active metal transfer control by utilizing enhanced ... · metal transfer, the periodical melt-ing...

WELDING RESEARCH

WELDING JOURNAL / AUGUST 2014, VOL. 93282-s

IntroductionMetal transfer, the periodical melt-

ing and droplet forming/detaching,plays a key role in producing qualitywelds in gas metal arc welding(GMAW) (Refs. 1, 2). Typically, themetal transfer is characterized bythree modes: short-circuiting, globu-lar, and spray (Ref. 3). Drop spraytransfer, a subcategory of spray trans-fer with discrete droplet detachments,is generally associated with good arcstability and low spatter and thus is

often considered the preferred trans-fer mode (Refs. 4–6). However, to pro-duce drop spray transfer, the currentneeds to be higher than the spray tran-sition current. The resultant high heatinput, metal vapors, and arc pressuresare not always preferred.

Researchers in the welding commu-nity have dedicated their efforts to de-veloping innovative arc welding meth-ods that produce stable drop spraymetal transfer with lower currents(Refs. 7–13). Pulsed gas metal arcwelding (GMAW-P) is able to produce

drop spray transfer at a wide range ofaverage current, lower than the spraytransition current, by applying apulsed current waveform (Ref. 14). Al-though the average current in GMAW-P can be significantly reduced, thepeak current still must be higher thanthe spray transition current (Refs.15–18). Further, in conventionalGMAW-P, one droplet per pulse(ODPP) transfer is desired, thus thepeak current and duration must beproperly selected to avoid multi dropsper pulse (MDPP) and one drop multi-pulses (ODMP). Hence, the processlacks controllability and robustness. Infact, for ODPP transfer in GMAW-P,the droplet diameter could be slightlylarger than the wire diameter if thepeak current is not higher than thetransition current and the pulsefrequency is relatively low. Strictlyspeaking, the transfer mode in thiscase should be called small globulartransfer.

Active metal transfer control by uti-lizing an excited droplet oscillation isan innovative method to achieverobust drop spray transfer in GMAW-Pwith reduced peak current lower thanthe spray transition current (Refs. 19,20). Its essence lies in theactive/intentional excitation ofdroplet oscillation: an exciting peaklevel is used to produce a relativelyhigh electromagnetic force to elongatethe droplet; once the current is inten-tionally reduced to the base level, theelectromagnetic force decreases signif-icantly such that the surface tensionacts as a spring force to drive the

Active Metal Transfer Control by Utilizing EnhancedDroplet Oscillation Part 1: Experimental Study

The effects of phase delay on droplet detachment and current waveform parameters onthe optimal detaching phase delay were investigated, and the lower limit of the detaching

current was determined

BY J. XIAO, G. J. ZHANG, W. J. ZHANG, AND Y. M. ZHANG

ABSTRACT Active metal transfer control utilizes an excited droplet oscillation to producerobust drop spray/small globular transfer with peak current lower than the transitioncurrent in gas metal arc welding. The excitation is intentionally generated by reducingthe current from a peak level, referred to as the exciting peak, to the base level.Another peak level, referred to as the detaching peak, is then applied after a time interval called the detaching phase delay to synchronize the detaching action with thebeneficial droplet momentum as a phase match such that the droplet detachment isenhanced by the beneficial momentum. In a recent study, the droplet oscillation wassignificantly enhanced through current waveform modification. The active metaltransfer utilizing the enhanced droplet oscillation, referred to as the enhanced activemetal transfer control, was further systematically studied in this investigation. The effect of the detaching phase delay on the droplet detachment was experimentally analyzed first, and the general property of the optimal detaching phase delay for themost enhanced droplet detachment was thus revealed. Consequently, the effect ofthe current waveform parameters on the optimal detaching phase delay wasexamined. Then a series of enhanced active metal transfer control experiments wereconducted by using corresponding optimal detaching phase delay. It was found thatthe minimum detaching peak current in the enhanced active metal transfer controlwas not only remarkably lower than the transition current, but also significantly lower

KEYWORDS• Enhanced Active Control • Metal Transfer • Droplet Oscillation • Detaching Phase Delay • Phase Match • Detaching Current

J. XIAO and Y. M. ZHANG ([email protected]) are with the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, China,and the Institute for Sustainable Manufacturing, University of Kentucky, Lexington, Ky. G. J. ZHANG is with the State Key Laboratory of Advanced Welding andJoining, Harbin Institute of Technology, China. W. J. ZHANG is with the Institute for Sustainable Manufacturing and Department of Electrical and Computer Engineering, University of Kentucky, Lexington, Ky.

