Arc Physics of Gas Tungsten andGas Metal Arc WeldingY.M. Zhang, University of Kentucky

THIS ARTICLE provides basic physicsneeded to understand two most widely usedarc welding processes: gas tungsten arc welding(GTAW) and gas metal arc welding (GMAW).The GTAW part is an update from the article“Arc Physics of Gas-Tungsten Arc Welding”authored by J.F. Key (of EG&G Idaho, Inc.)and published in Welding, Brazing, and Solder-ing, Volume 6 of ASM Handbook, 1993. Treat-ments for general arcs, arcs in other specificarc welding processes, and arc physics ingreater depths may be found in the SelectedReferences listed at the end of this article andfrom other literature. A general descriptionof arc welding processes is also given in thearticle “Overview of Welding Processes” in thisVolume.

Gas Tungsten Arc Physics

The GTAW process is performed using awelding arc between a nonconsumable tung-sten-base electrode and the workpieces to bejoined. C.E. Jackson defined a welding arc as“a sustained electrical discharge through ahigh-temperature conducting plasma producingsufficient thermal energy so as to be useful forthe joining of metals by fusion.” This definitionis a good foundation for the discussion thatfollows.The physics of GTAW are fundamental to all

arc processes and are more straightforward,because the complications of materials (forexample, filler and flux) transferred throughand interacting with the arc can be avoided.Geometrically, the arc discharge in GTAW isbetween a rod-shaped tungsten electrode and aplanar-shaped electrode, that is, the workpiece.Pure tungsten electrodes are less expensive

and, possibly, more environmentally compati-ble than those with rare earth or other oxideadditions. They are used for less critical welds,where tungsten contamination that could becaused by the molten electrode surface can betolerated. They are also used for alternatingcurrent (ac) welding of aluminum, copper,

magnesium, and thin sections of low-alloy andstainless steels.Analysis of the arc discharge is separated

into electrode regions and the arc column. Theelectrode regions are confined to very small dis-tances from the electrode surfaces, have veryhigh electrical and thermal fields, and havemuch higher current density, because of thecontraction of the arc to a small spot. As aresult, electrode regions for both the cathodeand the anode are difficult to analyze by diag-nostic measurements and theoretical computa-tion. This situation must be remedied for athorough understanding of the process, becausethe process parameters control the arc dischargeat the cathode, with the anode serving as theconnection to ground. The arc column, on theother hand, is relatively easy to analyze but isimportant primarily as a means to deduce arccharacteristics at the electrodes.Polarity. The GTAW process generally uses

a direct current (dc) arc, where the tungstenelectrode has a negative polarity. The tungstenelectrode thus becomes the cathode and theworkpiece becomes the anode. The polarity iscalled straight polarity, or direct current elec-trode negative (DCEN).Reverse polarity, or direct current electrode

positive (DCEP), is literally the reverse ofDCEN. The workpiece is the cathode and thetungsten electrode serves as the anode. Becausemost heat is generated at the anode in theGTAW process, DCEP is used for welding cer-tain thin-section, low-melting-point materialswhen DCEN would be likely to cause excessivepenetration or burn-through.Either ac or DCEP is used for removing an

oxide film from the surface of the weld poolor workpiece. This is particularly important inobtaining a sound joint in metals with a tena-cious oxide, such as aluminum. The oxide filmpromotes emission during the half-cycle (ac)when the workpiece is negative polarity. Asthe oxide is depleted, the emission moves to anew location that has a high enough oxide con-tent to sustain the discharge of electrons. Thearc root or cathode spot where the emission

occurs is highly mobile in ac or DCEP and, asa result, the arc is much less stable than inDCEN.Gas Shielding. In all cases, the arc and both

electrodes are shielding by gas, usually an inertgas or a gas mixture. Argon and argon-heliummixtures are used most often, although argon-hydrogen mixtures are used for some applica-tions. The GTAW process may simply use thearc to fuse the workpieces together withoutthe addition of filler materials (autogenous), orfiller may be added to the molten pool to fillgrooves in thicker weldments. A reasonableunderstanding of welding arc fundamentalsand the GTAW process requires a more thor-ough discussion of the electrode regions of thearc and the arc column.Electrode Regions and Arc Column. The

cathode and anode are similar in severalrespects. Both exhibit a voltage drop causedby a space charge that covers a very thin regionover their surfaces, and the arc is significantlycontracted on the surfaces. Figure 1 shows thatthe total arc voltage is partitioned between theelectrode drops and arc column. The relativemagnitude of these drops depends on weldingparameters and electrode material.The arc discharge requires a flow of electrons

from the cathode through the arc column to theanode, regardless of polarity or whether ac or

Fig. 1 Plot of relative arc voltage distribution versusrelative arc length between electrodes

ASM Handbook, Volume 6A, Welding Fundamentals and ProcessesT. Lienert, T. Siewert, S. Babu, and V. Acoff, editors

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dc is used. Two cases of electron discharge atthe cathode are discussed: thermionic emissionand nonthermionic emission, also called coldcathode or field emission.Thermionic emission results from joule heat-

ing (resistance) of the cathode by the imposedwelding current until the electron energy atthe cathode tip exceeds the work function(energy required to strip off an electron). Thiscase applies to the general case of DCEN,where the tungsten electrode is the emitter, orcathode. Pure tungsten electrodes must beheated to their melting point to achieve therm-ionic emission. Once molten, the equilibriumtip shape becomes a hemisphere, and a stablearc results from uniform emission over this sur-face. Thoria (ThO2), zirconia (ZrO2), or ceria(CeO2) are added to pure tungsten inamounts up to 2.2 wt% ThO2, 0.4 wt% ZrO2,or 3.0 wt% CeO2 to lower the work function,which results in thermionic emission at lowertemperatures and avoids melting the cathodetip. These electrodes typically have a groundconical tip, and thermionic emission is loca-lized to a cathode spot. Thermionic emissioncreates a cloud of electrons, called a spacecharge, around the cathode. If a second elec-trode at a higher potential is nearby (the work-piece, in this case), then the electrons willflow to it, thus establishing the arc.Nonthermionic, or field, emission creates an

