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ABSTRACT. This paper examines theweldability of zinc-coated steel sheets ina lap joint configuration without a jointclearance by pulsed Nd:YAG laser beamwelding. A mechanism for sufficient ex-haustion of zinc gas for the formation ofacceptable quality welds is proposed.

The pulsed laser beam weldingprocess is controlled by a variety of pa-rameters. These include average peakpower density (APPD), mean laserpower, traverse speeds and pulse dura-tion. The present study focuses on the ef-fects of these main processing parame-ters on weld quality. Laser beam weldswere produced in 0.7-mm-thick electro-galvanized and 0.7-mm-thick galvannealsteel sheets with rectangular powerpulses.

It was experimentally found that aver-age peak power density (APPD) is themost critical factor in governing pulsedlaser beam welding. Excessive APPDtends to result in cutting effects. Con-versely, insufficient APPD has a high ten-dency to cause incomplete penetration.The other factors include average powerand travel speed. High quality lap jointwelds can thus be produced mainly byproper selection of APPDs, mean powersand traverse speeds.

Introduction

The automotive industry uses a vari-ety of galvanized products to enhancethe durability of vehicle structures. Weldjoints in the automotive industry are nowgenerally made by spot welding, ofwhich, for safety reasons, a large numberof them are. If a high-power laser is usedfor welding, the joints would be mademore quickly and with better quality,which makes it attractive as an alterna-

tive joining technology for vehicle bodyfabrication and assembly. In addition, theadvantage of single-side access and nodirect contact with the workpiece whilewelding render this technique all themore desirable.

The studies on the weldability of zinc-coated steel sheet have been undertakento overcome zinc gas explosion problemmainly encountered while using CO2lasers. Many different techniques werereported to successfully produce accept-able lap joint seam welds in zinc-coatedsteel sheet, mostly using multi-kilowattCO2 lasers (Refs.1–19). The pulsedmethod using 2.5 kW CO2 laser with noroot opening first claimed to be success-ful in producing visually sound full-pen-etrated welds by Heydon, et al., in 1989(Ref. 1). In 1990, Hurley, et al., reporteda success again (Ref. 2). Unfortunately, inboth studies visually acceptable weldswere made with a very small window ofprocessing parameters, and the effects ofthe laser processing parameters on thewelding process were not sufficiently in-vestigated.

The use of a continuous wave (CW)laser on a joint with no clearance be-tween sheets, which was carried out in1991 by Bilge, et al., from General Mo-tors, was patented in the United States as

a novel way of welding zinc-coated steelwith a CO2 laser (Ref. 3). The special fea-ture is that the sheets are positioned andmoved vertically during welding whilethe laser beam is applied to sheets hori-zontally. This idea has been seen rarelyin other academic publications probablydue to the difficult positioning and move-ment of steel sheets.

The technique of prior zinc removalin the weld area on a joint with no clear-ance between sheets was applied by Pen-nington, et al. (Refs. 4–5). The treatedzone was replaced by nickel coating,which does not vaporize appreciably atthe steel fusion temperature to reduceporosity and provide corrosion protec-tion. The disadvantage with this tech-nique is the prohibitive additional pro-cessing cost.

The method using a joint clearancewas investigated by Akhter, et al., whoput spacers between sheets to provide apath for venting gas formed from heatingthe coating material (Refs.1, 6–11, 16).This method is useful for CO2 laser weld-ing of members coated with materialshaving a lower melting point than thebase metal. However, there is the diffi-culty of maintaining a constant jointclearance during the welding processdue to thermal distortion and the varia-tion of coating thicknesses.

The altered joint geometry technique,which offers controlled channels be-tween the sheets to exhaust zinc gas, wasused by Piane, et al., to laser weld zinc-coated sheet steels (Refs. 12–16). Alteredjoint geometry is usually created as con-vex and concave surfaces in the topsheet. It has the additional advantage thatweld quality is not sensitive to dimen-sional variation of the clearance formedbetween the sheets (Ref. 16).

Pulsed Nd:YAG laser beam weldinghas better energy coupling and a moreflexible beam delivery using fiber optics,

Pulsed Nd:YAG Laser Seam Welding of Zinc-Coated Steel

BY Y-F. TZENG

Successful welds without gas-formed porosity are made in tightly clamped lapjoints of zinc-coated sheet steel

KEY WORDS

Pulsed Nd:YAGLaser WeldingLap JointElectro-Galvanized SteelGalvanneal SteelZinc-CoatedSeam Weld

Y-F. TZENG is with Department of MechanicalEngineering, Chang Gung University, Taiwan.