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AUGUST 2014 / WELDING JOURNAL 283-s

droplet into oscillation. When thedroplet is moving away from the wiretip during oscillation, a detachingpulse can be applied such that thedroplet downward momentum can beutilized to enhance dropletdetachment and thus the neededdetaching peak current can bereduced.

Apparently, the synchronization be-tween the droplet downward motionand detaching pulse, referred to as thephase match, is a prerequisite foreffective utilization of the beneficialdroplet momentum. Further, thedetaching pulse needs to be synchro-nized with the droplet downward mo-tion during the first excited oscillationcycle to take maximum advantage ofthe downward momentum, becausedroplet oscillation damps rapidly. Ifnot otherwise specified, the droplet

oscillation discussed in this paper allimplies that it is in the first cycle afterexcitation.

The above original version of the ac-tive metal transfer control was firstproposed and verified in Ref. 19 wherea relatively simple current waveformshown in Fig. 1 was used to excite thedroplet oscillation. As can be seen, theexciting pulse is coupled into thegrowing pulse. However, an independ-ent exciting pulse may increase the os-cillation to be taken advantage of.Hence, in a recent study (Ref. 21), sig-nificantly enhanced droplet oscillationwith lower average current wasachieved by applying a modifiedcurrent waveform, as shown in Fig. 2,in which the growing and excitingpulses are separated and thuscontrolled independently. Therelatively narrow exciting pulse was

applied to oscillate the droplet onlywhen the droplet grew to the desiredsize.

The improvements from theenhanced active control are apparent.

1) Reduced average current. The de-coupling of the droplet excited by thegrowing enables independent selectionof current levels and durations. Thegrowing current no longer needs to beas high as the exciting peak current.Much lower current can thus be usedto grow the droplet such that the aver-age welding current can be reduced.

2) Better process controllability. Sincedroplet growing and exciting aredecoupled, the initial dropletsize/mass before exciting can be accu-rately controlled by adjusting thegrowing parameters such that thedroplet oscillation frequency iscontrolled. The exciting parameters

Fig. 1 — Current waveform for original active metal transfercontrol.

Fig. 2 — Modified current waveform for enhanced droplet oscillation.

Fig. 3 — Schematic diagram of experimental system. Fig. 4 — Current waveform for enhanced active metal transfercontrol.

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can thus be effectively adjusted to con-trol the droplet oscillation magnitude.

3) Maximized droplet oscillation mag-nitude. Decoupling also introduces anew important parameter: the excitingrising level IR, which is defined as thedifference between the exciting peakcurrent Ie and the current before excit-ing, which is the base current Ib: IR =Ie – Ib. It was found that a larger excit-ing rising level IR induced strongerdroplet oscillation when the other ex-citing parameters were the same. SinceIb is a minimized base level, IR is thusmaximized; consequently, the dropletoscillation energy was maximized.

4) Droplet pre-oscillation. If thegrowing current is high enough to pre-elongate the droplet, the droplet goesinto a pre-oscillation when the currentis switched from the growing currentto the base current. If the excitingpulse and the downward momentumof the droplet during the pre-oscillation were well synchronized, theoscillation activated by the excitingpulse, referred to as the main oscilla-

tion here, was further enhanced.Although this enhanced droplet os-

cillation with better controllability hasbeen achieved, the resultant activemetal transfer utilizing such enhanceddroplet oscillation, referred to as theenhanced active metal transfercontrol, has not yet been fully studied.To better characterize and understandthe enhanced active metal transfercontrol, the following terminologiesare needed: 1) The moment when theexcited droplet reaches its maximumelongation is referred to as the elonga-tion peak moment; 2) the momentwhen the droplet changes its movingdirection from upward (toward thewire) to downward (away from thewire) is referred to as the oscillationreversing moment; 3) the timeinterval between the end of the grow-ing pulse and the start of the excitingpulse is referred to as the excitingphase delay, denoted as Tp1; and 4) thetime interval between the end of theexciting pulse and the start of the de-taching pulse is referred to as the de-

taching phase delay, denoted as Tp2.Further, if the exciting/detachingpulse starts exactly at the reversingmoment during the pre-oscillation/main oscillation, the result-ant exciting/detaching phase delay isreferred to as the feature exciting/de-taching phase delay, denoted as Tp1and Tp2, respectively. Since it was veri-fied that the pre-oscillation under rela-tively low growing current wasrelatively weak, Tp1 is fixed and notdiscussed in this experimental study.

