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WELDING RESEARCH/9 UPPLEMENT TO THE WELDING jOURNAL, jULY 1993Sponsored by the American Welding Society and the Welding Research Council -

M e ta l Transfer in Pulsed Cur ren tGas M eta l Arc WeldingA static force balance analysis was used to estimate the melting rates of theelectrodes during pulsed gas metal arc welding

BY Ye-S. KIM AND T. W. EAGAR

ABSTRACT. In order to achieve onedrop per pulse operational conditionswith pulsed current GMAW, it is nec-essary to control both the drop size a tthe peak current and the melting rate ofthe electrode. In this study, a static forcebalance analysis was used to predict thedroplet size at the peak current and aweighted sum of the melting rates mea-sured under Direct Current ElectrodePositive (DCE?) welding was employedto estimate the melting rate with pulsedcurrent. Combining the static force bal-ance analysis and the weighted summethod, a model is proposed to predictthe optimal conditions of one drop perpulse operation. The model i s found tobe in good agreement with the experi-mental results when the base currentand the load duty cycle are small. Whenthe base current increases above 22 0 Aand the load duty cycle exceeds IO'?"using 1.6-mm-diameter steel electrodes,the predict ion of the model deviates sig-nificantly from the experimental results.The discrepancy between the modeland the experimental results i s discussedY. S. KIM is Assrstant Professor, Departmentof Metallurgy and Mat er~a ls cience, Honglk Unrv ersity, Seoul, Korea, and T. W. EAGAR

., 1s Co-D irector , Leaders for Man ufact uringProgram, Richard P. Simmons Professor ofMetallurgy, Department of Materials 5c;enceand Engineering, Massachusetts Institute ofTechnology, Cambridge, Mass.

and is shown to be due to tapering ofthe electrode tip at high welding cur-rents.Introduction

Since the introduction of pulsed cur-rent Gas Metal Arc Welding (GMAW-P)in 1962 (Ref. I ) , this method of weldinghas been used widely both in mechanizedwelding and in robotic welding. Withpulsed GMAW, a stable spray metal trans-fer mode can be obtained at low averagecurrents that would otherwise produceglobular transfer with large sporadic drops.Pulsing leads to stable spray metal trans-fer and formation of a uniform bead shape

KEY WORDSPulsed Current GMAWModelingMetal TransferElectrode Melt RateStatic Force BalanceWeighted Sum MethodDroplet SizeOptimum Pulsing Freq.Electrode TaperingMelting Rates

with shallow penetration. Recent irn-provements in power supply designs usingtransistor or frequency converter controlsalso provide better controllability of theprocess (Ref. 2).

?ulsing the current introduces addi-tional operational parameters, which in-clude peak current, base current, peakpulse time, and base pulse time, in addi-tion to the variables of DC welding, whichinclude electrode extension, welding cur-rent and welding voltage. These extra vari-ables cause difficulty in selecting optimumoperating conditions for pulsed currentwelding. A trial-and-error method is oftenused to determine these conditions. How-ever, the basic physics of metal transfer inpulsed current welding needs to be un-derstood in order to more successfuliycontrol the process.

There have been several attempts toanalyze pulsed current welding theoret-ically (Refs. 3-5). Samati (Ref. 5 ) pre-dicted the theoretical pulsing frequencyby div iding the electrode melting rate bythe mass of the drop, and showed goodagreement between these predictionsand experimental results. However, thisagreement i s anticipated since there i s arange of working solutions instead of asingle-va[ued pulsing condition asshown experimentally by Allurn (Ref. 3).

