arc characteristics of ultrasonic wave-assisted gmaw · ultrasonic wave-assisted gas metal arc...

375-sWELDING JOURNAL

WELDING RESEARCH

Introduction

Gas metal arc welding (GMAW) hasbeen widely employed in manufacturing,and has attracted extensive attention dueto its advantages, which include high effi-ciency, high flexibility, and adaptability forwelding most metals (Ref. 1). The stabilityand quality of GMAW is highly related tometal transfer, which is affected by manyfactors (Refs. 2, 3). Therefore, controllingand getting the ideal metal transfer is stilla challenge. Many researchers have donesome distinctive work in this field. In Ref.4, a new method to employ the meltingrate, heat input, and detaching droplet di-ameter as controlled variables to controlheat and mass transfer was proposed. TheTrifarc method used an extra wire with re-versed electric current, which was insertedbetween the electrodes of the standard tocontrol the molten droplet and pool (Ref.5). In Ref. 6, the researchers struck an ad-ditional arc on the droplet, which served asthe cathode to regulate the current distri-bution and metal transfer. Y. M. Zhang

and others (Refs. 7–10) decoupled the un-desired dependence of the metal transferon the welding current and used a laser tofacilitate droplet detachment from thewire tip. In addition, plasma diagnostics(Ref. 11), arc light and spectrum (Refs. 12,13), magnetic field (Ref. 14), mechanicalvibration (Refs. 15, 16), and arc sound(Refs. 17, 18) also have been used to con-trol the metal transfer process. In Refs. 19 and 20, the authors pro-

posed a new hybrid welding method, ul-trasonic wave-assisted gas metal arc weld-ing (U-GMAW). During the U-GMAWprocess, ultrasonic radiation force wasused to control the metal transfer. The ex-perimental results showed the dimensionof the droplet decreased and the transferfrequency increased, which led to deeperwelding penetration and finer grain crys-tallization. The welding arc has importantinfluences on the metal transfer process;however, the characteristics of the weldingarc in the novel U-GMAW process have

not been systematically studied and discussed.It is well known that the arc is the ulti-

mate source of the welding process andhas the decisive influence on metal trans-fer as well as fusion of the base metal. Dur-ing this research, it was also found that theultrasonic wave not only had an obviouseffect on the metal transfer but also on thewelding arc. Under the action of the ultra-sonic wave, it was obvious that the arccontracted and its energy density in-creased. The purpose of this study is to re-veal the relationship and the action mech-anism of the main ultrasonic parameterson the welding arc, such as the distance be-tween the ultrasonic radiator and theworkpiece, the ultrasonic vibration ampli-tude, and the action area of the ultrasonicradiator. The results are helpful in com-prehensively understanding the U-GMAW method.

Experimental Setup and Parameters

Experimental Setup

The U-GMAW system includes threecomponents, i.e., an ultrasonic powersource, a welding power source, and a hy-brid welding torch, as shown in Fig. 1 (Ref.19). The main body of the torch can be di-vided into two parts, i.e., the ultrasonictransducer and the ultrasonic horn. Theultrasonic transducer transforms electricenergy into ultrasonic vibration, and thenthe vibration is amplified by the ultrasonichorn. The ultrasonic wave radiates outfrom the end of the ultrasonic horn (ultra-sonic radiator in Fig. 1). The welding wireis fed through the axial hole of the trans-ducer and the horn, and the arc is ignitedbetween the wire tip and the workpiece.Since the wire does not contact any vi-brating component, no vibration is con-ducted directly from the wire to the arc.In order to conveniently observe the

arc, bead-on-plate welding experimentswere conducted. During welding, theworkpiece also acts as the ultrasonic re-flector. Since the dimension of the work-piece is much larger than the ultrasonic ra-diator, the workpiece can be regarded as

Arc Characteristics of Ultrasonic Wave-Assisted GMAW

The arc characteristics vary when an ultrasonic wave is added to the gas metal arc welding process