Since an effective utilization of themain oscillation depends on the selec-tion of detaching phase delay, the opti-mal detaching phase delay resulting inmaximum enhancement on thedroplet detachment was a focus of thisstudy. Further, since the goal is to re-duce the detaching peak current, theminimum detaching current for stabledrop transfer under the optimaldetaching phase delay was alsostudied.

Thereby, this work entailed the following:

Fig. 5 — Illustration of applicable range of Tp2.

Fig. 6 — Current waveform and metal transfer in experiment 3. Tp2 =2.8 ms.

Fig. 7 — Current waveform and metal transfer in experiment5. Tp2 = 3.2 ms.


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1) Examined the effect of detachingphase delay on the droplet detachment todetermine the optimal detaching phasedelay. Even though the detachingphase delay varies in a certain range,the phase match condition may still besatisfied. However, the resultantenhancement on the dropletdetachment should be of differentmagnitudes. The experimental resultsdemonstrated that the feature detach-ing phase delay can be considered asthe optimal selection. Further, the ro-bustness of the enhanced active metaltransfer control was also evaluated.

2) Examined the effects of currentwaveform parameters on the optimal de-taching phase delay. Apparently, the op-timal detaching phase delay is affectedby the growing and excitingparameters, because the oscillation pe-riod and initial phase depends onthese waveform parameters. Hence, itwas necessary to study their effects onthe optimal detaching phase delay.

3) Determined the lower limit of thedetaching current. Lower effectivedetaching current enlarges the param-eter range in the enhanced activemetal transfer control. Itscontribution to reduce the heat inputand weld fume is also appreciated. Inthis sense, the lower limit of thedetaching peak current/duration forstable spray/small globular transfer inthe enhanced active control needs tobe explored. A series of experimentswere conducted with optimal detach-ing phase delays to determine thelower limit of the detaching current.The results were compared with thoseof the original active control todemonstrate the advantage of the en-hanced active control.

ExperimentalSystem and ApproachExperimental System

Figure 3 shows the experimentalsystem including the welding cell, dataacquisition system, high-speedcamera, and controllers. The powersource works in constant currentmode. The arc length was controlled tobe stable by regulating the wire feedspeed based on arc voltage feedback.An embedded controller was designedto compute the current output wave-form and adjust the wire feed speed.Considering the droplet detachmentmay be very sensitive to the detachingphase delay, a new timer was designedto precisely control the current outputduration in 0.1-ms resolution. Thedata acquisition system and high-speed camera were triggered by thesame 5-V TTL signal, thus the record-ing of the actual current waveform andmetal transfer were synchronized. Therecording frequency was set at 5000Hz. All the welding experiments wereconducted as bead-on-plate weldingwith 3 mm/s travel speed and 15L/min pure argon shielding gas. Thebase metal was mild steel; the wire wasER70S-6 with 0.8 mm diameter; andthe distance from the contact tip tothe workpiece was set at 12 mm. If nototherwise specified, the arc length inthe experiments was approximately 6 mm.

Approach

Two current waveforms were usedto conduct the experiments: 1) the one

shown in Fig. 2, denoted as Wave1; 2)the one inherited from Wave1 byinserting a relatively low detachingpulse with a detaching phase delay, de-noted as Wave2 and as shown in Fig. 4.The forced detaching pulses in thesetwo waveforms were used to eliminatepossible droplet mass accumulation,which may affect the analysis of exper-imental results. In practical implemen-tation of the enhanced active metaltransfer, the forced detaching pulsewas not required. Since the forced de-taching pulse is fixed, all detaching pa-rameters discussed below imply thosefor the first lower detaching pulse inWave2.