electron discharge with a very high electricfield, typically exceeding 109 V/m. This intenseelectric field literary pulls electrons out of a rel-atively cold or unheated cathode. This wouldnot appear to be applicable to welding untilone considers that for reverse polarity or DCEP,a condensation of positive ions from the arccolumn can build up in a very thin (1 nm, or0.04 min.) layer over the cathode surface, creat-ing a very high localized electric field eventhough the cathode voltage drop may onlyamount to several volts.An oxide layer, which is always present

on the cathode surface in an actual weld, facil-itates the discharge with a source of lowerwork-function electrons. When the oxide layeris very thin (on the order of one atom layer),emission occurs via a tunneling mechanismthrough the film to an emitting site. Thickeroxide films exhibit locally conducting spots atthe end of filamentary channels through theoxide. Large currents flow in these channels,which are on the order of 100 nm (4 min.) indiameter and have lifetimes of 0.001 to 1 ms.The cathode cleaning action, which is one of

the principal reasons to use DCEP or ac, resultsfrom stripping away the oxide film at the emit-ting sites by very small and intense jets of metalvapor and debris. It becomes obvious that apractical implication of the short lifetime ofthese cathode spots is a generally unstable arcthat is due to the necessity of continual move-ment of the cathode spot to undepleted regionsof oxide film. Arc instability is undesirableand DCEP or ac is only used when cathodiccleaning or the minimizing of heat input to the

workpiece is a higher priority than optimizingweld bead shape and location.Anode. Welding process parameters (for

example, current and voltage) control the arc dis-charge at the cathode. The electron flow entersthe anode through the anode spot and constitutes85% of the energy going into the weld pool, thusmaking current density the single most importantwelding parameter that determines pool shape.However, events at the anode can only be con-trolled indirectly by controlling the cathode.Anode spot stability does depend somewhat onshielding gas composition and the shape of theanode (that is, weld groove).Current density and heat input measurements

at the anode help to explain how process

parameters that are largely controlling eventsat the cathode will, in turn, influence the shapeand melting rate of the weld pool. The relativecontributions of heat transfer to the workpiece,in terms of the GTAW process, are shown inFig. 2.The Thomson effect represents the energy

lost by electrons as they move from higher tolower temperatures. The sum of work function,Thomson effect, and anode fall gives an elec-tron contribution to heat transfer of approxi-mately 84%. The remaining 16% is due tothermal effects (that is, conduction, convection,and radiation). There are small heat losses fromthe pool that are due to evaporation of metalions and radiation. Figure 3 shows that the

Fig. 2 Relative heat-transfer contributions to workpiece with gas tungsten arc welding. (a) Contribution of individualparameters to anode heat input. (b) Heat output at cathode (workpiece) relative to weld pool heat loss

Fig. 3 Plot of electron and thermal contributions to heat transfer. A, total arc power (standard deviation, s, of 0.8mm, or 0.031 in.); B, electron contribution (s = 0.7 mm, or 0.028 in.); C, thermal contribution. Weld

parameters: current, 10 A; voltage, 10 V; time, 10 s; shielding gas, argon; electrode angle, 30�

250 / Arc Welding Processes

energy distribution for this particular caseapproaches a Gaussian distribution (that is, nor-mal distribution curve). High helium contentsin the shielding gas have produced data thatare typically better fit by a Lorentzian curve,indicating a narrower current density distribu-tion (that is, a more-contracted arc at the anodespot).Arc efficiency, in addition to those variables

that have an effect on it, is an extremely impor-tant term in the heat-transfer analysis of weld-ing. It gives the percentage of heat dissipatedin the arc that actually is captured by the work-pieces and is available for melting. Arc effi-ciency, as a function of all GTAW weldingparameters and many materials, has been deter-mined experimentally and found to be nomi-nally 75%. The variables having the greatesteffect on arc efficiency are arc voltage andanode material. For those variables, the effectis usually no more than þ�5%. Other parametershave a negligible effect.Arc Column. The electron discharge

between the electrodes partially ionizes theshielding gas in its path, thus making the arccolumn a conductor, or plasma. Overall, thearc column is neutral and is composed of elec-trons, positive gas and possibly metal ions,and neutral gas atoms.Ironically, fundamental measurements of arc

properties are most easily made in the arc col-umn, although the actual effect of these proper-ties on the electrode region of an actual weldmust still be inferred. Nevertheless, it is usefulto understand fundamentals of the arc thatrelate essential welding variables (for example,current, voltage, electrode gap, choice ofshielding gas, and electrode shape) to arc tem-perature, current density distribution, and gasflow structure at the anode surface.Effect of Cathode Tip Shape. For the gen-

eral case of straight polarity, DCEN, the tip ofthe tungsten alloy cathode is ground to a pointand then truncated somewhat to prevent thesharp tip from burning off and contaminatingthe weld. The included angle of the cone andthe diameter of the truncation under some weld-ing conditions have a significant effect on weldpool shape. Figure 4 shows examples of theeffect of these two parameters on weld poolshape.For a stationary spot-on-plate weld shielded

by pure argon, the weld depth-to-width ratioincreased with an increasing vertex angle upto 90� and with an increasing truncation diame-ter. The arc became less “bell shaped” andmore “ball shaped” as the vertex angle or trun-cation diameter increased. These results shouldbe a valid indication of the effect of cathode tipshape for pulsed current welding, which pro-duces a series of overlapping spot welds.A study of bead-on-plate welds (Fig. 5) made

with constant current and velocity indicated asimilar but less pronounced trend. These condi-tions produce a tear-drop molten pool shapewhen viewed from above, compared to a circu-lar shape for spot or pulsed current welding.

