ultrasonic vibrationassisted keyholing plasma arc welding · vibration measurement was as follows:...

WELDING RESEARCH

AUGUST 2017 / WELDING JOURNAL 279-s

Introduction Nowdays because a variety of newhigh-performance materials are usedin manufacturing, there is a big de-mand for advanced welding processesand equipment for joining medium-thick plates with high efficiency (Ref.1). Plasma arc welding (PAW) is of ap-plication potential in joining medium-thick metal plates because it has ad-vantages such as a big arc column stiff-ness, concentrated heat energy, largeratio of weld depth to width, and nar-row heat-affected zone (Refs. 2–4).Compared to other high-density ener-gy beam (laser and electron beam)welding processes, the PAW processand equipment are low in cost, easy inoperation and maintenance, andstrong in adaptability (Refs. 2, 5).However, conventional PAW still hassome limitations, especially an insuffi-cient keyholing capability for penetrat-ing plate thickness as well as poor dy-namic stability of the keyhole and

weld pool (Ref. 6), which restrict theapplication of keyhole PAW. To solve these problems, re-searchers recently developed somemodified PAW processes. Vredeveldtdesigned and manufactured a newtype of PAW torch, which added a cool-ing duct between the plasma gas andshielding gas ducts (Ref. 7). The gas in-side the cooling duct flowed radiallytoward the root of the arc column atthe torch bottom. Such added radialgas flow was supplied through aporous ring so that the radial diameterof the plasma arc was further reducedat the orifice exit. In this way, the heatdensity of the plasma arc was in-creased by the additional radial gasflow. Mahrle et al. developed a laser-assisted PAW process, where the com-monly solid tungsten electrode was re-placed by a hollow one with an inter-nal diameter of 1.2 mm, and a low-power laser was projected through thehollow tungsten electrode and passed

through the plasma arc (Ref. 8). Thelaser interacted with the plasma arc,the latter was enhanced and stabilized,and the welding speed was increased.However, such modifications requiredspecial design and fabrication of theplasma torch to integrate additionalradial gas flow or laser beam, which in-creased the complexity of equipmentand operations. Zhang et al. proposed a quasi-keyhole strategy in PAW operating ina mode “one keyhole per pulse” (Ref.9). For the current waveform with twodeclining substages, when the weldingcurrent changed from the peak levelto the base level, the first substagewas with a variable slope while thesecond one was with a fixed slope. Wuet al. developed a controlled pulsekeyholing PAW process by employinga specially designed welding currentwaveform (Refs. 10–12). At the drop-ping stage of the welding currentfrom the peak level to the base level,two substages of current decreasingwith different slopes were added sothat the sustainment and closure sta-tus of the keyhole were able to be ac-tively controlled. But the increasednumber of the waveform parameterscaused some difficulty of process opti-mization. Ultrasonic vibration, as a kind ofmechanical energy, has advantages ofeasy application and high energy usageefficiency (Refs. 13, 14). Sun et al.used ultrasonic energy in gas tungstenarc welding (GTAW) (Refs. 15, 16).The GTAW torch was modified by in-cluding the ultrasonic transducer andthe radiator. The tungsten electrodewas inserted through the concentrichole of the ultrasonic radiator, and ul-

Ultrasonic VibrationAssisted KeyholingPlasma Arc Welding

The interaction of ultrasonic vibration with the plasma arc results in improved heatpressure features and the keyholing capability of the plasma arc

BY C. S. WU, C. Y. ZHAO, C. ZHANG, AND Y. F. LI

ABSTRACT A process variant of plasma arc welding (PAW), i.e., ultrasonic vibrationassisted keyholing PAW process, was developed. The tungsten electrode connected with a speciallydesigned ultrasonic transducer directly transmitted ultrasonic vibration into the plasmaarc. The welding tests on stainless steel plates demonstrated that under the same conditions ultrasonic vibration can effectively improve the keyholing capability of the plasmaarc so that an open keyhole can be produced with lower welding current and higherwelding speed. The measurement results showed that the interaction of ultrasonic vibration with the plasma arc resulted in a further constriction of the arc column and a15–31% increase in the plasma arc pressure under the conditions used in this study.