Wave1 allowed full observation ofthe enhanced droplet oscillation tomeasure key variables depicting theoscillation such as the feature detach-ing phase delay, the oscillation period,the damping speed, and the springconstant of the droplet. The verticalcoordinate of the droplet’s lower edgewas automatically calculated by offlineimage processing to track the dropletoscillation and detachment. The aver-age of the concerned oscillationvariables thus were measured from therecorded current waveforms and theobtained dynamic droplet positioncurves. For Wave2, it was used to con-duct the resultant enhanced activemetal transfer control experiments bypresetting a certain detaching phasedelay. In all the experiments below,the growing parameters were fixed at80 A with 20 ms duration, the basecurrent is at 30 A, and the excitingphase delay is 2 ms. Thus, the initialdroplet diameter before the exciting


Fig. 10 — Detached droplet velocity under different Tp2.


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pulse in all the experiments was con-trolled to be 1.2 mm approximately,which is slightly larger than the wirediameter. Therefore, the metal transferdiscussed in this paper should becalled small globular transfer.

Effect of Detaching Phase Delayon Droplet Detachment

Estimation on Detaching Phase Delay

To determine the optimal detachingphase delay for maximizedenhancement on droplet detachment,the enhanced active metal transferwith different Tp2 around the featurevalue T*

p2 needs to be studied. Denotethe applicable detaching phase delayrange as

Tp2∈[T*p2 – δ1, T*p2 + δ2] (1)

where δ1 and δ2 define the tolerancerange. δ1 corresponds to the period P1illustrated in Fig. 5, while δ2

corresponds to P2 and part of P3. Itcan be seen that the droplet keepsmoving toward the weld pool duringperiod P2 and P3. The phase matchcondition may be satisfied by applyingthe detaching pulse within P2 and P3.Hence, δ2 is estimated to be longerthan T1/4, but shorter than T1/2,while δ1 should be much shorter. Intheory, if the detaching pulse isapplied within P1, during which thedroplet is still moving toward the wire,the droplet oscillation would be signif-icantly weakened, thus the dropletcannot be detached by a relatively lowdetaching peak current. The reason forpotential tolerance δ1 is that it gener-ally takes about 1-ms rising time forthe welding current to be fullyswitched from the base to peak.Hence, for an adequately short periodahead to the reversing moment, thewelding current probably has notreached the level that can effectivelyaffect the droplet motion. In thissense, δ1 is estimated to be 0.5 ms be-cause the current generally reaches 80A in approximately 0.5 ms, and thenthe current becomes sufficient to gen-erate relatively larger electromagneticforce but the droplet starts to moveaway from the wire. Thus, the phasematch may not be affected.

Experiments and Results

The feature detaching phase delayTp2 under 80 A/20 ms growing pulseand 120 A/3 ms exciting pulse wasfirst measured to be 3.16 ms by usingWave1. Then, experiments 1–10 using

Wave2 were conducted to examine theeffect of detaching phase delay on thedroplet detachment in the enhancedactive metal transfer control. Theexciting peak current was fixed at 120A with 3-ms duration while thedetaching current was fixed at 140 A,which is much lower than thetransition current of 165 A. Its dura-tion is set at 6 ms. The detachingphase delay changes around the meas-ured Tp2, as listed in Table 1.

Figures 6–9 show the typical metaltransfers of experiment 3, 5, 7, and 9,where Tp2 corresponds to P1, P2,reversing moment, and P3,respectively. The time takes the firstframe as the reference. To better eval-uate the droplet detachment under adifferent detaching phase delay, theaverage droplet velocity in 1 ms afterits detachment was measured from therecorded high-speed image sequences.The measurement results are shown inFig. 10.

Analysis

Applicable Range of DetachingPhase Delay

It can be seen from Fig. 10, the ap-plicable detaching phase delay Tp2varies from 2.6 to 5 ms. For 3.6–5.0ms Tp2, longer than the feature valueT*p2 3.16 ms and corresponding to pe-riod P2 and P3, the droplet can beeffectively detached. When the detach-ing phase delay was further increasedto 5.5 ms in experiment 10, thedroplet cannot be detached. δ2 wasthus close to 2 ms. For 2.6–3.0 ms Tp2,

Fig. 11 — Tp2 measured from experiments 11–22. Fig. 12 — Standard deviation of Tp2 in experiments 11–22.