Fluid and heat flow within the pool is lessuniform front-to-rear in a tear-drop-shaped pooland probably has a greater influence on poolshape than electrode tip shape.When the arc is used in a weld groove,

the relative shapes of the cathode tip andthe anode groove become more important. Thearc discharge from the cathode will seek apath to ground with the lowest electrical resis-tance. For a stable arc properly centered inthe groove (for example, a root pass), the short-est path to ground should be between the cath-ode tip and the bottom of the groove (Fig. 6).This will require the cathode vertex angle tobe somewhat less than the included angle ofthe groove and/or a sufficiently wide grooveto ensure that the shortest path to ground isfrom the cathode tip to the groove bottom andnot, for example, from the electrode shoulderto the groove wall, as the case would be witha 90� electrode in a 10� narrow groove. Weld-ing in a groove places a higher priority onarc stability and location than on maximumpenetration.To understand the effects of tip shape, tem-

perature distributions in the plasma weremeasured. Figure 7 shows that as the cathodevertex angle increases, the plasma radius ofthe arc column increases at midgap andbecomes more constricted near the anode. Thequantitative interpretation of these resultsrequires theoretical modeling, which has yet tobe completed.

Effect of Shielding Gas Composition. TheGTAW process typically uses either an inertgas, such as argon, or an inert gas mixture, suchas argon and helium, to shield the arc and theweld from atmospheric contamination. Occa-sionally, a slightly reactive gas mixture, suchas argon with up to 15 vol% H2, is used. (The15 vol% limit is based on safety considera-tions.) Shielding gas composition has a ratherstrong effect on arc temperature distributionand, under certain conditions, a significanteffect on weld pool shape.Figure 8 shows how shielding gas affects arc

voltage. The curves would all be displaceddownward for shorter arc lengths, but the rela-tive positions would be maintained. Figure 9shows the effects of both cathode vertex angleand shielding gas composition on weld poolshape for spot-on-plate welds. Increasing addi-tions of helium to argon show a remarkableincrease in penetration when using a 30� vertexangle. However, the effect is much less evidentwhen using a 90� vertex angle.To understand these phenomena, arc temper-

ature distributions for a variety of shieldinggases and mixtures, electrode shapes, current,arc voltages (electrode gaps), and anode materi-als have been measured in order to clarify weld-ing arc fundamentals. Welding arcs arecomposed of electrons, positive gas ions, andneutral gas atoms. Some measurement techni-ques give the temperature of one species (elec-trons), whereas other techniques give the

Fig. 4 Fusion zone profile for spot-on-plate welds as a function of electrode tip geometry using 100% Ar as ashielding gas. Weld parameters: current, 150 A; duration, 2 s

Fig. 5 Fusion zone profile for bead-on-plate welds as a function of electrode tip geometry using 100% Ar as ashielding gas. Weld parameters: current, 150 A; welding speed, 3 mm/s (0.12 in./s)

Arc Physics of Gas Tungsten and Gas Metal Arc Welding / 251

temperature of another species (neutral atoms).If the arc is in local thermodynamic equilibrium(LTE), all techniques should give the sametemperature.Although the assumption of LTE used to be

considered completely valid, contemporaryinvestigations have suggested that this is notalways the case and that some of the older mea-surements may be somewhat in error. Absolutevalues of arc temperature are only needed toestablish boundary conditions for detailedarc modeling of temperature-dependent proper-ties. What is of more importance to thewelding engineer or technologist is the relativeeffect of essential variables on heat input tothe workpiece. Arc temperature measurementis one useful indication of how these variablesaffect the arc.When compared to Fig. 7(a), Fig. 10 shows

that large additions of helium to argon decreasepeak temperatures slightly and increase theplasma diameter in the plasma column. Thearc appears to become a broader and more iso-thermal heat source. The lower peak tempera-ture is reasonable, because a combination ofthe high ionization potential of helium and rel-atively low currents of welding arcs gives

an arc column that is only slightly ionized.Figure 7 showed that a large vertex angle hada similar, but less pronounced, effect on arctemperature when adding helium to the shield-ing gas (that is, the axial peak temperaturedecreased and the plasma diameter increased).Figure 11 shows that doubling the current

from 150 to 300 A produces an increasein plasma diameter (that is, that portion ofthe arc above approximately 8000 K, or13,900 �F). This effect occurs regardless ofthe shielding gas composition.The arc length or gap between the electrodes

is yet another process parameter that must beconsidered, especially for mechanized welding,where it can be kept reasonably constant.Because arc voltage increases roughly in pro-portion with arc length, longer arcs have higherarc voltages and consume more energy for thesame current. However, this increased energyis generally lost through radiation to the envi-ronment surrounding the weld and does noteffectively supply additional heat to the work-pieces. Mechanized welding generally usesrather short arc lengths (2 to 3 mm, or 0.08 to0.12 in.), whereas manual welding uses a longerarc length. Unfortunately, relationships that

establish a direct correlation between the tem-perature distribution in the arc column and theweld pool shape still have not been established,primarily because weld pool shape depends onother factors, such as compositionally depen-dent molten metal properties.Flow Structure. Shielding gas is used to dis-

place reactive gases in the atmosphere from thevicinity of the weld. Inert gases are preferredfor the GTAW process, because they minimizeunwanted gas-metal reactions with the work-pieces. A uniform laminar flow of gas fromthe gas cup would be ideal and, in fact, is usu-ally achieved as long as there is no welding arc.However, the arc discharge rapidly heats thegas in the arc column, and thermal expansioncauses plasma jets. The lower temperature nearthe cathode tip in Fig. 11 is an indication of ajet pumping in cooler gas. This becomes animportant factor at high currents, because thesejets can depress the surface of the weld pooland alter heat transfer to it. The rapid gasexpansion can cause the flow to deviate fromlaminar and, in extreme cases, the flow canbecome turbulent. Turbulence tends to mixatmospheric contaminants into the arc, oftenwhere they can do the most harm: at the surface