KEYWORDS • Ultrasonic Vibration • Plasma Arc Welding (PAW) • Keyholing Capability • Plasma Arc Pressure • Arc Constriction

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WELDING JOURNAL / AUGUST 2017, VOL. 96280-s

trasonic vibration from the radiatorwas applied to the arc coaxially. Thetest results in welding of AISI Type304 stainless steel showed that the ul-trasound could significantly increasethe weld penetration. Fan et al. introduced ultrasound ingas metal arc welding (GMAW) to im-prove the metal transfer process (Refs.17, 18). In their method, the mainbody of the welding torch was an ul-trasonic vibration piezo-electric trans-ducer and the filler material was fedthrough a concentric hole along itsaxis length. The ultrasonic wave wasthen radiated from the end of the ul-trasonic radiator and reflected by thesurface of the workpiece. This reflect-ed wave interacted with the incidentwave, thus forming an acoustic radia-tion field in the region of the arc col-umn. With action of the ultrasoundfield, the gas metal arc was more con-tracted and became brighter, and itslength was decreased. Under the samewelding conditions, the presence ofthe ultrasonic field changed the globu-lar transfer mode to the short-circuit-ing mode (Ref. 18). For ultrasound-assisted GTAW andGMAW, the ultrasonic radiator with amuch larger diameter than the tung-sten/wire was used to transmitacoustic vibration into the arc column.It was found that the ultrasonic ener-gy and the contraction degree of thearc are enhanced with the increase inthe diameter of the ultrasonic radiator(Refs. 18, 19). Because ultrasound-assisted GTAW

and GMAW produced satisfactoryprocess effectiveness, is it feasible toapply ultrasonic vibration in the PAWprocess to improve the keyholing capa-bility of the plasma arc? For one thing,PAW has its own features. The PAWtorch itself has a special structure, andthe tungsten electrode is set back in-side the torch (Refs. 2, 20). It is impos-sible to employ the ultrasonic radiatorwith a size larger than the diameter oftungsten/wire electrode, as in the cas-es of GTAW and GMAW. On the otherhand, unlike gas tungsten arc and gasmetal arc, the plasma arc itself has al-ready been constricted by the orificesize (mechanical constriction) and thewater-cooled nozzle (thermal constric-tion). Both mechanical and thermalconstrictions greatly increase the cur-rent density in the plasma arc column.

Thus, the electromagnetic constrictionamong the current lines inside the arccolumn is further enhanced, and thetemperature and heat intensity aregreatly improved (Refs. 2, 21). For thetriple-constricted plasma arc with highheat intensity, is the additional ultra-sonic vibration able to play a role infurther improving the heat-pressurefeatures and the keyholing capabilityof the plasma arc? It is a topic worthyof investigation. In this study, a specially designedultrasonic transducer was directly con-nected with the tungsten electrode, ul-trasound was transmitted into theplasma arc via the electrode itself, andthe ultrasonic vibration-assisted key-holing PAW system was developed.The welding tests on stainless steelplates were carried out to examine if

Fig. 1 — Schematic of ultrasonic vibrationassisted PAW system.

Fig. 3 — Measurement setup of ultrasonic vibration.

Fig. 2 — Photograph of ultrasonic vibrationassisted PAW torch.

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the additional ultrasonic vibration wasable to play a positive role in improv-ing the heat-pressure features and thekeyholing capability of the plasma arc.

Experimental Setup

Experimental System

Figure 1 shows the schematic dia-gram of an ultrasonic vibration-assist-ed keyholing PAW system, which in-cludes three main parts: the ultrasonicvibration unit, the plasma arc weldingmachine, and the sensing and controlunit. The ultrasonic vibration systemincludes the ultrasound power source,the ultrasonic transducer, and the am-plitude transformer. Under the electri-cal driving of the ultrasound powersource, the ultrasonic transducer gen-erated the ultrasonic wave that was