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which renders it a promising replace-ment for both CW CO2 laser beam weld-ing and conventional spot welding.However, complexity when using thepulsed mode of this laser for welding isincreased due to the introduction of morevariables in the process, namely pulseenergy, pulse duration, pulse repetitionrate, mean power, peak power and pulseshaping.

Few experimental results have beenreported on the quality of seam welds inzinc-coated steel made with a pulsedNd:YAG laser (Ref.17). Good qualitywelds, which require no joint clearancefor gas blowout, were reported as a suc-cess by Norris, et al. (Refs. 6, 16, 18–19).However, the reason for their successand the mechanism of Nd:YAG laserbeam seam welding of zinc-coated steelsheet is far from being fully understood.It follows that the systematic study of theweldability of zinc-coated sheet steeltightly clamped in a lap joint usingpulsed Nd:YAG laser is required in orderto boost confidence in industrial appli-cations. This is what this paper aims toprovide.

Pulsed Nd:YAG Laser Parameters

Figure 1 illustrates a schematic of thelaser power output for a series of constantenergy pulses in a self-designed shape.To help clarify the approach used in thisstudy, nomenclature associated withNd:YAG laser material processing is de-fined below.

PP = [EP/TP] (1)

(2)

PM = EP x PRR = [EP/TF] (3)

CD = [TP/TF] (4)

where PP is average peak power (kW), EPpulse energy (J), TP pulse duration (ms),PD average peak power density (kW/m2),A laser spot area (m2), PM mean laserpower (kW), PRR pulse repetition rate(1/s), TF pulse-to-pulse time (ms) and CDduty cycle.

Pulsed Welding of Zinc-Coated Steel

Proposals for successful welding ofzinc-coated steel in a lap joint without ajoint clearance using pulsed Nd:YAGlaser are as follows:

A pulsed Nd:YAG laser beam is ableto penetrate the workpiece deeper than aCW CO2 laser beam due to its higher av-erage peak power density and its shorter

wavelength of 1.06 µm, which results ina higher heat absorption by the substrate.Depending on the setup of the thermalinput rate, pulsed laser welding can va-porize the substrate to give a good key-hole effect. This creates a vent that mightaid in exhausting high-pressure zinc gasand thus reduce the risk of explosiveejection of material.

As illustrated in Figs. 2A and B, zinc,which boils at 900°C, will generate vaporboth in the fusion zone and in the sur-rounding heated area. The quantity ofzinc vapor is dependent on the coatingthickness at the interface between thesheets to be lap welded and also on thepulsed laser heating cycle, which deter-

mines the extent of the surrounding zoneabove 900°C. This quantity of zinc vaporhas to be exhausted. The fusion zone isat approximately 1600°–3000°C, andhence, the saturated vapor pressure ofthe zinc (as described by the Clausius-Clapeyron equation) will be 66 ∼ 1060atmosphere pressure.

In the work of Akhter (Ref. 20), thezinc vapor was exhausted between steelsheets by arranging a joint clearance. Hecalculated the joint clearance to be ap-proximately 0.1 mm for successful gasevacuation. In the present work, thesheets were tightly clamped, a conditionimposed by the requirements of industry.Thus, the only route for exhausting the

PE

T ADp

p=

×

Fig. 1 — Schematic of the output power pulses with illustrations of the defined average peakpower, the mean power, the pulse duration and the pulse-to-pulse time.

Fig. 2 — Schematic of zinc gas movement: A — Between the steel sheets; B — through the key-hole.

zinc vapor is through the weld pool alongwith the iron vapor formed in the key-hole. The high pressure of the zinc at theleading edge of the weld will distort thelocation of the keyhole forward. But if thematerial solidifying at the rear is sound,the weld produced will be sound.

Pulsed Nd:YAG laser beam welding ofzinc-coated sheet steel is essentially a se-ries of periodic spot welds partially over-lapping each other to form a weld seam.The partial overlapping of the successfulpower pulses refills the preceding fusionzone and thus could effectively reducepores left in the fusion zone by previouspulses. This is illustrated in Fig. 3, whichschematically shows the development ofpartially overlapping spot welds bypulsed laser beam welding. Therefore,appropriate partial overlapping of weldspots, plus the previously good keyholeeffect, may be capable of producing a de-sirable weld. In addition, zinc gas can besignificantly reduced by periodic power-offs in the pulse trains of the laser beam.As a result, the production of visuallysound welds on zinc-coated sheet steelmay be possible.