(a) (b) (c)

30°

80°90°

30°

80°90°

30°

80°90°

Fig. 6 Effects of electrode tip geometry on the path length to ground in weld grooves of various shapes. (a) 75� V-groove. (b) 40� U-groove. (c) 10� narrow groove

Fig. 7 Effect of vertex angle on gas tungsten arc welding arc column temperature distribution with 100% Ar used as shielding gas. (a) 30� electrode vertex angle. (b) 90� electrodevertex angle. Welding current, 150 A

252 / Arc Welding Processes

of the molten weld pool. Holographic interfer-ometry and Schlieren photography have beenused to characterize gas flow.Figures 12 to 14 show examples of laminar

and turbulent flow. The flow from a commer-cial-design gas cup for three current levels isshown in Fig. 12. Laminar flow occurs wherethe fringes are generally straight and parallel.Turbulent flow is indicated by very wavy or cir-cular fringes. Increasing current tends to makethe laminar region somewhat broader andthicker, effectively increasing the area shieldedfrom atmospheric contamination.Figures 13 and 14 result from experiments

with gas cup shapes that were designed toimprove shielding. Figure 13 shows that a con-verging cone would be a very poor choice forthe GTAW process, as indicated by the verysmall area of laminar flow and the extreme tur-bulence in the surrounding region.Figure 14 is a venturi shape, which provides

a large laminar flow region for all current levelsand excellent shielding. This design may besomewhat better than commercial designs, butweld contamination studies should be con-ducted to verify this possibility.Arc Length Control. As mentioned earlier,

the arc length or gap between the electrodes isan important process parameter that must beconsidered. In particular, the distribution ofthe arc (thus arc force and heat distributions)is also determined by the arc length togetherwith the arc current. Unlike the tungsten shapeand flow gas that can be easily controlled, thearc length fluctuates and needs to be controlledfor precision joining, where GTAW has itsadvantages.The most convenient way to control the

arc length is to use the arc voltage as a feed-back of the arc length. This is because a changein the arc length would result in the samechange in the arc column. In a first-order

approximation, the voltage of the arc columnis considered to be proportional to thearc length (length of the arc column), and thevoltage drops at the anode and cathode are con-sidered to be constant for the given shieldinggas and workpiece material. Hence, the changein the arc voltage is considered to be propor-tional to the change in the arc length, andcontrolling the arc length at a constant wouldbe the same as controlling the arc voltage at aconstant.For precision sensing and control of arc

length, especially when the arc length is smallor the current may change, the aforementionedfirst-order approximation may not always besufficient. The complexity has been demon-strated through the following:

� For the same arc length, the voltage of atungsten-copper arc in argon decreases rap-idly before the current increases to 50 Aand decreases significantly before the cur-rent increases to 100 A (Ref 1).

� When the current is the same, the arc voltagedecreases before the arc length increases toapproximately 1 mm (0.04 in.) and thenincreases gradually when the arc length fur-ther increases (Ref 2, 3).

To use the arc voltage, V, as a precision feed-back/measurement of the arc length, l, studiesare needed for the particular anode (workpiece)to measure the arc voltage at different currentsand arc lengths in order to establish an accuratemodel for the specific material being welded:

Fig. 9 Effect of electrode tip geometry and shielding gas composition on weld pool shape for spot-on-plate welds.Welding parameters: current, 150 A; duration, 2 s

Fig. 10 Plot of gas tungsten arc welding arc column temperature distribution as a function of anode distance and arcposition. Welding parameters: electrode vertex angle, 30�; current, 150 A; shielding gas, 10Ar-90He

Fig. 8 Plot of arc voltage versus arc current forselected inert shielding gases. Welding

parameters: anode, titanium; cathode, tungsten; polarity,direct current electrode negative; arc length, 12.7 mm(0.050 in.)

Arc Physics of Gas Tungsten and Gas Metal Arc Welding / 253

l ¼ f V; Ið Þ (Eq 1)

where I is the current. However, despite thepossibility that such a model may be estab-lished and the model is deterministic, it is likelythat this model will be nonlinear, as has beendemonstrated in the previously referencedstudies.In the previous studies, because of the lack

of capabilities to measure the weld pool sur-face, the arc length was difficult to accuratelymeasure, and the gap between the tungstenand the unmelted workpiece was typicallyused as the measurement of the arc length.When the arc length is small and the weldingcurrent is large (such that the differencebetween the weld pool surface and the

workpiece surface is large), the arc lengthshould be measured more accurately. To thisend, technology recently developed to measurethe three-dimensional weld pool surface maybe used (Ref 4).The nonlinearity issue seen in Eq 1 may

be overcome by replacing the arc voltageusing an argon atomic line intensity (Ref 5).Figure 15 shows the linear relationshipbetween argon atomic line intensity andarc length. Ideal linear relationships are foundin the entire arc length range that is typicallyused. Of course, the measurement of the lineintensity requires the use of an optical-electricsensor with a band-pass filter, and it is morecomplex than the measurement of the arcvoltage.