amplified by the amplitude trans-former and then transmitted to thetungsten electrode. The plasma arcwelding system consisted of the digi-tally controlled TPS-5000 plasma gen-erator and PTW-3500 plasma weldingtorch. During the welding process, thewelding current and arc voltage weresampled via different sensing units inreal time. Simultaneously, the keyholeexit at the underside of the workpiecewas imaged by a CCD camera (OK-AM1101A) to determine if a keyholechannel that penetrated through thewhole workpiece thickness was formedor not, as well as measured the key-hole behavior and dimension whensuch a fully penetrated keyhole is es-tablished. The CCD camera wasequipped with a narrow band filter(with a central wavelength of 1060nm, bandwidth of 20 nm, and trans-

parency at 85%) and a neutral filter. As shown in Fig. 1, the camera wasaimed at the backside of the workpiecewith an angle of 70 deg relative to thebottom surface of the workpiece. Thecaptured image signals were digitized bythe image grabber and processed by thecontrol computer. During the weldingprocess, the workpiece traveled at thewelding speed, while the torch and thecamera were stationary. Figure 2 shows the assembly of theultrasonic transducer, the amplitudetransformer, the tungsten electrode,and the PAW torch. Because the end ofthe amplitude transformer was me-chanically linked with the tungstenelectrode, the latter vibrated axiallywith the same frequency as the ultra-sound. The ultrasonic vibration fromthe tungsten electrode acted on theplasma arc, so that the latter’s charac-teristics were changed.

Measurement of the UltrasonicVibration

When the ultrasonic system wasmanufactured and assembled, themeasurement of the ultrasonic vibra-tion at the end of the tungsten elec-trode was performed to determine ifthe vibration parameters met the de-sign standard. To this end, a laserDoppler vibrometer (LDV) was em-ployed to aim at the end of the tung-sten electrode in a coaxial way, asshown in Fig. 3. The principle of thevibration measurement was as follows:The beam of a helium neon laser waspointed at the vibrating object and

Fig. 4 — Measured results of the ultrasonic vibration system (200 W). A — Frequency and amplitude; B — section of processed waveform.

Fig. 5 — The vibration amplitude under different output powers of the ultrasonic system.

A B


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scattered back from it. The vibrationamplitude and frequency were extract-ed from the Doppler shift of the re-flected laser beam frequency due tothe motion of the surface. The outputof an LDV is generally a continuousanalog voltage that is directly propor-tional to the target velocity compo-nent along the direction of the laserbeam. Figure 4 shows the measurementresults. As designed, the electrodetungsten vibrated at a frequency of 25( 0.5) kHz, and the vibration ampli-tude was about 17 m under the pow-er of 200 W — Fig. 4A. Figure 4Bshows a section of the waveform cap-tured from the processed wave. As theoutput power of the ultrasonic powersource increased, the vibration ampli-tude of the tungsten electrode wentup, as demonstrated in Fig. 5. The aimof this study was to employ a low butsufficient ultrasonic power to enhancethe keyholing capability of the plasmaarc. After some tests, it was found thatthe ultrasonic power source with 500W output can provide enough acousticinfluence on the plasma arc at a lowcost.

Welding Test Conditions

As aforementioned, the CCD cam-era was employed to observe the key-hole exit from the backside of theworkpiece. During the PAW process,the image of the keyhole exit was cap-tured if an open keyhole was estab-lished. The welding tests were carriedout on 304 stainless steel plates of 4mm thickness. Both the plasma gasand the shielding gas were pure argon,and their flow rate was 2.6 and 20L/min, respectively. The torch orificewas 3.2 mm diameter with a length of3 mm. The tungsten electrode was 5mm in diameter with a setback of 2mm. The torch standoff was 5 mm.The output power of the ultrasonicsystem was 500 W. Other parametersare listed in Table 1.

Results

Keyhole Initiation and Stability

For Test Cases 1 and 2, the samewelding current and welding speedwere used, but the only difference was

whether the ultrasonic vibration wasexerted or not. Figure 6 shows the cap-tured keyhole exit images for Test Cas-es 1 and 2. Under the same weldingconditions, no open keyhole was es-tablished in conventional PAW with-

out action of ultrasonic vibration, asshown in Fig. 6A. When the ultrasonicvibration was exerted, an open key-hole was formed so that the sequentialimages of the keyhole exit were ob-served — Fig. 6B. This means that ul-

Fig. 6 — The captured keyhole exit images: A — Test Case 1, conventional PAW; B — TestCase 2, UPAW.