It is seen in Fig. 3 that the more closelythe overlapping of the spot welds, thesmoother the central longitudinal sectionof the weld profile, i.e., a more continu-ous fusion zone is obtained at the base ofthe weld. The following equations de-scribe the overlapping of the spot weldsin pulsed laser beam welding:

(5)

(6)where PER = the percentage of overlap inthe X-axis direction; S´ = V x TF, thelength in a single spot not overlapped bysuccessive welding spots; S = W + V x TP,the major diameter of spot weld formedfrom a laser spot plus movement duringa pulse; V = the travel speed; and W = theminor diameter of the spot weld.

For pulsed laser beam welding appli-cations, the general constraint 0 ≤ PER < 1applies to Equation 6 because there is arelative motion between the pulsed laserbeam and the workpiece. Equation 6 canthus be rearranged as follows:

(7)It can be deduced from Equation 7 thatthe travel speeds suited to longitudinalwelding applications with a constantlaser spot setup depend on the mean out-put power. The use of higher mean powerleading to smaller difference between the

0 VW

T TF P

< ≤−

.

= − ×+ ×

×1

V TW V T

100%F

P

PS S

S100%

= 1-S

S

ER =− ′[ ] ×

′

×100%

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Fig. 3 — Schematic diagram of a typical rectangular laser power pulse train and a correspond-ing series of partially overlapping spot welds with porosity inside.

Fig. 4 — Suggested idea for strategic control of pulsed Nd:YAG laser parameters.

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pulse-to-pulse time and the pulse dura-tion indicates higher travel speeds. In re-ality, for pulsed laser beam weldingprocesses, PER can be estimated to be atleast greater than 50% in order to pro-duce a weld with consistent full penetra-tion. Therefore, the operating range ofthe travel speeds can be predicted to fallwithin the following constraint:

(8)Referring to Figs. 2 and 3, the relationshipbetween the parameters can be ex-pressed in various mathematical equa-tions as follows:

(9)

= PP x CD (10)

(11)

= PD x A x CD (12)

PM = PD x A x TP x PRR. (13)

It is observed from Equations 9 and 13that for a given laser power PM there arevarious combinations of EP and PRR, andtheir relationship is inversely propor-tional. This observation applies to theother relevant mathematical expressionsand it indicates the flexibility and com-plexity in the selection of pulsed para-meters. The question that arises is how toselect a satisfactory combination of theparameters to enable an efficient and ef-fective pulsed laser welding application.

For spot welding by single laser powerpulse, the most crucial processing para-meters are average peak power density(PD) governing the thermal input rateupon the laser treated area, and pulse du-

ration (TP) controlling the interactiontime between the laser radiation and theworkpiece. While switching to thepulsed laser beam welding applicationsby pulse trains, other crucial processingparameters have to be taken into carefulconsideration. They are mean laser pow-ers (PM) and travel speeds (V). The ratio(PM/V) basically determines the extent ofthe average thermal input per unit weld.

PE

T AA

T

TMP

P

P

F=

×× ×

P E PRRE

T

T

TM PP

P

P

F= × = ×

0 VW

2T TF P

< <−

.

Fig. 5 — Example of the real rectangular power pulses.

Fig.6 — A typical pulsed laser weldability envelope for producingvaried quality welds for the material M1.

Fig. 7 — A typical pulsed laser weldability envelope for producingvaried quality welds for the material M2.

The mathematical relationship is ex-pressed as

(14)where Eave is average thermal input perunit weld for the laser spot size dS. Thevalue also determines the overlapping ofspot welds. This is understood by com-bining Equation 6 and Equation 14

(15)The Lumonics JK701 Nd:YAG laser ma-chine used in this study has a low meanoutput power (400 W), as such the oper-ating welding speeds are expected to bemuch slower compared to a high-powerCW CO2 laser. Therefore, to meet the de-mand of a high-power intensity for theseam welding applications of lasers, theimportance lies in the selection of suit-able APPDs. To achieve a suitable APPD,the laser spot size has to be carefully se-lected due to the restriction of the lowmean power available. It is also sug-gested that the laser spot be set on thefocal point for the smallest spot size(about 0.8 mm) leading to a wider choiceof APPDs. As for the types of shielding

gas, their compositions and flowrates are not considered in the de-sign of welding process parametersdue to their negligible effects on zincgas exhaustion (Ref.17).