Gas Metal Arc Physics

The gas metal arc welding (GMAW) processis performed using a welding arc established ina shielding gas between a continuously-fed con-sumable wire electrode and the workpieces orbase metal to be joined. Part of the shieldinggas that is continuously supplied around thewire is ionized to form the arc column and toestablish the arc; the rest surrounds the arcand molten metal to shield them from the atmo-sphere. The most widely used variant ofGMAW is flux cored arc welding, where thewire electrode is solid metal and cored with fluxof specific chemical powders.The most fundamental difference between

GMAW and GTAW is the presence of themetal-transfer process; that is, the consumableelectrode must be melted and the liquid metalof the melted consumable wire must transferfrom the electrode into the pool being formedby molten metal from the workpieces and

Fig. 11 Plot of gas tungsten arc welding arc column temperature distribution relative to anode distance and arcposition. Welding parameters: electrode vertex angle, 30�; current, 300 A; shielding gas, 100% Ar

Fig. 14 Effect of geometry on venturi gas cup laminarand turbulent flow as detected by real-time

holographic interferometry

Fig. 12 Effect of geometry on commercial gas cuplaminar and turbulent flow as detected by

real-time holographic interferometry

Fig. 13 Effect of geometry on converging cone cuplaminar and turbulent flow as detected by

real-time holographic interferometry

Fig. 15 Argon atomic line intensity versus arc length

254 / Arc Welding Processes

electrode. Both the melting of the electrode andhow the melted electrode metal detaches fromthe electrode into the weld pool play fundamen-tal roles in determining the behavior of the arc,the quality of the resultant welds, and the pro-ductivity of the process.The detachment of the melted electrode is

controlled by forces acting on the liquid dropletformed by the melted electrode. Of theseforces, the electromagnetic force often plays adominant role in determining the mode of themetal-transfer process and how the detachmentis completed. The primary variable that controlsthe electromagnetic force is the welding cur-rent, while other factors such as electrode diam-eter, shielding gas, polarity, and so on alsoaffect the electromagnetic force, but they arenot conveniently adjusted during welding.Another fundamental difference between

GMAW and GTAW is that the material of theelectrode varies with the application, becauseit will be melted and transferred into the basemetal to become part of the weld metal. Also,the electrons are emitted from the materialbeing welded in typical direct current electrodepositive GMAW applications. Because thematerial that emits the electrons varies withapplication, the cathode voltage varies fromapplication to application. Further, the electronstend to emit from areas of lower electron volt-age in the weld pool, the location of the cathodein relation to the anode (wire tip) may change,and the arc column may not always be straightif the reduction in the voltage for electron emis-sion is greater than the increase in the arc col-umn voltage. Because the location of the areaof lower electron voltage in relation to the wiretip may change during welding, the arc (path,column, distribution) may be subject to change.In particular, because of cyclic changes in thearc length due to metal-transfer variations, thearc length in GMAW typically must be longenough to tolerate these periodic changes. Thisincreased length of the arc also promotes thechange of the arc. As a result, the arc voltagefluctuates in addition to the periodic change.For constant voltage (CV) power supplies, thecurrent also fluctuates accordingly. Such fluc-tuations make it more difficult to sense by useof the arc signals than in GTAW.The GMAW process generally uses a dc arc,

where the electrode wire has a positive polarity.The wire thus becomes the anode and the work-piece becomes the cathode. The polarity iscalled reverse polarity, or DCEP. The purposeof DCEP is to detach the droplets formed bythe melted wire. This is because the componentof the electromagnetic force generated by thecurrent along the wire axis direction typicallyhas the same direction as the current in argon-rich gases. Hence, when the current flows awayfrom the droplet, the electromagnetic force is adetaching force. Further, DCEP can removeoxide films from the surface of the weld poolor workpiece. The oxide film promotes emis-sion. As the oxide is depleted, the emissionmoves to a new location that has sufficient

oxide content to sustain the discharge of elec-trons. The arc root or cathode spot where theemission occurs is highly mobile. Togetherwith the oscillation of the weld pool surfacecaused by impact from the detached droplets,the arc is typically not completely stable. Thearc voltage fluctuates, and for CV power supplythe fluctuation in the arc voltage will cause thecurrent to fluctuate.In DCEP, the wire is primarily melted by the

anode heat whose power is IVanode, where I isthe welding current, and Vanode is the anodevoltage. The heat directly imposed on the work-piece by the arc is IVcathode. For GMAW appli-cations, the workpiece and wire are similarmaterials, and the Vcathode/Vanode ratio isapproximately 2 (Ref 6). Hence, if GMAWis performed using straight polarity, the wireis expected to be melted at double speed, sothe deposition speed is doubled. That is,straight polarity helps GMAW to increasethe deposition rate or reduce the heat input.However, in DCEN, the electromagnetic forcesprevent the droplet from being detached.Hence, variable polarity or ac GMAW has beenproposed that uses DCEN to speed the wiremelting and DCEP to detach the droplet.In all cases, the arc and both electrodes are

shielded by gas, usually an inert gas, a gas mix-ture, or pure CO2. The inert gas is typicallyargon, and the gas mixture is Ar-O2 or Ar-CO2. A gas mixture is used to improve the pen-etration capability and weld bead, and pure CO2

is used for reduced costs and increased deposi-tion and arc temperature.A reasonable understanding of welding arc

fundamentals and the GMAW process requiresa more thorough discussion of electrode melt-ing and droplet detachment/metal transfer,including the physics and physics-basedcontrols.Wire Melting. In GMAW (Fig. 16), a con-

sumable wire is fed to the contact tube, whichis typically connected to the positive terminalof the power supply. A shielding gas is suppliedto surround the wire and is restricted bythe nozzle to deliver to the local area that sur-rounds the wire. When the wire touches thenegatively charged workpiece (connected tothe negative terminal of the power supply),the tip of the wire is rapidly melted, forming agap between the wire and the workpiece, andan arc is ignited across this gap. The anode ofthe arc directly melts the wire, and meltedmetal forms a droplet at the tip of the wire.The formation and detachment of the dropletis typically referred to as the metal-transferprocess.Before droplet detachment and metal transfer

are analyzed, the melting of the wire in GMAWmust be understood. Basically, the wire ismelted by the anode and resistive heat, whichare proportional to the current and its square,respectively. However, the resistive heat alsodepends on the length of the wire extension(E). The heat that melts the wire is often simpli-fied as:

Qwire ¼ IVanode þ rA

� �EI2 (Eq 2)

where r is the resistivity of the wire material,and A is the cross-sectional area of the wire.In a first-order approximation, r is treated as aconstant. If greater accuracy is needed, thedependence of resistivity on temperature, thatis, r(T) and @r(T)/@T 6¼ 0, can be considered,and an equivalent resistivity can be calculatedas the average resistivity over the wire exten-sion to replace the constant resistivity in Eq 2:

r ¼�Z

E0rðTðtÞÞdt

�=E (Eq 3)

However, a more accurate calculation of thisequivalent resistivity requires the following:

� r(T), the dependence of resistivity on tem-perature for the given wire material

� T(t), the dependence of temperature on posi-tion t(0 � t � E) in the wire extension forthe given wire material, diameter, current,and wire extension

One may find that extremely high precisionfor the heat that melts the wire is often notrequired because the temperature in the liquiddroplet is not uniform, and the average temper-ature of the droplets changes with the currentand droplet volume in addition to other metalparameters. Hence, the wire metal melt cannotbe accurately determined from the heat.Arc Length/Voltage Control. The length of

the arc column from the droplet to the cathodeon the weld pool surface is the arc length. Thewelding voltage is the sum of the voltage dropon the wire extension, the anode voltage drop,the arc column voltage, and the cathode volt-age. The arc length is an approximate indicatorfor the distance from the wire tip to the weldpool surface and must be controlled at anappropriate level to prevent unintentional

Fig. 16 Illustration of gas metal arc welding processes

Arc Physics of Gas Tungsten and Gas Metal Arc Welding / 255

short-circuiting (too short an arc length) or burn-ing of the contact tube (too long an arc length). Tothis end, the arc length must be feedback con-trolled. Because the voltage drop on the wireextension is typically much smaller than the arccolumn voltage, and the cathode and anode volt-age drops are both approximately constant, thearc column voltage can be approximately con-trolled by controlling the welding voltage.The major parameter that determines the arc

column voltage is the arc length, although thewelding current also affects the arc columnvoltage. A first-order approximation is:

V ¼ V0 þ kl (Eq 4)

where V0 = Vanode + Vcathode + Vwire and k areboth considered constant, although Vwire and kare both current-dependent and Vwire is alsowire-extension-dependent. Hence, the weldingvoltage can be used to indicate the arc length,and the control of the arc length can be con-verted into the control of the welding voltage,which can be easily measured.The arc length control is based on the follow-

ing melting-feeding balance equation:

dl

dt� c1Qwire � vf (Eq 5a)

or

dV

dt� kðc1Qwire � vf Þ (Eq 5b)

where vf is the nominal wire feed speed, and c1is a constant that depends on wire material,diameter, and droplet temperature.When the contact-tube-to-work distance sud-

denly increases, the arc length immediatelyincreases, such that the welding voltage alsoincreases immediately. In this case, the feed-back control system will reduce the voltage,and the needed dV/dt is negative. Based onEq 5(b), Qwire must decrease. From Eq 2, thecurrent must be reduced. Once the currentdecreases, dV/dt < 0 will be realized, and thevoltage will decrease.Metal Transfer. The wire is continuously

melted, but the metal melted from the wiremay not transfer continuously/immediately intothe base metal/weld pool. Instead, it accumu-lates at the tip of the wire to form a droplet, pri-marily because of the surface tension at thedroplet (liquid)/wire (solid) interface that func-tions as a retaining force for the droplet. Themetal-transfer process is often loosely used torefer to the entire process associated with wiremelting, droplet formation and detachment,and merging with the base metal. Dropletdetachment is specifically referred to in thissection.The droplet is transferred into the base metal

through three major modes/types described sub-sequently: short-circuiting, globular, and spraytransfer. There are two major established the-ories for when and how the droplet is detached:pinch instability and static force balance.

Detailed discussion and application of eithertheory to determine when and how the dropletis detached is beyond the scope of this article.This article only introduces the principle ofstatic force balance.When applying the static force balance the-

ory, all forces imposed on the droplet are pro-jected to the axis of the wire and arecategorized into detaching forces (pointingaway from the liquid-solid interface) andretaining forces (pointing toward the liquid-solid interface). In typical cases (an argon-richshielding gas and DCEP), the major detachingforces are the electromagnetic force, whichcan be considered to approximately increasequadratically with the current, and the gravita-tional force, which increases proportionallywith the mass of the accumulated mass in thedroplet; the major retaining force is the surfacetension at the liquid-solid interface, along withsome supportive force from the momentum ofelectrons and ions. For a given wire materialand diameter, the surface tension is approxi-mately fixed, and the major parameters thatdetermine when and how the droplet isdetached are the welding current and arc length:

� Short-circuiting transfer is characterized bythe occurrence of short-circuiting betweenthe droplet and weld pool. When the wirefeed speed is low, such that the currentneeded to melt the wire is small, the electro-magnetic force will be small. Unless a con-dition exists for the droplet to grow to alevel such that its gravitational force,together with the electromagnetic force,

exceeds the surface tension, the droplet willtouch/short-circuit with the weld pool beforetransferring into the weld pool. Figure 17shows a typical short-circuiting metal-transfer process. It is found that much spattermay be generated in this process.

� Globular transfer is characterized by rela-tively large droplets whose diameters aresignificantly greater than that of the wire.Two conditions are needed for this to occur:the arc length, l (Fig. 16), is sufficient toallow the droplet to grow to a diameter thatis significantly larger than that of the wire,and the current is relatively small, such thatthe corresponding electromagnetic force isnot sufficient to form a competent detachingforce that exceeds the surface tension with-out a droplet of relatively large diameter toprovide a sufficient gravitational force.Two typical globular transfers are shown inFig. 18.