Fig. 7 — Stable open keyhole is established with lower welding current in UPAW: A —Test Case 3, PAW, welding current 100 A; B — Test Case 4, UPAW, welding current 90 A.

Table 1 — Welding Conditions

Test Case Welding Current Welding Speed Ultrasonic Vibration No. (A) (mm/min)

1 80 100 No 2 80 100 Yes 3 100 110 No 4 90 110 Yes 5 95 100 No 6 95 115 Yes 7 110 120 No 8 110 120 Yes

A

B

A

B


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trasonic vibration can enhance thekeyholing capability of the plasma arcunder the same welding conditions.During the ultrasonic vibration-assist-ed PAW (U-PAW) process, a penetratedkeyhole was established first at instantt = 4.60 s, and the keyhole exit ex-panded its dimension until the mo-ment t = 5 s. Then, the size of the key-hole exit remained the same, whichmeans the quasi-steady state wasachieved. Because ultrasonic vibration can in-crease the keyholing capability of theplasma arc, a lower level of weldingcurrent may be used to get an openkeyhole status and complete joint pen-etration in U-PAW if other weldingconditions are the same. To validatethis point, Test Cases 3 (PAW, weldingcurrent 100 A) and 4 (U-PAW, welding

current 90 A) were made. For thesetwo tests, the welding speed was 110mm/min. In conventional PAW with awelding current of 100 A, an open key-hole was first established at instant t =8.75 s, but it could not be maintainedcontinuously because it was intermit-tently closed and opened — Fig. 7A.However, in U-PAW with a weldingcurrent of 90 A, an open keyhole wasnot only first established at instant t =4.38 s, but it also kept a sustainedopen status once it was formed — Fig.7B. It is obvious that under the sameconditions, ultrasonic vibration can ef-fectively improve the keyholing capa-bility with reduced welding currentbut the weld penetration is guaran-teed. On the other hand, if the weldingcurrent is the same, a higher welding

speed may be used in U-PAW. Test Cases5 (PAW, welding speed 100 mm/min)and 6 (U-PAW, welding speed 115mm/min) were performed. Under thewelding current of 95 A, open keyholewas formed first at instant t = 6.50 s inconventional PAW with a welding speedof 100 mm/min — Fig. 8A. Whereas inU-PAW with a welding speed of 115mm/min, an open keyhole was formedfirst at instant t = 5.20 s — Fig. 8B. Al-though a sustainable open keyhole wasachieved in both test cases, U-PAW canrun at a 15% higher welding speed thanwith PAW, and open keyhole wasformed earlier. It is clear that ultrasonicvibration can enhance the keyholing ca-pability through increasing the weldingspeed used under the same welding conditions. With a little bit higher values of bothwelding current and welding speed, TestCases 7 and 8 were further conducted tocheck out the effectiveness of ultrasonicvibration in enhancing the keyholing ca-pability. As shown in Fig. 9A, the firstopen keyhole was formed at instant t =4.50 s, but it was closed until instant t =10 s in PAW. However, in U-PAW, thefirst open keyhole was formed at in-stant t = 3.75 s and it maintained theopen keyhole status continuously —Fig. 9B. To compare the images in a quasi-steady state, i.e., the images with-in the period of 12.5–22.5 s in PAW andthose within the period of 7.5–22.5 s inU-PAW, it can be seen that the size ofthe keyhole exit was larger in U-PAWthan that of PAW. Under the same con-ditions, ultrasonic vibration can acceler-ate the establishment of an open key-hole, and make the established openkeyhole reach quasi-steady state quickly.

Plasma Arc Pressure

As previously mentioned, exertionof ultrasonic vibration on the plasmaarc can increase its keyholing capabili-ty. If the keyholing capability is en-hanced, the pressure-heat features ofthe plasma arc must be improved byultrasonic vibration. To examine thispoint, experiments were performed tomeasure the pressure distribution ofthe plasma arc. The stagnation pressure measure-ment apparatus was used to detect thepressure of the plasma arc between atorch and an anode. The apparatusconsisted of a water-cooled copper

Fig. 8 — Stable open keyhole is established with higher welding speed in UPAW: A — TestCase 5, PAW, welding speed 100 mm/min; B — Test Case 6, UPAW, welding speed 115mm/min.