In this study, a theoretical frameworkis described for pred iction o f the rangeof optimum pulsing frequencies. Themethod uses a combination of the

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(a) globular (c) streamingwith tapered tip (d) streaming

A . STICK-OUT 36 !lm* STICK-OtiT 26 Mi30 , STICK-OUT I6 Mi4

20B 250 300 350 400 450

WELDING CURRENT (AMPERE)

Fig. 1- chematic representations of metal transfer modes as weld- F;g. 2- elt ing rates of steel electrodes (1.6 mm diameter)shieldeding current increases from A to D. with Ar-2%02

droplet size predicted from the staticforce balance theory, and the melti ngrate from the weighted sum of mel tingrates at the peak and base currentsequivalent to DCwelding. This theoret-ical model is then compared with theexperimental results obtained usingsteel, Ti-6Al-4V and aluminum elec-trodes.Theoretical Framework forPulsed Current GMAW

As one increases the current duringDC welding in argon-rich atmospheres,the meta l transfer mode changes fromglobular to spray. Wi th further currentincreases in the spray cur rent regime,the anode spot increases in size until itbegins to cl imb the sides of the solidcylindrical electrodes. The condensa-tion heat produced by the current onthese vertical surfaces causes mel ting ofthe cylinder edges (Ref. 11 . At suffi-ciently high currents, this produces a ta-pered solid electrode tip as seen in Fig.1. In order to obtain one liquid metaldrop wi th a size similar to the electrode

diameter at every pulse, the ope ratingconditions must be such that significanttapering does not occur at the tip of theelectrode. If tapering occurs, the pulsedcurrent process degenerates into stream-ing metal transfer m ode (Ref. 7 ) and itbecomes difficult to obtain one dropwith each pulse.Among the four pulsing parameters,which include peak current, peak time,base current and base time, the pursingfrequency and the load duty cycle wereused as the operational parameters ofinterest instead of the more commonlyused peak time and base time. Pulsingfrequency i s defined as l/(peak time +base time) and load duty cyc le as (peaktime) / (peak time + base time) X 10 0( Y o ) .The employment of pulsing fre-quency and load duty cycle as the op-erational parameters eliminates some ofthe complexity of adjusting the process.For instance, if the load duty cycle iskept constant, the pulsing frequency canbe changed without affecting the aver-age weidi ng current, which may lead toa relatively constant electrode meltingrate. In this manner, i t is possible to de-

termine a range of opt imum pulsing fre-quencies at a constant electrode me lt-ing rate.In pulsed current GMAW, a theoreti-cal pulsing frequency is obtained by d i-viding theelectrode melting rate with cur-rent pulsing by the mass of one drop :theoretical pulsing frequency =

where:rnPuiye is the electrode meltingrate with current pu~sing,vd~op(lp)s thepredicted volume of the drop at the peakcurrent, and pd is the density of the drop.The average melting rate for a squarewave current may be estimated as theweighted sum of the DC melting rate at

the peak current and at the base current.

b: load duty cycleh(1,,):C melting rate at peak current

I0 100 -0 IS0 -a 120--0 000 -0 060 --8 030 -a ma ~ , I , I ~ I , G

I60 240 320 400 480 560WELDING CURRENT (AMPERE)

Fig. 3 - he equilibrium droplet size from a 1.6-mm-diameter steelelectrode calculated from the static balance theory at two differentargon gas speeds (70 m/s and 100 m/s) around drops.

current frequency frequwncy

pulsing frequency (fp)Fig. 4- chematic diagram of weld current pulsing

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Fig. 5- verall layout of wejding equipment.z4a. a 3ze.a 4ae.a 4ea.0 s6a.e

PEAK CURRENT (AMPERE)Fjg. 6- heoret;cal pulsing frequency for steel electrodes shieldedwith Ar-2%02 as a function of peak current.

m(l,,): DC melting rate at base currentAs shown in our previous work (Ref.81, the melt ing rate undergoes a transi-tion as the welding current increases asshown in Fig. 2. This transition is relatedto formation of the taper. Since fully de-veloped tapers have less tendency toform in pulsed current welding, the DCmelt ing rate measured in the pretransi-tion region has been extrapolated to thepeak current levels in order to estimatethe melting rate at the peak current.The droplet size in pulsed currentwelding may be determined at the peakcurrent using the static force balancemodel. Figure 3 shows the results of thiscalculation. The higher the peak current,the smaller will be the droplet size. Thedetails of this calculation can be foundelsewhere (Ref. 7 ) .When the pulsing frequency is in-creased above the theoretical pulsing re-quency of Equation 1 with other opera-tional parameters held constant, notevery pulse can detach one drop. In otherwords, the droplet size and the meltingrate remain the same; theoretically it i simpossible to produce more drops thanpreaicted by the theoretical frequency

