxide is not thought to be a hazard inGMAW.

Carbon MonoxideCARBON DIOXIDE SHIELDING used with the GMAW processwill be dissociated by the heat of the arc to form carbonmonoxide. Only a small amount of carbon monoxide iscreated by the welding process, although relatively high

f


GAS MET A L ARC W E L DIN G 153

concentrations are formed temporarily in the plume offumes. However, the hot carbon monoxide oxidizes tocarbon dioxide so that the concentrations of carbon monoxide become insignificant at distances of more than 3 or4 in. (75 or 100 mm) from the welding plume.

Under normal welding conditions, there should be nohazard from this source. When welders must work overthe welding are, or with natural ventilation moving theplume of fumes towards their breathing zone, or wherewelding is performed in a confined space, ventilation adequate to deflect the plume or remove the fumes and gasesshould be provided (see ANSI Z49 .1, Safety in \Velding andCutting).

METAL FUMESTHE WELDING FUMES generated by GMAW can be controlled by general ventilation, local exhaust ventilation, orby respiratory protective equipment as described in ANSIZ49.1. The method of ventilation required to keep thelevel of toxic substances within the welder's breathingzone below threshold concentrations is directly dependentupon a number of factors. Among these are the materialbeing welded, the size of the work area, and the degree ofconfinement or obstruction to normal air movementwhere the welding is being done. Each operation should beevaluated on an individual basis in order to determinewhat will be required.

Acceptable exposure levels to substances associatedwith welding, and designated as time-weighted averagethreshold limit values (TLV) and ceiling values, have beenestablished by the American Conference of GovernmentalIndustrial Hygienists (ACGIH) and by the OccupationalSafety and Health Administration (OSHA). Compliancewith these acceptable levels of exposure can be checked bysampling the atmosphere under the welder's helmet or inthe immediate vicinity of the welder's breathing zone.Sampling should be in accordance with ANSI!AWS F1.1,Method for Sampling Airborne Particulates Generated byWelding and Allied Processes.

RADIANT ENERGYTHETOTAL RADIANT energy produced by the GMAW process can be higher than that produced by the SMAW process, because of its higher arc energy, significantly lowerwelding fume and the more exposed arc. Generally, thehighest ultraviolet radiant energy intensities are producedwhen using an argon shielding gas and when welding onaluminum.

The suggested filter glass shades for GMAW, as presented in ANSI Z49.1 as a guide, are shown in Table 4.19.To select the best shade for an application, first select avery dark shade. If it is difficult to see the operation properly, select successively lighter shades until the operation is

AWS WHB-2 90 II 0784265 0010937 T II


Table 4.19Suggested Filter Glass Shades for GMAW

Welding Current, A

Under 6060-160

160-250250-500

Lowest Shade ComfortNumber Shade No.

7 910 1110 1210 14

NOISE-HEARING PROTECTIONPERSONNEL SHOULD BE protected against exposure tonoise generated in welding and cutting processes in accordance with paragraph 1910.95 "Occupational NoiseExposure" of the Occupational Safety and Health Administration, U.S. Department of Labor.

ELECTRIC SHOCK

sufficiently visible for good control. However, do not gobelow the lowest recommended number, where given.

Dark leather or wool clothing (to reduce reflectionwhich could cause ultraviolet burns to the face and neckunderneath the helmet) is recommended for GMAW. Thegreater intensity of the ultraviolet radiation can causerapid disintegration of cotton clothing.

LINE VOLTAGES TO power supplies and auxilliary equipment used in GMAW range from 110 to 575 volts. Weldersand service personnel should exercise caution not to comein contact with these voltages. See precautions listed inANSI Z49.1, Safety in Welding and Cutting.

SUPPLEMENTARY READING LISTAldenhoff, B. J., Stearns, J. B., and Ramsey, P. W. "Con

stant potential power sources for multiple operation gasmetal arc welding." Welding[ournalS3(7):425-429; July1974.

Althouse, A. D., Turnquist, C. H. Bowditch, W. A. andBowditch, K. E. Modern welding. South Holland, ILL;The Goodheart - Willcox Company, Inc., 1984.

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