� In spray transfer, if the current is sufficientthat the detaching electromagnetic force isclose to or exceeds the retaining surface ten-sion, the droplet will detach without a needfor additional gravitational force. The dia-meters of the droplets will thus be similarto (drop spray) or much smaller than thatof the wire. Figure 19 shows a typical dropspray metal transfer.

The occurrence of spray transfer is primarilydetermined by the current level for the givenwire diameter and material, and the arc length,l, is irrelevant as long as it is not extremelyshort. The current level that determines if a

Fig. 17 Typical short-circuiting metal-transfer process

Fig. 18 Schematic of typical globular metal transfer. (a)Axial globular transfer. (b) Nonaxial globular

transfer Fig. 19 Typical drop spray metal transfer

256 / Arc Welding Processes

spray transfer could occur is referred to as thetransition current, It, and it is approximately230 A for steel wire of 1.2 mm (0.05 in.) diam-eter. At I < It, the gravitational force will play arole in providing sufficient detaching force. If lis sufficient, globular transfer theoreticallycould occur at any I < It, and the diameter (vol-ume) of the droplets detached decreases as Iincreases; otherwise, short-circuiting wouldoccur. For a given l, the transfer tends tochange from short-circuiting to globular as Iincreases.Static Force Balance and Arc Climb. The

wire is continuously melted, but the resultantdroplet typically detaches from the wire andtransfers into the base metal discontinuously.The static force balance can be a good start tounderstanding the metal-transfer process.The major forces acting on the droplet

include gravitational force, electromagneticforce (Lorentz force), aerodynamic drag force,surface tension, and momentum force. Theforce due to gravity can be expressed as:

Fg ¼ mdg ¼ 4

3prd3rg (Eq 6)

where md is the mass of the droplet, rd is thedroplet radius, r is the droplet density, and gis the acceleration of the gravity. The surfacetension is given as:

Fs ¼ 2pRs (Eq 7)

where R is the electrode radius, while s is thesurface tension coefficient. The aerodynamicdrag force can be expressed as:

Fd ¼ 1=2CdAdrpvp2 (Eq 8)

where Cd is the aerodynamic drag coefficient,Ad is the area of the drop seen from above,and rp and vp are the density and fluid velocityof the plasma, respectively. The momentumforce can be expressed as:

Fm ¼ vs _md (Eq 9)

where vs is the wire feed speed, and _md is thechange of the droplet mass. The electromag-netic force, Fem, is given by:

Fem ¼ m0I2

4p1

2þ ln

riru

� �(Eq 10)

where m0 is the magnetic permittivity, I is thewelding current, ri is the exit radius of the cur-rent path, and ru is the entry radius of the cur-rent path; ri and ru are related with the processof the droplet status. Before the droplet startsto be detached, ru is the same as the radius ofthe wire and is thus a constant. However, oncethe droplet is being detached, ru decreases.The increase of Fem thus accelerates the detach-ment of the droplet. In the conventionalGMAW process, the droplet is not detachedwhen the retaining force, Fs, is still sufficientto balance the detaching force, Ft:

Ft ¼ Fg þ Fd þ Fm þ Fem (Eq 11)

During the metal-transfer process, the majorvariables that affect the detaching force arethe droplet mass and the current, as can be seenfrom Eq 6 to 10. Because the surface tension isthe major retaining force and it is fixed for thegiven wire, the droplet can only be detachedeither by waiting for the droplet to grow intoa larger size, such that the gravitational forceis sufficient to break the balance; waiting forthe droplet to touch the weld pool, such thatan additional detaching force—surface tensionbetween the droplet and weld pool—is added;or by increasing the current to increase the elec-tromagnetic force.The phenomenon of the transition current

implies that the effect of the electromagneticforce as the detaching force changes rapidlyaround the transition current. The rapid increaseof the electromagnetic force reduces the depen-dence on the gravitational force to overcomethe surface tension at the liquid-solid interfaceon the wire tip. Such a rapid increase in theelectromagnetic force at detaching is due tothe rapid climb of the arc (anode) from the bot-tom of the droplet toward the neck of the drop-let (the solid-liquid interface). The climb of thearc increases the exit radius of the current inEq 10. When the arc climbs rapidly as the cur-rent increases, the exit radius of the current inEq 10 increases rapidly, such that the requiredgravitational force component is much smaller.(Figure 20 demonstrates the arc climb phenom-enon.) As a result, the diameter of the dropletdecreases rapidly in the narrow range aroundthe transition current, from being much largerthan that of the wire to being much smaller thanthat of the wire.Metal-Transfer Control. Metal transfer

plays a critical role in determining the qualityof the welds produced. In conventional GMAWwith a continuous waveform CV power supply,the continuous waveform current is dictated bythe wire feed speed. If the wire feed speed islarge, such that the resultant current is higherthan the transition current, that is, I � It, thedroplets are automatically detached smoothlyat relatively small diameters as spray transfer,and the metal transfer is typically not an issueof concern unless the current is extremely high,producing a rotating transfer. However, if I < Itis required, the metal transfer could cause pro-blems. If the arc length is long, the large dro-plets pending at the tip of the wire wouldwander about the wire axis and cause not onlyan arc instability but also uncertainties in thetravel direction of the detached droplet. Dro-plets thus merge into the weld pool at differentlocations, forming irregular weld beads. If thearc length is short, resulting in short-circuitingtransfers, spatters would be produced. Hence,a continuous waveform current is typically notused in the large current range I < It. A numberof methods have been developed to control themetal-transfer process if the needed current issmaller than the transition current.