BY C. L. FAN, C. L. YANG, S. B. LIN, AND Y. Y. FAN

ABSTRACT

Ultrasonic wave-assisted gas metal arc welding (U-GMAW) is a newly devel-oped welding method. Under the action of the ultrasonic wave, the characteris-tics of the welding arc make an obvious change. Compared with the conventionalGMAW arc, the U-GMAW arc is more contracted and becomes brighter, and itslength is decreased. The arc length varies wavelike with the height of the ultra-sonic radiator. The reason is that the amplitude of the stationary ultrasonic wavepressure varies with the phase difference between the incident wave and the re-flected wave. Under the same conditions, the ultrasonic energy and the contrac-tion degree of the arc are enhanced with the increase in the diameter of the ul-trasonic radiator and the ultrasonic vibration amplitude. In addition, the arclength in both GMAW and U-GMAW increases with increasing voltage. But atthe same voltage, the arc length in U-GMAW is shorter than with GMAW, andthe difference increases with the increasing voltage. For U-GMAW, the unit in-crease in arc length with increased voltage is only about one-third that of conven-tional GMAW.

KEYWORDS

Gas Metal Arc WeldingArc CharacteristicsUltrasonic Wave

C. L. FAN ([email protected]), C. L. YANG,and S. B. LIN ([email protected]) are with theStake Key Laboratory of Advanced Welding andJoining, Harbin Institute of Technology, Harbin,China. Y. Y. FAN is with Dongfang Electric Ma-chinery Co., Ltd., Deyang, China.

Fan Supplement December 2013_Layout 1 11/14/13 4:06 PM Page 375

an ideal infinite reflector. With the reflec-tion, a stable interference acoustic field isformed between the radiator and theworkpiece, and the arc is burning insidethis acoustic field.

The ultrasonic power source was aCSHJ-1000 with the output power of 1000W. A digital control, constant voltagepower supply (Kemppi, Promig 500) wasemployed, and the welding process wasperformed under the direct current elec-trode positive (DCEP) condition. Duringthe experiments, the welding torch wasfixed and the workpiece was moved at aconstant speed. The base metal was mildsteel, and 1.2-mm-diameter ER70S-6 wirewas chosen as the electrode. Pure argon

was used as the shieldinggas. The other welding pa-rameters are shown inTable 1.

High-speed photogra-phy and a laser shadowingmethod were used to in-vestigate the welding arc,

as shown in Fig. 2. The camera was an Op-tronis CamRecord D5000×2. The wave-length of the laser beam used as the backlight is 808 nm, and a bandpass filter withthe same wavelength was mounted in frontof the lens to strain off the arc light. Witha shutter speed of 20 μs, the camera’sframe rate is 3000 f/s at an image resolu-tion of 512 × 512 pixels.

Experimental Parameters

There are four main ultrasonic parame-ters related to the experiments, i.e., f (ultra-sonic frequency), H (distance between theultrasonic radiator and the workpiece), ξ

(ultrasonic vibration amplitude), and D (di-ameter of the ultrasonic radiator), as shownin Fig. 3. In this study, according to the fre-quency of the ultrasonic piezoelectric trans-ducer, f was fixed at 20 kHz. The workingvoltage and current of the ultrasonic powerwere 220 V and 0.5 A, respectively. Of allthese parameters, f and ξ are mainly con-cerned with the state of the incident wave.

The characteristic parameters of thearc shape are L (arc length), Dp (projecteddiameter), and DR (root diameter) (Ref.21), as shown in Fig. 4. During the GMAWprocess, because of the interference of themetal transfer, the arc shape is not invari-able. Many factors, such as the form andtransition of the droplet, have influenceon it. By long-term observation, it wasfound that during a droplet transition pe-riod, Dp and DR were changed violentlywhile the variation of L was very slight.Therefore, in the following study, L is se-lected as the main characteristic parame-ter to indicate constriction of the arc.

DECEMBER 2013, VOL. 92376-s

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Fig. 1 — Schematic of U-GMAW (Ref. 19). Fig. 2 — Schematic diagram of the experiment system.