Table 1 — Used Detaching Phase Delay inExperiments 1–10

No. Tp (ms) Corresponding Period

1 2.2 P12 2.6 P13 2.8 P14 3.0 P15 3.2 Reversing moment6 3.6 P27 4.0 P28 4.5 P29 5.0 P3

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which is shorter than the feature value3.16 ms and corresponds to the periodP1, the droplet can also be effectivelydetached. Hence δ1 was determined tobe 0.56 ms, agreeing with the previousestimation.

Optimal Detaching Phase DelayAlthough the droplets all can be de-

tached by only 140-A peak currentwhen the detaching phase delay rangesfrom 2.6 to 5 ms, the detaching abilityof the detaching pulse, in anotherwords, the enhancement on thedroplet detachment from the dropletoscillation, are of different amplitudes.Hence, the optimal detaching phasedelay that most enhanced the dropletdetachment can be determined fromFig. 10. As can be seen, the detacheddroplet velocity is quite similar at themaximum level with 3 and 3.2 ms Tp2,while the other longer and shorter Tp2both result in weaker droplet detach-ment. It is thus clearly confirmed thatthe feature detaching phase delay Tp2corresponding to the oscillationreversing moment is the optimal for

the enhancedactive metal trans-fer control.

The differencein the detacheddroplet velocity ex-ists because the ef-fect of the electro-magnetic forceproduced by thedetaching pulse onthe dropletchanges with thedetaching pulsestarting moment.

1) Weakening effect. During P1, therising electromagnetic force mayweaken the droplet oscillation slightlyor considerably, depending on thetime length ahead to the reversing mo-ment that divides P1 and P2. If theahead time is adequately short, the os-cillation weakening effect is negligible.

2) Accelerating effect. During P2,where the droplet was compressed andmoving away from the wire, thedroplet would be effectivelyaccelerated by the detachingelectromagnetic force, and thus thebeneficial momentum was increased.

3) Detaching effect. During P3,where the droplet has been elongated,the downward momentum of thedroplet can be combined with the elec-tromagnetic force to detach thedroplet.

If the detaching pulse is exactly ap-plied at the reversing moment, the ac-celerating time is maximized such thatthe beneficial downward momentumis maximized and a sufficient durationextending the detaching pulse into P3will add further detaching force.

Hence, the detached droplet velocityunder 3.2 msTp2, close enough to thefeature value 3.16 ms, is measured atthe maximum level. For 3 ms Tp2, thedetaching start moment is only 0.16ms ahead to the reversing momentsuch that the weakening effect is negli-gible while the accelerating time is notcompromised. The droplet velocityunder 3 ms Tp2 is thus also at the max-imum level. For further decreased Tp2,such as 2.8 and 2.6 ms, the weakeningeffect increases significantly, thus thedetached droplet velocity decreasessharply. On the other hand, if the de-taching pulse starts in P3, the dropletdownward momentum only comesfrom the excited droplet oscillationbut not further increased by thedetaching pulse. The dropletdetachment is thus weaker, and thevelocity of the detached droplet with 5ms Tp2 is quite smaller than those with3.6–4.5 ms Tp2.

Process RobustnessAlthough the results from

experiments 2–9 have already provedthat the detaching current can beeffectively reduced by using theenhanced active metal transfercontrol, the robustness of thecontrolled process has not been stud-ied. Since the phase match is the pre-requisite condition for the enhancedactive metal transfer control, this ro-bustness is ensured primarily by 1)sufficiently small fluctuation of the op-timal detaching phase delay undergiven current waveform parameters;2) adaptiveness of the droplet detach-ment to varying detaching phasedelay. Hence, the standard deviation

Fig. 13 — Current waveform and droplet position in experiments14–16. A — Experiment 14; B — experiment 15; C — experiment 16.

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of T*p2 in each experiment from 2 to 9

was calculated by

(2)

It was found that the maximumstandard deviation of T*

p2 in these ex-periments was only about 0.1 ms.Such a fluctuation in T*

p2 issufficiently low. In fact, as can be cal-culated from Fig. 10, with 3.0 ms Tp2that is 5.1% shorter than theoptimal/feature value 3.16 ms, thedetached droplet velocity is still atthe maximum level. For 3.6 ms Tp2that is 13.9% longer than the optimalvalue 3.16 ms, the droplet velocity isonly reduced by 0.9%. Even when Tp2is further increased by 44% to 4.5 ms,the detached droplet velocity is onlyreduced by 9%. Overall, the two con-ditions for strong robustness of theenhanced active metal transfercontrol are both satisfied. Of course,the above discussion is limited to therobustness under given excitingparameters. The standard deviationof Tp2 among different sets ofexciting parameters is furtherexplained in the next section.