Based on the previous discus-sions, the strategy for governing themain pulsed laser processing parame-ters used in the experimental trials isshown in Fig. 4. It is noted that the spotarea A is kept constant because of thereason stated before. The attributes ofvariables being “independent” or “de-pendent” in the mathematical equa-tions are arbitrarily defined from thestandpoint of ease in operating apulsed Nd:YAG laser machine and inexperimental trials.

It is noted in Fig. 4 that just con-trolling the selected levels of three in-dependent parameters PM, EP and TPleads to a series of induced dependentparameters (PD, CD and PRR). PD is de-termined simply by selecting theproper combination of EP and TP onthe laser controller for a constant spotsize A. The strategy used in controllingthe pulsed laser processing parametersherein enables a systematic compari-son of experimental trial results.

Materials andExperimental Methods

The materials used are M1 and M2,whose specifications are as follows:

1) M1: Electro-galvanized sheetsteel (EZ), 0.7 mm thick with 7.5 µmpure zinc coating on both sides.

P 11

W ds EE

TE

11

W ds EE

C

ERave

P

P

P

ave

PD

= − × × +

=

− × × +

.

EP T PRR

d V

Ed V T

Pd V

aveP P

S

P

S F

M

S

= × ××

=

× ×=

×

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Fig. 8 — The processing map showing the effects of the APPD on the weld quality usingpulsed Nd:YAG laser.

Fig. 9 — A — Top view; B — transverse section ofa visually acceptable seam weld produced in gal-vanized steel M1 at 2 mm/s using low-mediumAPPD (3.73 x 109 W/m2, EP = 22.5 J, TP = 12 ms),and mean power 396 W.

Fig. 10 — A — Top view; B — transverse sectionof a visually acceptable seam weld produced ingalvanized steel M1 at 1.5 mm/s using low-medium APPD (3.73 x 109 W/m2, EP = 7.5 J, TP =4 ms), and mean power 363 W.

A

A

B

B

2) M2: Galvanneal sheet steel (IZ),0.7 mm thick with 6.0 µm zinc coatingon both sides. The Galvanneal zinc coat-ing is typically 10.8% iron, 0.4% alu-minium, with balance around 89% zinc.

The chemical compositions in weightpercentage of the steel substrate M1, andM2 are displayed in Table 1. The laser pro-cessing parameter set up is shown in Table2. The key experimental strategy of settingup the pulsed laser processing parametersis to maintain a constant duty cycle andaverage peak power density, while vary-ing the pulse duration and the pulse en-ergy for consistent comparisons of the ex-perimental results. Before welding, thesurfaces of the specimens were cleanedup using acetone solution to remove dirtand oil. The laser beam welding experi-ments were conducted using 60 x 100-mm zinc-coated steel sheet specimens.The clamping arrangement is crucial asdescribed previously. It has to be ensuredthere is no joint clearance between theclamped sheets and the weld distortiondoes not create an opening while welding.

Experimental Resultsand Discussions

Weldability of Zinc-Coated Steel

The entire experimental results aresummarized in Table 3 and show that it ispossible to weld lap joints in tightlyclamped specimens of zinc-coated steelsheet with a pulsed Nd:YAG laser. Figures6 and 7 display the typical operating win-dows resulting in acceptable welds withvisually sound appearance, no internalcracks, and no zinc gas blow-holes or pit-ting on the top surface for the materialsM1 and M2, respectively. The reason forthe success of welding is due mainly tothe venting of the zinc vapor through thekeyhole. The venting mechanism for zincgas has been discussed in the previoussection. This is a function of weld poolsize vs. overlap or travel speed. The suc-cessful venting process is slow, approxi-mately between 1 to 3mm/s, as seen in

Table 3, due to the use of low av-erage laser power available in thestudy.

Effects of Average Peak PowerDensity on Weldability

Table 3 shows the APPD valuegreatly affects the results of weldson zinc-coated steel sheet. Forease in controlling the pulsedlaser welding process, it may berequired to construct the process-ing map for APPD effects. It canserve as a useful guide for indus-trial practitioners to effectively ex-ecute welding applications.