given by Equation 1 . Therefore, the the-oretical puking frequency i s the theoret-ical maximum pulsing frequency (TMPF)that should be applied to the system. O nthe other hand, as the pulsing frequencyis decreased below the TMPF, each pulsecan still produce one drop over a lim itedrange of lower frequencies, but thedroplet size becomes larger than theequilibrium droplet size at the TMPF. Ifthe pulsing frequency is decreased fur-ther, droplet transfer frequency at the DCbase current will eventually becomefaster than the applied pulsing frequency.Hence the droplet transfer frequency atthe DC base current sets the lower limitof the one drop per pulse region. Whenthe pulsing frequency is lower than thelimit, the drop wil l be detached in twomodes: one controlled by the base cur-rent and the other controlled by the peakcurrent. Therefore, withi n one cycle o fpulsing, several drops may be detachedand the size of the droplets wi ll becomenonuniform.

Figure 4 schematically shows theconcepts of the preceding paragraph-The droplet transfer frequency to pulsefrequency ratio on the vertical axis is de-

fined as the actual drople t transfer ratedivided by the applied pulsing fre-quency. When the droplet to pulse fre-quency ratio is equal to one, each pulseproduces one drop. This is the optimumpulsing frequency region for practicalwelding. When the droplet to pulse fre-quency ratio is larger than 1.O,he nat-ural frequency becomes larger than thepulsing frequency, hence insufficientpulse frequency is present. Finally, whenthe droplet to pulse frequencv ratio isless than 1.O, ulsing becomes so fastthat not every pulse can produce a drop,hence the pulse frequency is excessive.Experimental Procedures

Mil d steel (AWS E7Os-3), aluminumalloy (AA I 100, AA5336), and titaniumalloy (Ti-6Al-4V) were used in the ex-perimental portion of this study. Theshielding gases were pure argon andargon-2O/0 oxygen. The welding equip-ment included a constant current-typepower supply, a transistorized currentregulator, and a voltage-controlled elec-trode feed with a low inertia motor. Thepower supply could provide a total out-

a f a 20 30 *aPULSING FREQUENCY(ISEC)

Fig. 7- ptimum pulsing frequency regions for steel electrodesshielded w ith Ar-2%0 2. The base current was 180 A and the loadduty cycle was 5%.

CURRENT :eoAPEAKCURRENT. ::o ALOAD DUTYCYCLE 5 %

0.- , I I I I I 1 I ,a 16 20 3a 40 sa

PULSINGFREQUENCY VSEC)Fig. 8- rople t size variation in the range of optimum pulse fre-quency for steel electrodes w ith Ar- 2% 02 shielding . The peak cur-rent is 500 A.

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--Table 1 -Cond itions used for Pulsed Curr ent Weld ingPeak current (A) Base current (A) Frequency(Hz) Duty cycle YO

Mild steel 3CO,400, 5CO 180,200,220,260 5 to300 5, 10,20Aluminum 300,400,500 170,200 3 to150(1100)

put power of over 1200 A.The transistorized current regulatorused in this study can supply DC cur-rent with less than l% ripp le (Ref. 9).This system uses transistors to controlthe welding current and is capable ofpulsing the DC current to a max imumof 5 kHz for small superimposed signals.The equipment can cont rol pulsing pa-rameters, peak current, base current,peak time and base time, independentlyfrom a function generator included withthe controller. An a lumina tube was in-serted into the contact ti p of a commer-cial welding gun leaving only 5 mm forcontact length rather than the normalcontact length of 24 mm. A transversingweld table was used so that the wel d guncould remain at a fixed position. Figure5 shows the overall layout of the weld -ing equipment.Analysis of metal transfer was per-formed using high-speed videographywith a backlighted shadow graphicmethod (Ref. 10). This method excludesmost of the intense arc light and trans-mits most of the laser ligh t by a spatialfilter that i s placed at the focal poi nt ofthe objective lens. The high-speed videocamera i s capable of producing imagesat a maximum I 000 full frame picturesper second (pps). The dropl et transfer