Table 1 — Experimental Parameters

Wire Feed Welding Welding Flow Rate of the Distance from the Contact Distance from the Nozzle Speed Voltage Speed Shielding Gas Tube to the Workpiece to the Workpiece

(wfs, m/min) (V) (mm/min) (L/min) (mm) (mm)

3.5 27 300 25 24 11

Fig. 3 — Parameters of the ultrasonic system.

Fig. 4 — Parameters of the arc shape.


Results and Discussion

The Effect of the Ultrasonic Parameters

The Distance between the Ultrasonic Radiatorand the Workpiece

Figure 5 shows the differences betweenthe conventional GMAW arc and the U-GMAW arc. From the figures, it can beseen that compared with conventionalGMAW, the U-GMAW arc is contractedand becomes brighter. The relationshipbetween L and H is shown in Fig. 6, andthe fitting curve is wavelike rather thanlinear, which typically represents the in-fluence of the acoustic field. It can also beseen that the contraction degree of the U-GMAW arc varies with H. Under the H of20 and 30 mm, the arc is shorter than thatunder the other H values.

The incident ultrasonic emitted fromthe radiator can be expressed as waveEquation 1 (Ref. 22):

pi = pAcos(ωt – φ1) (1)

where pi is the ultrasonic pressure, pA isthe amplitude of pi, ω is the angular fre-

quency, t is time, and ϕ1 refersto the initial phase of the ultra-sonic wave.When the incident ultrasonic

wave hits the reflector (theworkpiece) surface, the re-flected wave is generated. Ifneglecting the attenuation of

the ultrasonic wave, the reflected wave canbe expressed as Equation 2, where pr is thereflected wave pressure, ϕ2 is the phase ofthe reflected ultrasonic wave, and ϕ2 = ϕ1+ ψ. ψ is the phase difference between theincident wave and reflected wave.

pr = pAcos(ωt+ ϕ2) (2)

The virtual ultrasonic field between theultrasonic radiator and the workpiece isthe interfered result of the incident ultra-sonic wave and the reflected ultrasonicwave, which can be called the stationaryacoustic field. This field is given in Equa-tions 3 and 4, where pC is the amplitude ofthe stationary ultrasonic wave pressure,and ϕ refers to the phase of the stationaryultrasonic wave.

p = pi + pr = pCcos(ωt – ϕ)pC2 = 2pA2 + 2pA2cos(ψ) (3)

From the equations, it can be seen thatthe stationary ultrasonic wave is still an ul-

trasonic wave with the same vibration fre-quency as the incident ultrasonic wave.The amplitude of the stationary ultrasonicwave depends on the phase difference (ψ)between the incident wave and the re-flected wave, instead of the algebraic sumof their absolute values.

The most important influence factor ofψ is the distance between the radiator andthe workpiece, i.e., H. Under some spe-cific H values, ψ is even multiple times ofp. The incident ultrasonic wave and the re-flected wave reach the peak value simulta-neously, and the stationary ultrasonicwave has double amplitude of the incidentultrasonic wave where the stationaryacoustic field is a syntonic field with themaximum energy density. While undersome other specific H values, ψ is odd mul-tiple times of p, the incident ultrasonicwave and the reflected wave canceled out,which means the amplitude of the station-ary wave is zero and the ultrasonic fieldenergy density is minimum.

The particles inside the normal arc,such as the electrons and ions, are movedby the drive of the electric field betweenthe anode and cathode. But the situationchanged inside the U-GMAW arc; the ad-ditional ultrasonic wave will influence themotion state of the particles. Beside themotion caused by the electric field, theparticles are forced to oscillate aroundtheir equilibrium position 20,000 timesper second. This vibration increases theinstantaneous velocity and the collision

ϕϕ ϕϕ ϕ

=++

arctansin sincos cos

(4)1 2

1 2


WELDING RESEARCH

Fig. 5 — Comparison of conventional GMAW and U-GMAW arcs. A — Conventional GMAW arc; B — U-GMAW arc.