Fig. 9 — Comparison of keyhole exit images in PAW and UPAW: A — Test Case 7, conventional PAW; B — Test Case 8, UPAW.

A

B

A

B


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block with dimensions of 140 mmlength 60 mm width 22 mm thick-ness. The 0.5-mm-diameter measure-ment orifice was set in the copperblock to lead the arc pressure into thesensor. At the bottom of the copperblock, concentric to the measuring ori-fice with a short and thin pipe, a dif-ferential pressure sensor (PRC-905)with a measuring range of 0–5 kPa wasassembled. A data acquisition card(PCI 8613) was set to sample the dataduring the test. At a position, themeasurement was repeated ten times,and then the averaged value was ob-tained. During the test, the weldingtorch was stationary while the copperblock was controlled via a precisionthree-dimensional slide platform, sothat the relative movement betweenthe welding torch and the sensor wasobtained, and the distribution of theplasma arc pressure at different posi-tions was obtained. In this experiment, the plasma arclength, i.e., the distance between thenozzle end and the water-cooled cop-per block surface, was 5 mm. The ul-trasonic power was set as 500 W, andthe frequency of the ultrasonic systemwas 25 kHz. For this test case, boththe plasma gas and the shielding gaswere pure argon, and their flow ratewas 2.6 and 20 L/min, respectively.The welding current took three levelsof 80, 100, and 120 A.

Figure 10 compares the plasma arcpressure distribution in U-PAW (withultrasonic vibration) and in conven-tional PAW (without ultrasonic vibra-tion). With ultrasonic vibration, thearc pressure in U-PAW is always higherthan that in conventional PAW. At thecenter of the anode corresponding tothe plasma arc axis, the pressure dif-ference between U-PAW and PAW was

largest, and thegap between thetwo curves rapid-ly narrowed asthe distance awayfrom the centeraxis rose. For thepeak value of theplasma arc pres-sure at the centeraxis, the differ-ence between U-PAW and PAWbecame less whenthe welding cur-rent increasedfrom 80 to 120 A.When the weld-ing current was80, the peak val-ue of the plasma arc pressure wasabout 1390 and 1060 Pa for U-PAWand PAW, respectively, and the exer-tion of ultrasonic vibration made it in-crease by 31%. When the welding cur-rent was 120 A, the peak value of theplasma arc pressure was about 2098and 1820 Pa for U-PAW and PAW, re-spectively, and the exertion of ultra-sonic vibration made it increase by15%.

Discussion The test results in Figs. 6–9demonstrated that U-PAW was able toachieve an open keyhole with a lowerwelding current or a higher weldingspeed under the same welding condi-tions. This implies that the interac-tion between the ultrasonic vibrationand the plasma arc enhanced the key-holing capability of the plasma arc.The underlying mechanism may beexplained as follows: A plasma arc is amixture of three particles, i.e., atoms,

electrons, and ions. The particles in-side the plasma arc are driven tomove at high speed by the electricfield strength (potential gradient),the particle density gradient, and thetemperature gradient (Ref. 22). Whenultrasonic vibration is applied to aplasma arc, the particles are also ex-erted an extra force so that they vi-brate too, as shown in Fig. 11. Suchan extra vibration at high frequencyis equivalent to the particle volumeexpanding along the vibration direc-tion so that the contacting interfaceand the colliding probability of parti-cles increase, which results in an in-crease of thermal conductivity of theplasma arc. The increment of thermalconductivity causes a forced coolingaction to the plasma arc, which re-quires the plasma arc to generatemore heat to compensate for thiscooling effect. Thereby, the plasmaarc has to automatically constrict itssize at the transverse cross section todecrease its heat loss according to the

Fig. 10 — Plasma arc pressure distribution in PAW and UPAW: A — 80 A; B — 100 A; C — 120 A.