Effect of Exciting Parameters onOptimal Detaching Phase Delay

As can be seen, the feature detach-ing phase delay Tp2 is the optimal formaximal enhancement on the dropletdetachment. On the other hand, thisdelay Tp2 depends on the oscillationperiod and initial phase. If the initial

phase is the same,Tp2 increases withthe oscillation pe-riod. Since the os-cillation period hasbeen studied andgrowingparameters deter-mine theoscillation periodthrough thedroplet mass (Ref.21), this sectionturns attention to the exciting param-eters, which may significantly affectthe initial phase. Such effects can bedemonstrated by the phase offset be-tween the exciting end moment andthe elongation peak moment, denotedas Toff. Depending on the exciting pa-rameters, the phase offset may be neg-ative, zero, or positive, correspondingto different initial phases in theexcited oscillation.

Experiments 11–22, using Wave1,were conducted to study the effect ofthe exciting parameters on the optimaldetaching phase delay. A completemetal transfer cycle being studied hereincludes four substages: droplet grow-ing, exciting, oscillating, and forced de-taching. The varying parameters arelisted in Table 2. It can be seen that theexciting peak current Ie ranges from 80to 140 A. For each used exciting peakcurrent, the exciting peak duration Te isset at three levels: 2, 3, and 4 ms, whichare the recommended selections fromthe previous work (Ref. 21).

The optimal detaching phase delayTp2 in experiments 11–22 wasmeasured. To minimize the

measurement error, averages were cal-culated from ten consecutive metaltransfer cycles in each experiment. Theresults are shown in Fig. 11. It can beseen that the optimal detaching phasedelay increases with the exciting peakcurrent. This is because the initialdroplet mass at the exciting endmoment increases with the excitingpeak current. The standard deviationsof Tp2 in each of these experimentswere also calculated. Figure 12 showsthat the standard deviations of Tp2 inexperiments 11–22 were all sufficientlysmall. The maximum standard deviationis only approximately 0.1 ms. Such a lowfluctuating level further suggests suffi-cient robustness of Tp2 under given cur-rent waveform parameters.

However, with the same excitingpeak current, the optimal detachingphase delay decreased when the excit-ing peak duration increased from 2 to4 ms, even though the initial dropletmass under a longer exciting peak du-ration was slightly greater. This resultclearly confirms the existence of thephase offset between the exciting endmoment and elongation peak

∑( )( )σ = −∗ ∗

=

1N

T i Tp p2

i 1

N

Fig. 14 — Demonstration of phase offset in experiments 14–16.A — Experiment 14; B — experiment 15; C — experiment 16.

A

C

B

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moment, which means the initialphase of the oscillation wassignificantly affected by the excitingpeak duration. It also can be seen thatthe three lines in Fig. 11 are approxi-mately parallel, indicating that thephase offset is mainly determined bythe exciting peak duration but almostindependent to the exciting peakcurrent.

Take the results from experiments14–16 to demonstrate the phaseoffset. The recorded currentwaveforms and measured dynamicdroplet position curves given by Fig.13, especially its zoom-in view Fig. 14,clearly demonstrate the phase offsetphenomenon. The first data point ofthe curves in Figure 14 represent thestart moment of the exciting pulse.For experiment 14, the exciting peakduration was 4 ms. Fig. 14A showsthat the droplet starts to spring backbefore the end of the exciting pulsesuch that Tp2 < T1/2. For experiment15, the exciting peak duration was 3ms. It can be seen in Fig. 14B that the

end of the exciting pulse and the startof droplet oscillation are well synchro-nized. There is no obvious phaseoffset. Thus,Tp2 = T1/2. In experiment16, the exciting peak duration was 2ms. It can be seen in Fig. 14C that thedroplet elongation continues toincrease after the end of exciting dueto the droplet inertia. In this case, Tp2> T1/2.