rate was measured for 10 s and an aver-aged droplet transfer rate for each weld-ing condit ion was calculated: Thedrop let size was measured from the stil limage on the screen once every secondfor I 0 and averaged. The variation indroplet size and frequency is estimatedto be 5% in most cases.Melting rates of the electrode weremeasured using a tachometer that was incontact with the moving wire electrode.The output voltage of the tachometer andof the current shunt, w hich was filteredby a lo w band pass filter, was recordedwith a high-speed recorder.The ranges of operational variables forpulsed current welding used in this studyare shown in Table1. Based on the initialpulsing frequency, which was determinedusing predictions from the theoreticalmodel developed in this study, the puls-ing frequencies were changed in order todetermine the range of pulsing frequen-cies of one drop per pulse. This range ofthe pulsing frequencies was judged pri-marily from the recordings of arc voltageand pulse current on a high-speedrecorder and was later analyzed more ac-curately using high-speed videography.With the high-speed videography, thedroplet transfer frequency and the dropletsize were determined.

0 0300 .a00 l ' l 1 I ' ; ' l

168 240 320 488 +ea 560PEAK CURRENT (AMPERE)

Fig. 9- om~parison etween droplet size from the static force bal-ance theory c~ n d inimum droplet size in pulsed current welcling forsteel electrocles with Ar-2%02 shielding.282-s I JULY 1993

Pulsed Curren t GMAW withSteel ElectrodesEffect of Peak Current

Figure 6 shows the TMPF calculatedfrom Equation 6 as a function of peakcurrent at various levels of base current.The TMPF increases as the peak currentincreases because the melting rate of theelectrode increases due to the increasein average current and the decrease i ndroplet size. Using this TMPF as a refer-ence frequency, a series of puls ing fre-quencies was tested experimentall y inorder to determine the range of one dropper pulse with other pulsing conditionsremaining constant.Figure 7 shows the regions of puls-ing frequency with a 180-A base currentat three different peak currents: 300,400and 500 A. The load duty cycle used was5%. As seen in the figure, as the peakcurrent increases, the width of the one-pulse-one-drop (OPOD) region in -creases. When the peak current i s 300A, the OPOD region is very narrow ( 4to 6 Hz). When the peak current in-creases to 400 A, the range widens to 4to 12 Hz and, finally, when the peak cur-rent is 500 A, the range expands to 4 to38 H z. This expansion of the OPOD re-gion as the peak current increases wasalso observed at different base currents.This expansion of the OPOD region, es-pecially the increase of TMPF wit h peakcurrent, is due to the increase in the elec-trode melting rate and the decrease inthe droplet size as the peak current in-creases. The lower bound pu lsing fre-quency, 4 Hz, was not affected by thepeak current. This value agrees well w it hthe measured natural DC droplet trans-fer frequency of 3.5 Hz at a current o f

1 BASE CURRENT : 180 ALOAD DUTY CYCLE : 5%

/"

&a

H:PREDICTION:EXPERIMENTAL

WELDING CURRENT (AMPERE)Fig. 10- elting rate of steel electrodes at three different peak cur-rents wi th Ar-2%0 2 shielding.