A B

Fig. 6 — Relationship between H and L.

Fig. 7 — Ultrasonic horns with different end diameters.

Fan Supplement December 2013_Layout 1 11/14/13 4:06 PM 77

probability of the particles, which willsurely increase their thermal conductivity.In other words, more heat will be trans-ferred to the workpiece, and this will breakthe equilibrium of the general heat pro-duction and dissipation. According to theprinciple of minimum voltage (Ref. 2), thearc has to constrict to reduce the heat loss.On the other side, the arc must increasethe electric field strength to remain the in-creasing particle collision probability andgenerate more heat to compensate for theheat loss. The increasing electric fieldstrength leads to the increasing powerdensity along the arc, and that is why theU-GMAW arc is brighter than the GMAWarc. A similar phenomenon has also beenobserved during ultrasonic-assisted gastungsten arc welding (U-GTAW) (Ref.23). However, the arc length in the GTAWprocess is stable between the nonconsum-able electrode and the workpiece. This isdifferent from the U-GMAW process.If the H value is changed continuously,

as shown in Fig. 6, ψ will vary synchro-nously, which causes continuous variationof the stationary acoustic field energy den-sity. Under some certain H values (e.g., H= 20 or 30 mm), the incident ultrasonic

wave and the re-flected wave res-onated, and the sta-tionary acousticfield reaches to themaximum energydensity, thus theconstriction degreeof the arc reaches tothe peak value.From Fig. 6, it canbe seen that whenthe H value equals20 or 30 mm, thecontraction degreeof the arc reachesmaximum, and thearc length is onlyabout 43–48% ofthat of the conven-

tional GMAW arc (Fig. 5A), respectively,and the variation curve of the L-H reachesthe trough area.Though the degree of arc contraction is

maximized under the condition of H= 20or 30 mm (a little bit weaker when H= 30mm), but from the welding point, H = 30mm is much better. That’s because duringthe GMAW process, spatter is unavoid-able, and if spatter attaches on the radia-tor surface, it would affect the emission ofthe ultrasonic wave, so a higher H value ismore reasonable. Otherwise, a smaller Hmeans the horn is closer to the arc and thetemperature of the horn is higher, whichwill weaken the stability and efficiency ofthe ultrasonic radiation.

Diameter of the Ultrasonic Radiator

In order to test the influence of the ul-trasonic radiator dimension on the arc,five horns with different end diameters(D) were machined, as shown in Fig. 7. Forthe limit of the spray nozzle dimension,the largest D was 28 mm. The test resultsare showed in Fig. 8.In Fig. 8, the arc length reduced mo-

notonously with the increase in D. Higher

D means a larger radiation area, whichsurely would improve the ultrasonic en-ergy density and the degree of arc con-traction. In addition, the degree of arccontraction was greater when D changedfrom 24 to 28 mm than when it changedfrom 20 to 24 mm. That’s because when Dwas at a higher level, the increase in theannular radiation area caused by the sameincreasing diameter was higher.

The Ultrasonic Vibration Amplitude

The ultrasonic system has three levelsof vibration amplitudes, i.e., ξ = 25, 28,and 30 μm. Figure 9 shows the influence ofthe vibration amplitudes on the weldingarc. The degree of arc contraction in-creased with the increasing vibration am-plitude, but compared with H and D, theeffect of the ultrasonic vibration ampli-tude was not very strong.The level of the ultrasonic vibration

amplitude indicates the degree that theparticles are forced to oscillate aroundtheir equilibrium position, so it is not dif-ficult to understand that greater ξ causeshigher arc contraction.

The Effect of the Arc Voltage

In order to find out the influence of thearc voltage, a series of experiments wasdone based on the conditions of H = 30mm, D= 28 mm, and H = 30 m. In orderto ensure the same conditions for GMAWand U-GMAW, the test was taken on thesame workpiece, as shown in Fig. 10. Atthe middle of the process, the ultrasonicwave was applied.The arcs of conventional GMAW and

U-GMAW at different welding voltagesare showed in Fig. 11, and their differ-ences are summarized as follows:1. At the same voltage, the arc of U-

GMAW is invariably shorter than that ofGMAW, and the difference in length in-creases with the increasing voltage. In ad-dition, the U-GMAW arc is distinctly


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Fig. 8 — Relationship between D and L. Fig. 9 — Relationship between ξ and L.