Minimization of Detaching Current

Experiments

Minimization of the detaching cur-rent increases the application rangeand controllability of the active metaltransfer. Further, a lower detachingcurrent also contributes to reducingthe heat input, weld distortion, andweld fume. The minimization studywas conducted with experiments using

Wave2, with optimal detaching phasedelays obtained from experiments11–22. For each used exciting peakcurrent, the metal transfer under dif-ferent detaching current and durationwas analyzed. While the exciting peakduration was fixed at 3 ms, theexciting peak current changes from 80to 140 A at 20-A increments. For eachused exciting peak current, the detach-ing current decreases from 150 A. Thedetaching peak duration changes inthree levels: 3, 4, and 6 ms. As a result,a large number of experiments wereconducted, and the minimum detach-ing peak current/duration in theenhanced active metal transfer controlwas thus determined. Because the ex-periments were extensive, only thosedemonstrating the minimum detach-ing current/duration are listed in Table3. In the table, “Yes” means thedroplet can be robustly detached bythe used detaching parameters, while“No” means the droplet almost cannotbe detached at all. “Partially” meansthe droplet can be detached, but notalways assured.

Minimum Detaching Peak Duration

Not only the detaching peakcurrent should be minimized, shorterdetaching peak duration is also appre-ciated for reducing the heat input. Tothis end, the minimum detaching du-ration was further analyzed from theexperimental results. According to theresult of experiment 30, the dropletoscillation under 140-A exciting peakcurrent was sufficiently strong. As aresult, stable drop transfer was

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Fig. 15 — Minimum detaching current under different exciting peak currents.

Fig. 16 — Droplet oscillation and detachment in enhanced active metal transfercontrol. Ie = 120 A, Te = 3 ms, Id = 120 A, Td = 6 ms, 1 ms per frame.

Table 2 — Exciting Parameters in Experiments11–22

No. Ie (A) Te (ms)

11 140 412 140 313 140 214 120 415 120 316 120 217 100 418 100 319 100 220 80 421 80 322 80 2

Table 3 — Parameters and Results in Experiments 23–36

No. Ie (A) Id (A) Td (ms) Detached

23 140 120 6 Yes24 140 115 6 No25 120 120 6 Yes26 100 125 6 Yes27 100 120 6 Partially28 80 135 6 Yes29 80 130 6 Partially30 140 120 4 Yes31 120 125 4 Yes32 100 130 4 Yes33 100 125 4 Partially34 80 140 4 Yes35 80 135 4 Partially36 140 150 3 No

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achieved under only a 120-A detachingcurrent with 4-ms duration. However,it was seen from experiment 36 thatthe exciting peak current was also 140A, while the detaching current signifi-cantly increased to 150 A, but thedroplet could not be detached despitethe sufficient droplet oscillation anddetaching current, because the detach-ing peak duration was reduced from 4to 3 ms. Hence, the lower limit of ap-plicable detaching peak duration in theenhanced active metal transfer controlwas determined to be 4 ms.

Minimum Detaching Peak Current

From experiments 23–36, the mini-mum detaching peak currents underdifferent exciting parameters were de-termined — Fig. 15. The typical metaltransfer of experiments 25 and 31were selected to give representativeviews of the droplet oscillation and de-tachment in the enhanced active metaltransfer control, as shown in Figs. 16and 17, where the time intervalbetween each frame is 1 ms. It can beseen from frames 2–4 in these two fig-ures, the droplets are effectively elon-gated by the exciting pulses, and thedetaching pulses are then applied rightafter the droplets start to move awayfrom the wire tips; therefore, the oscil-lation was fully utilized and thedroplets were detached at lowcurrents. In particular, frame 11 inFig. 17 shows that the current startsto reduce to the base level so that theelectromagnetic force starts todecrease significantly; however, thedroplet still can be detached, indicat-

ing that the droplet inertia here wassufficiently strong to overcome thesurface tension, since the droplet wasfirst oscillated by the exciting pulseand then further accelerated by the de-taching pulse.

It can be seen from Fig. 15 that theminimum detaching current 120 A (ex-periments 23, 25, 6 ms detaching peakduration) in the enhanced active metaltransfer is significantly lower than 165A, which is needed in conventionalGMAW-P to detach droplets with thesame size, and is also the transition cur-rent tested under the same welding con-ditions as described earlier. For only 4ms detaching peak duration, the detach-ing peak current needed in conventionalGMAW-P (single pulse) to detachdroplets at the same size increased toover 180 A; however, in the enhancedactive control, it was only 125 A (experi-ment 31).