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0 I 0 20 3aI40

PULSING FREQUENCY (ISEC)0 6 I6 24 32 40

PULSING FREQUENCY (ISEC)Fig. 7 7 - ulsing frequ ency regions of steel electrodes at a base cur- F;g. 72- u/s;ng requency regions of steel electrodes at a base cur-rent of 200 A with Ar-2% 02 shie lding. rent of 22 0 A A r-2% 02 shie ld ing.180 A. However,the TMPF, which areindicated by the arrows in the figure, donot co inci de with the measured maxi-mum pulsing frequency.Figure 8 shows the variation o f thedroplet size as the pulsing frequencychanges at a peak current of 500 A. Asthe pulsing frequency increases, thedrop let size decreases unt il i t reaches aminimum value. This corresponds to theequi libr ium droplet size at which the de-taching forces at the peak current arejust equal to t he retain ing surface ten-sion force. Figure 9 compares the equi-lib rium droplet size from the static forcebalance theory wi th the experimentallymeasured minimum droplet sizes at dif-ferent peak currents. The prediction andthe exper imental results agree with er-rors of less than 2 1O0/0. These resultsshow that the static force balance the-ory can be used to predict the dropletsize at various peak currents.The discrepancy of the TMPF fromthe experimental results may be causedby two possibilities as one can see fromEquation 1: either the equilibriumdroplet size is in error or the melting ratepredicted from the weighted summethod is in error, or both. Since the

predicted equilibrium droplet sizeagrees reasonably well with the experi-mentally measured minimum dropletsize, the actual melting rate duringpulsed current weld ing was measuredto compare with the melting rate pre-dicted from the weighted sum method.The melting rates measured at differ-ent peak currents along wi th the melt-ing rates predicted from th e weightedsum method of Equation 1 are shown inFig. 10- As menti oned in the previoussection, the melting rate for the peak cur-rent i s calculated from the curve extrap-olated from the pretransition me lting ratecurve of the DCEP we ld ing process. Asseen in the figure, the measured elec-trode melting rate is higher than the cal-culated melting rate predicted by Equa-tion l . When the increased melting rateunder current pulsing is used in Equa-tion l , the TMPF at 180 A base current,500 A peak current, and 5?& oad dutycycle is calculated to be 30 Hz, whichis closer to the experimentally observed3 7 Hz. Therefore, this increased melt-ing rate under pulsed current weld ingmust cause a significant por tion of thediscrepancy between the TMPF and themeasured maximum pulse frequency.

Effect of Base CurrentFigures 1 1, 12 and 1 3 show the ex-perimental results of the droplet to pulsefrequency ratio as a function of the puls-ing frequency at base currents o f 200,220 and 260 A, respectively. When thebase current is 200 (Fig. 11) and 180 A(Fig. 71, the predicted TMPF lie with inthe OPOD region. As the base current

i s increased to 220 A as in Fig. 12, theTMPF starts to shift outside of the OP ODregion. Wi th peak currents of 400 and500 A, the TMPF are within the regionof OPOD, but with a peak current of 300A the TMPF becomes smaller than thelower frequency of the OPOD region.When the base current i s increased to260 A, the TMPF of al l peak currents be-comes smaller than the measured lowerlimit frequency of the OP OD region.These large deviations of theoreticalprediction from the experimental mea-surements can be explained from the re-sults of the droplet size measurementsin our previous studies (Ref. 7 ) . Around210 A in DCEP wel din g the measureddroplet size becomes significantlysmaller than the droplet size predictedby the static force balance theory due to

8 ZB 40 60 80 l E 0 I20 I40PULSING FREQUENCY [ISEC)

Fig. 13- ulsing frequency regions of steel eiectrodes at a base cur-rent of 260 A Ar-2%U2 shielding.

2 ~ 8 2sa ma 3% 400 456 s ~ aPEAK CURRENT (AMPERE)

Fig. 14- he minim um droplet size of steel electrodes at three dif-ferent base currents Ar -2 %0 2 shielding.