Fig. 10 — Schematic of the comparative experiment.


brighter and more stable.2. Arc length for both conventional

GMAW and U-GMAW increased with theincreasing welding voltage, but the growthrate of the conventional GMAW arclength is much higher than that of the U-GMAW arc. When the welding voltagereached 28 V, the conventional GMAWarc approached the nozzle (Fig. 11H). Ifthe voltage increased continuously, the arcwould retreat into the nozzle and damagethe welding torch. In contrast, the U-GMAW arc remained a reasonable lengthwithin a large voltage range, even whenthe voltage reached 33 V. This is why onlyU-GMAW arc pictures were given whenthe voltage went beyond 28 V.


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Fig. 11 — Arc shape at different welding voltages (wire feed speed (wfs) = 4.5 m/min; C stands for con-ventional GMAW; U stands for U-GMAW). A — 21 V; B — 22 V; C — 23 V; D — 24 V; E — 25 V;F — 26 V; G — 27 V; H — 28 V; I — 29 V; J — 30 V; K — 31 V; L — 32 V; M — 33 V.

A

C

E

G

I

L

D

F

H

J

M

K

B


Figure 12 reveals the relationship be-tween the arc length and the welding volt-age. Both the fitting curves of conven-tional GMAW and U-GMAW are linear.The fitting curve equations presented inthe figure show that for conventionalGMAW, the unit increase of the arc lengthis about 0.99 mm/V, while for U-GMAW,it is about 0.35 mm/V, which is only aboutone-third that of the former. Also, it canbe deduced that the sum of cathode andanode fall voltages in U-GMAW is about4.3 V less than that in the GMAW process.

In the experimental conditions, thewelding material and the plate materialare both low-carbon steel, which is a cold-cathode material. So, the electrical parti-cle emission mechanisms are mainly elec-tric field and collision emissions. In theexperiments, the welding current wassmall, no more than 200 A. In the GMAWprocess, the particle collision could notprovide enough charged particles. As a re-sult, a high voltage was needed to establisha strong electric field, and the electric fieldemission could help maintain the stabilityof the welding arc.

In the U-GMAW process, the weldingarc is compressed and the electric field in-tensity is improved. As a result, the parti-cles in the arc are accelerated, and the col-lision is more intense. More chargedparticles are provided in the collisionemission process, so that the sum of theanode and the cathode voltage is reducedin the U-GMAW process.

Conclusions

1. Under the action of the ultrasonicwave, the U-GMAW arc is contracted and

brighter and its length isdecreased. The arclength varies wavelikewith the height of the ra-diator, but enhances withthe increasing diameterof the radiator and theultrasonic vibration am-plitude monotonously.2. The amplitude of thestationary ultrasonicwave pressure is associ-ated with the phase dif-ference (ψ) of the inci-dent wave and thereflected wave. Some-times when ψ is evenmultiple times of p, thecontraction degree ofthe arc reaches maxi-mum; when ψ is oddmultiple times of p, the

contraction degree of the arc is negligible.3. The arc length of both conventional

GMAW and U-GMAW increases with in-creasing voltage. At the same voltage, thearc length of U-GMAW is invariablyshorter than that of GMAW, and the dif-ference increases with the increasing volt-age. For U-GMAW, the unit increase ofthe arc length to the voltage is only aboutone-third that of conventional GMAW.

Acknowledgments

This work was financially supported bythe National Natural Science Foundationof China under grant No. 51275134 and50975063.

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Fig. 12 — Relationship between welding voltage (U) and L.


arc characteristics of ultrasonic wave-assisted gmaw · ultrasonic wave-assisted gas metal arc...

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