It also can be seen from Fig. 15 thatthe minimum detaching current in theenhanced active metal transfer controldecreased as the exciting peak currentincreased. However, such decrease be-comes less significant as the excitingpeak current increases and eventuallystops. The selection of the excitingpeak current should be based on theresultant reduction in the minimumdetaching peak current such that thegreater of the exciting peak currentand corresponding minimum detach-ing current be minimal.

Processes Comparison

As a comparison, the original activemetal transfer control was studiedusing the current waveform shown in

Fig. 18, denoted as Wave3. Differingfrom Wave2, the droplet growing andexciting in Wave3 are coupledtogether. In the original active metaltransfer control, the growing/excitingcurrent needs to be relatively high toguarantee effective droplet elongation.Hence, its growing/exciting currentwas set at 140 A, which equals thehighest exciting peak current used inthe enhanced active controlexperiments. Its growing/exciting du-ration was set at 15 ms to keep thedroplet mass before detaching beingthe same with that in the comparablyenhanced active control. The minimumdetaching current to achieve stabledrop transfer under 4 and 6 ms dura-tion was found to be 150 and 140 A,respectively. While their counterpartsin the enhanced active metal transfercontrol are only 125 and 120 A,respectively. The advantage of the en-hanced active metal transfer control inreducing the detaching peak currentwas thus clearly demonstrated.

Although the new enhanced activemetal transfer control shows obviousadvantage in reducing the heat inputand fume generation, the transfer fre-quency and deposition rate is compro-mised, because the droplet diametergenerally needs to be slightly largerthan the wire diameter to guaranteesufficient droplet oscillation energy.Fortunately, the deposition rate is notthe key issue in applications requiringlow heat input, such as the welding ofthin sheets. On the other hand, well-established short-circuiting transferprocesses, such as Surface TensionTransfer (STT) and Cold Metal Trans-fer (CMT), also provide low heat input

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Fig. 17 — Droplet oscillation and detachment in enhanced active metal transfercontrol. Ie = 120 A, Te = 3 ms, Id = 125 A, Td = 4 ms, 1 ms per frame.

Fig. 18 — Current waveform for original active control.


to the base metal; however, either thewelding power source or the wirefeeder of these technologies iscomplexly designed and thus costly.The enhanced active metal transfercontrol just needs a common CCpower source and a computer to con-trol its output amperages via a generalanalog interface.

ConclusionsActive metal transfer control

utilizing enhanced droplet oscillationwas systematically studied. The effectof the detaching phase delay on thedroplet detachment was examined,and the optimal detaching phasedelay for maximum enhancement onthe droplet detachment wasdetermined. The effect of the excitingparameters on the optimal detachingphase delay was then examined. A se-ries of experiments were conductedto determine the lower limit of thedetaching current using the optimaldetaching phase delays. The mainconclusions are as follows:

1) Moderate deviation of thedetaching phase delay from thefeature detaching phase delay maystill yield an effective phase match toutilize the droplet oscillation, but thefeature detaching phase delay resultsin maximum enhancement on thedroplet detachment. It is thus deter-mined to be the optimal selection forenhanced active metal transfercontrol.

2) The existence of the small rangeof the detaching phase delay, withinwhich the maximum enhancement isstill achieved, ensures the needed ro-bustness for enhanced active metaltransfer control.

3) Given other waveformparameters, the optimal detachingphase delay decreases as the excitingpeak duration increases because theinitial phase of the excited oscillationchanges with the exciting peakduration.

4) The minimum detaching currentto achieve stable small globular trans-fer in enhanced active metal transfer isdetermined to be not only significantlylower than that in conventionalGMAW-P, but also much lower thanthat in the original active metal trans-fer process. The range of applicablecurrent waveform parameters is thuseffectively enlarged. The advantage ofenhanced active metal transfer controlis also clearly confirmed.

This work was financially supportedby the State Key Laboratory ofAdvanced Welding and Joining,Harbin Institute of Technology,Harbin, China, and the NationalScience Foundation under grantCMMI-0825956. Jun Xiao greatly ap-preciates the scholarship from theChina Scholarship Council (CSC) thatfunded his visit to University of Ken-tucky to conduct this research.

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References


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