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250 300 350 400PEAK CURRENT (AMPERE)

Fig. 15 -Partial taperingat the tip of thesteel electrode when shielded Fig. 16- he minimum drople t size of steel electrodes at three difwith Ar-2%02 The base current is 22 0 A and the peak current is 400 ferent peak currents when shielded wi th Ar -2%02. The base currenA. is 780 A and the load duty cycle is 10%.

tapering of the electrode. Therefore, w ithpulsed welding conditions in whi ch ta-pering of the electrode occurs, theTMPF, which are calculated by thedroplet size predicted from the staticforce balance theory, will be smallerthan the measured droplet transfer fre-quency. Figure 14 shows the droplet sizemeasured at different base currentswhen tapering occurs as in Fig. 1 5 . Itcan be seen that the min imu m dropletsize is smaller than that predicted by theDC (nontaper) prediction at the 220 basecurrent. Thus, it i s believed that it is theformation of a taper that causes the pre-dicted TMPF to be smaller than that mea-sured experimentally. The tendency fortapering of the electrode increases asboth base currents and peak currents in-crease.

Effect of Load Duty CycleWhen load duty cycle is increased to

lo%, tapering of the electrode occurseven at l ow base currents. For instance,with 10% load duty cycle, tapering i sobserved at a base current of 180 A anda peak current of 400 A. With such ahigh load duty cycle, the electrode ta-pers during the peak current period anddoes not return to a cylindr ical shapeimmediately after the current is loweredto the base current. This phenomenoni s especially easy to observe at pulsingfrequencies near the lower boundary ofthe optimum pulsing frequency region.Since a small amount of tapering canexpand the OP OD region by creatingdecreased minimum droplet sizes, thetapering of the electrode can be benefi-

0 20 40 60 80 I00PULSING FREQUENCY (ISEC)

Fig. 17- he pulsing frequency region of steel electrodes shieldedwi t h A r - 2 x 0 , ~t three different peak currents. Tho wiclt t~ f the opti-mum pulsing frequency region hss increased signilic.intly a t 10%locid duly cycle.

cia1 if the degree of tapering i s smalenough such that droplet sizes similato the electrode size can be obtained.Figure 16 shows the measured decreasein dr oplet size due to the partial ly developed taper seen in Fig. 1 5 . Whenthere is partial tapering of the electrode,the OPO D region is increased significantly as shown in Fig. 17. The pulsefrequency working range at a base cur-rent of 18 0 A and a peak current of 400A with 10% load duty cycle i s approxi-mately twice as wide as that with 5%load duty cycle, wh ich produces no partial tapering.As the load duty cycle is further in-creased u p to 20%, the OPO D regionincreases significantly because thedroplet sizes are further reduced by thewell-developed taper on the electrode.

COAO DUTY CYCLE sm.BASEE A K C U R R E N T u AURRENT 2 M A0 PEAK CURRENT 500

S0 100 150 290 250 300PULSING FREOENCY ( ISEC)

Fig. 18- he pulsing frequency region o f steel electrodes at 20%load duty cycle Ar -2 %0 , shielding. The base current is 220 A.


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Fig. 22- apering o i Ti-6AI-4V electrode wi th argon shielding. A- eginning of the tapering; B -an established taper of the electrode. Thebase current is 200 A and the peak current is 500 A. The load duty cycle is 70%.

Fig. 23. This may be due to the fact thatthe peak currents used in this study weretoo high for aluminum. The low surfacetension of aluminum causes the detach-ing force to create too much disturbancein the liquid drop. Wi th lower peak cur-rents and with a lower load duty cycle,there may b e a range of optim um puls-ing frequencies for aluminum welding.Further Observations w ithPulsed Current GM AW

From the observations made in thisstudy, several important aspects of thepulsed current welding process can beidentified. Firstly, peak current has themost significant effect on the OPODrange, as seen in Fig. 7. In general, thehigher the peak current, the wider the

OP OD range. However, when peak cur-rent is increased too much, tapering ofthe electrode will occur, leading to astreaming transfer mode in which thedroplet size is too small to cont rol. Taper-ing may be suppressed bv using a shield-ing gas consisting of Ar-He mixtures.

Secondly, whe n weldin g wit h steelelectrodes using carbon d ioxide as ashielding gas, the application of pulsedGM AW w ill not provide any advantagesin controlling droplet size. Since thedroplet size remains nearly the same andthe mo de of metal transfer is repelledtransfer (Ref. 1 I ) , pulsing of current wi llnot produce projected sprav transferwhen welding steel electrodes shieldedwith carbon dioxide.