C

A B


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minimum voltage theorem in arcphysics (Ref. 23). That means the ul-trasonic vibration makes the plasmaarc produce an extra constriction. On the other hand, when ultrasonicvibration interacts with a plasma arc,the extra force exerted onto the parti-cles will change the direction of thecomposite force on the particles. Asshown in Fig. 12A, the electromagnet-ic force Fe and the plasma jet force Fp

constitute a composite force FPAW inthe plasma arc. But for ultrasonic vi-bration-assisted plasma arc, the extraacoustic force Fu and the force FPAW

constituted a new composite force FU-PAW, as demonstrated in Fig. 12B.The addition of Fu caused FU-PAW with asmaller angle with respect to the plas-ma arc axis so that the radial compo-nent of FU-PAW makes the plasma arcproduce further constriction. Due to the interaction between ul-trasonic vibration and plasma arc, thetwo fold further radial constriction ofthe plasma arc gives rise to an increasein temperature, heat intensity, andflow velocity inside the plasma arc.Thus, the plasma arc pressure is in-creased, and the keyholing capability isenhanced. The CCD camera was also employedto observe the plasma arc column gen-erated between the torch and the water-cooled copper block. The cap-tured raw images of the plasma arcwere processed, and the plasma arccolumn beyond a certain gray valuewas extracted to characterize the mainpart of the plasma arc column withhigher temperature and heat intensity.Figure 13 shows the extracted plasmaarc column in PAW and U-PAW under

the same conditions. It is clear that ex-ertion of ultrasonic vibration made theplasma arc column be constricted un-der the same level of welding current.Such a constriction increased both theflow velocity of the plasma jet and theheat density inside the plasma arc.Therefore, ultrasonic vibration madethe plasma arc have a higher pressureand heat density, which enhanced itskeyholing capability. The welding current has a predom-inant effect on the arc pressure andheat intensity. In fact, the arc pres-sure is directly proportional to thesquare of the welding current (Refs.22, 23). When the welding current islow, the constriction extent of theplasma arc is not so high. Thus, thereis room for further constriction by ul-trasonic vibration. But the furtherconstriction room becomes less whenthe welding current is higher becausethe welding current deduced constric-tion itself is already large enough.This is the reason why the incrementof arc pressure due to ultrasonic vi-bration gets lower if the welding cur-rent is higher. The additional ultra-sonic vibration can result in a15–31% increase in the plasma arcpressure under the conditions in thisstudy.

Conclusions The following conclusions may beobtained: 1) The ultrasonic vibration-assistedkeyholing PAW system was developed.A specially designed ultrasonic trans-ducer was directly connected with thetungsten electrode, and ultrasound

was transmitted into the plasma arcvia the electrode itself. 2) The ultrasonic vibration can en-hance the keyholing capability of theplasma arc. Under the same condi-tions, the additional ultrasonic vibra-tion can produce an open keyhole witha lower welding current and a higherwelding speed. 3) Compared to conventional PAW,the establishment of an open keyholewas accelerated and the quasi-steadystate was reached in a shorter time inultrasonic vibration-assisted PAW. 4) The additional ultrasonic vibra-tion can result in a 15–31% increase inthe plasma arc pressure under the con-ditions in this study. That is the rea-son why U-PAW has a strong keyhol-ing capability. 5) The interaction of ultrasonic vi-bration with the plasma arc resulted infurther constriction of the plasma arccolumn. However, the underlying mech-anism needs further investigation.

Fig. 11 — Schematic of particle vibration in UPAW.

Fig. 12 — Comparison of the force actedon particles in the plasma arc: A —PAW; B — UPAW.

A

B


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The authors are grateful for the fi-nancial support in this research fromthe National Natural Science Founda-tion of China under Grant No.50936003.

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Fig. 13 — The dimensions of plasma arc column with/without ultrasonic vibration.

References

CHUANSONG WU, Ph.D ([email protected]), CHENG ZHANG, and YONGFENG LI are with the Institute of Materials Joining, Shandong University, Jinan, China. CHENYU ZHAO is with FAWVolkswagen Automotive Co. LTP.

Acknowledgments


ultrasonic vibrationassisted keyholing plasma arc welding · vibration measurement was as follows:...

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