Pulsed Arc Control. This is a method toachieve the desired spray transfer over a widerange of average current by using a squarewaveform with a background current, Ib < It,to maintain the arc and a peak current, Ip � It,to detach the droplet, as shown in Fig. 21. Theapplications of GMAW that require differentlevels of heat input are thus greatly extended.Surface Tension Transfer (STT, Lincoln Elect-

ric). This technology was developed from anunderstanding of phenomena associated withshort-circuiting transfer. By detecting the prog-ress of the short-circuiting process, the currentwaveform is controlled/optimized differentlyin different metal-transfer stages to minimizethe production of spatters. Figure 22 shows itstypical current waveform.Cold Metal Transfer (CMT). Gas metal arc

welding typically feeds the wire forward con-tinuously by using a motor (wire feeder) at adistance from the welding torch to minimizethe weight being carried and to maximize thetorch accessibility. The CMT uses a secondmotor installed close to the torch to drawthe wire back after the droplet touches theweld pool. As a result, the droplet can be suc-cessfully separated from the solid wire andtransferred into the weld pool, and the arc isre-established during the separation process, asshown in Fig. 23. During the separation andarc re-establishment, the current is controlledat a minimal level to minimize the productionof spatter.Double-Electrode GMAW. In conventional

GMAW, the current melts the wire. By addinga bypass electrode and bypass loop (Fig. 24),such that part of the current passes throughthe bypass loop, there will be two cathodes,one on the base metal and another on thebypass electrode. The current flowing to theworkpiece becomes lower than the melting cur-rent. As a result, the arc root on the wirebecomes more distributed, and the spray trans-fer is achieved with a base-metal current muchsmaller than the transition current (Ref 7).Heat Input Control. Many GMAW applica-

tions require that a certain amount of metal bedeposited into the workpiece with a minimalheat input. This problem can be posed as

Fig. 20 Arc climb phenomenon. (a) Below transitioncurrent. (b) Above transition current

Arc Physics of Gas Tungsten and Gas Metal Arc Welding / 257

achieving a maximum efficiency ratio, definedas the effective heat over heat input ratio:

l ¼ He=H (Eq 12)

where H is the heat input into the workpiece,and the effective heat, He, is the heat that meltsthe wire.This can be simplified by omitting the arc

column heat and resistive heat. In conventionaldirect current electrode positive GMAW:

He ¼ VaI=v (Eq 13)

H ¼ VaþVcð ÞI=v (Eq 14)

where Va is the anode voltage drop, Vc is thecathode voltage drop, and v is the travel speed.Their ratio is:

l ¼ Va= Va þ Vcð Þ (Eq 15)

In GMAW, the materials of the wire and basemetal are similar, and Vc � 2Va (Ref 6). Hence,for conventional direct current electrode posi-tive GMAW, the efficiency ratio is approxi-mately 33%.In direct current electrode negative GMAW:

He ¼ VcI=v (Eq 16)

H ¼ Va þ Vcð ÞI=v (Eq 17)

The efficiency ratio is approximately 66%.However, in electrode negative (EN) mode,the electromagnetic force prevents the dropletfrom being detached. To resolve this issue, thepolarity can be switched between EN and elec-trode positive (EP), such that the efficiencyratio is increased using EN but the droplet isdetached using EP. The resultant processes arevariable-polarity GMAW and ac GMAW.In double-electrode GMAW, if the bypass

electrode is nonconsumable, such as a tungsten:

He ¼ VaI=v (Eq 18)

H ¼ VaI þ VcIbmð Þ=v (Eq 19)

where Ibm is the base-metal current, and themelting current I = Ibp + Ibm, that is, the sumof the base-metal current and bypass current.The efficiency ratio is:

l � I= I þ 2Ibmð Þ (Eq 20)

If Ibp = Ibm, then l = 50%. If Ibp = 4Ibm, thenl = 5/7 = 71%.For consumable double-electrode GMAW

where the bypass electrode is a wire:

He ¼ VaI þ VcIbp� �

=v (Eq 21)

H ¼ Va þ Vcð ÞI=v (Eq 22)

Fig. 21 Typical pulsed gas metal arc welding process

Fig. 22 Schematic of surface tension transfer process and current waveform

258 / Arc Welding Processes

The efficiency ratio is:

� � 33%þ 2

3

� �IbpI

� �(Eq 23)

If Ibp = Ibm, then l = 66%. If Ibp = 4Ibm, thenl = 86%.

REFERENCES

1. K. Goldman, Electric Arcs in Argon, Phys-ics of the Welding Arc, Institute of Welding,London, 1966

2. K. Ando and J. Nishikawa, “Studies onAnode and Cathode Energy of TIG Arc,”IIW Document 211-158-68, 1968

3. J.F. Lancaster, Energy Distribution inArgon-Shielded Welding Arcs, Br. Weld.J., Vol 1, 1954, p 412–416

4. H.S. Song and Y.M. Zhang, Measurementand Analysis of Three-Dimensional

Specular Gas Tungsten Arc Weld Pool Sur-face, Weld. J., Vol 87 (No. 4), 2008,p 85s–95s

5. P.J. Li and Y.M. Zhang, Analysis of ArcLight Mechanism and Its Application inSensing of GTAW Process, Weld. J., Vol79 (No. 9), 2000, p 252s–260s

6. E.J. Soderstrom, K.M. Scott, and P.F.Mendez, Calorimetric Measurementof Droplet Temperature in GMAW,Weld. J. Res. Suppl., Vol 90, 2011, p 77-sto 84-s

7. K.H. Li and Y.M. Zhang, Metal Transfer inDouble-Electrode Gas Metal Arc Welding,J. Manuf. Sci. Eng. (Trans. ASME), Vol129 (No. 6), 2007, p 991–999

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Fig. 24 Nonconsumable double-electrode gas metal arc welding (GMAW). GTAW, gas tungsten arc welding

Fig. 23 Cold metal-transfer process (metal inert gas/metal active gas dip-transfer arc process). Courtesy of FroniusUSA LLC, 2007

Arc Physics of Gas Tungsten and Gas Metal Arc Welding / 259