Thirdly, when welding with steelelectrodes using helium as the shielding

gas, pulsed current GM A W may pro-duce projected metal transfer in the nor-mal DC range of repelled globular trans-fer. The repelled metal transfer mode atlo w welding current transforms into theprojected spray transfer mode as weld-ing current increases. Therefore, if thepeak current used i s greater than thetransition current of repelled -projectedtransition, pulsed current GM AW wi llproduce a projected transfer mode. Thesame reasoning can be applied whenwelding with titanium electrodesshielded wit h argon, w hich exhibit thesame transition phenomenon as theweldin g current increases.Conclusions

A theoretical model of pulsed currentweldin g is developed to predict rangesof one pulse per one drop pulse fre-

Fig. 23- econdarymetal transfer with alu-

minum electrodes inpulsed current weld-

ing. The shielding gasis pure argon.

quency. Experimental results confirmthis approach.

The width of the op timum pulsing fre-quency region increases as the peak cur-rent increases. This i s due to the fact thatthe range of drople t sizes available andthe me lting rate increase as the peak cur-rent increases.

The static force b alance theory canpredict the droplet size at a given peakcurrent provided that there is no signifi-cant tapering at the tip of the electrode.The melting rates under pulsing cur-rent conditions are greater than mel tingrates calculated usin g a weighte d sumof the melti ng rate (for DC currents) atthe peak current and at the base current.The workable ranges of base current andload duty cycle can be expanded whentapering of the electrode can be sup-pressed. This may be achieved by addingheliu m and/or carbon diox ide to theargon gas,

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AcknowledgmentsThis research was funded by a grant

from the United States Department ofEnergy under contract number DE-FG02-85ER-13331.

References1. Needham, J. C. 1962 . Control of trans-fer i n aluminu m consumable elect rode weld-ing. Phys ics of We lding Arc , The Inst i tute ofWe lding, London, pp. 1 14-1 24.2. Shirnada,W., an d Uka i, J . Effects ofpulsed current control on welding qual i ty i rn-provement. I IW Docum ent # Xll-B-11-81.

3. Quint in o, L., and Al lurn, C. 1. 1984 .Pulsed GM AW : interact ion between processparameters- ar t II. Weld ing and M eta l Fab-r icat ion, Vo l . 4, pp. 126-129 .4 . Quin t ino , L., and A l lu rn , C. J . 1984 .Pulsed GM AW : interact ion between processparameters- art I . Welding an d Meta l Fab-rication, Vol. 3, pp. 85-89.5. Samati, Z. 1986. Automatic pulsed M IGwelding . Me tal Construct ion, Vol . 18, No.1,pp. 33R-44R.6. Lesnewich, A. 1958. Cont ro l o f me l t -ing ra te and meta l t ransfer i n gas -sh ie ldedmetal-arc welding, Part I- ont ro l of elec-t rode me l t ing ra te . Weld ing Journa l37(8):343-5 to 353-S.7 . Kim, Y. S., and Eagar, T. W. Analys is

of me tal transfer i n gas me tal arc weld ing.We lding Journal 71 6):269-s to 278s..8. Kim, Y. S., and Eagar, T.W. 1989 . Tem-perature d is tr ibu t ion and energy ba la nce inthe electrode durin g GM AW . Proc. of Trendsin W eldin g Research, Gat l inburg, T N.9. Eickhoff, S. T. 1988. Gas-metal arcwelding in pure argon. M.S. thesis, MIT , Cam-bridge, Mass.10 . Alle ma nd, C.D., Schoed er, R., Ries,D.E.,and Eagar, T.W. 1985 . A method of f i lm-ing m eta l t rans fer in the we ld ing arc . Weld-ing Journal 64(1):45-47.11. Kim, Y. S. 1989. Me tal transfer in gasmetal arc welding. Ph.D. thes is , MIT , Cam-bridge, Mass.

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