The processing map in Fig. 8shows the higher the APPD, themore likely it is that the coated steelsheet vaporizes, leading to cutting.Conversely, the lower the APPD,the more likely the presence of incom-plete penetration. It is noted in Fig. 8 theappropriate zone widens with increasingpulse duration. This phenomenon doesnot imply that the operating window of thetravel speed behaves likewise since it in-volves a more complicated balance of thekeyholing effects and the zinc gas blow-out phenomenon at different processingconditions. However, there is not muchdifference in the operating speeds due toall being slow.

The processing map applies to the twotypes of zinc-coated steel sheets investi-gated in the study, as there is not muchvariation in the coating. As noted in Fig.8, there are four arbitrarily defined levelsof APPD values. They are high, upper-medium, lower-medium and low level. Itcomprises five different zones each withtheir own distinctive processing effects.They are discussed below.

Zone A — Cutting effects region,APPD ≥ 7.46 x 109 W/m2, whereby theAPPD is higher than that required for theproduction of sound welds. This resultsin violent and rapid ejection of moltenmetal.

Zone B — Appropriate zone, 5.47 x

109 W/m2 ≥ APPD ≥ 3.73 x 109 W/m2,whereby sound welds can be producedby matching the APPD in this region withother suitable mean powers and travelspeeds.

Zone C — Incomplete penetration re-gion, APPD ≤ 2.98 x 109 W/m2, wherebythe APPD is lower than that required forthe production of sound welds.

Zone D — Transitional zone, 7.46 x109 W/m2 ≥ APPD ≥ 5.47 x 109 W/m2,whereby the APPD is close to zone A. Acutting effect tends to result.

Zone E — Transitional zone, 3.73 x 109

W/m2 ≥ APPD ≥ 2.98 x 109 W/m2,whereby the APPD is approaching zone C.Incomplete penetration of welds tends toresult.

Effects of Mean Power and Travel Speed on Weldability

It was confirmed through experimen-tation that mean power and travel speedare two other major factors governing thepulsed laser seam welding process be-sides APPD. It is observed in Fig. 6, at396 W mean power level, the weld beadquality will reach what is regarded as the

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Fig. 11 — Transverse section of a visually acceptableseam weld produced in galvanized steel M1 at 1.8mm/s using upper-medium APPD (5.47 x 109 W/m2,EP = 22J, TP = 8ms), and mean power 363 W.

acceptable quality for welding speedsranging from 1.6 to 2.5 mm/s. Beyond thecritical value of 2.5 mm/s, the surface ofthe weld pool would usually be brokenand pittings or holes would be left behindgiving rise to the condition termed “zincgas expulsion.” It is due to too low ther-mal input rate at which keyholing effectis not sufficiently developed, in conjunc-tion with incomplete penetration. As thewelding speed is decreased below 1.6mm/s, serious root concavity and deepslumping appeared on the bottom andthe top of the weld bead, respectively. Itis however due to a overly high thermalinput rate. Hence, for a given mean laserpower, a narrow range of welding speedsthat will produce acceptable welds pre-vail. The same explanation applies to theother arranged mean powers of 363, 330and 297 W.

Effects of Pulsed Duration on Weldability

It is reported that the variations in thepulse duration with constant pulse energyfor the spot welding applications result inappreciable effects on the weld dimen-sion and quality (Refs. 21–24). This is dueto the alterations in the peak power den-sity. But, for the pulsed laser seam weld-ing application, the effects caused by thechanges in the pulse duration are ob-served to be insignificant on the weld-ability envelope. It is due mainly to itsshort range (0.5–20 ms) available fromthe laser controller and the low meanpower (400 W) available. For the seamwelds processed by the same meanpower and the APPD, the variations inpulse duration eventually changes thepulse repetitive rate (PRR) of power pulsesand the resultant heating process dynam-ics. As such, the increase in pulse dura-tion leads to somewhat faster operatingspeeds due to stronger heating effects.

Conclusions

The problem with welds in lap jointswithout a joint clearance in zinc-coatedsteel sheet being damaged by zinc gasexplosion during laser beam welding hasbeen resolved. The use of 400-WNd:YAG laser operating in pulsed modehas proven to be successful. The targetfor a better understanding of the relatedprocessing parameters affecting total per-formance of pulsed laser welding processhas been achieved. However, comparedto work previously done with a CO2 laserby other researchers, the operatingspeeds using Nd:YAG laser are found tobe lower. This might be improved byusing high-power lasers.

The experimental study shows thatthe operating window for the welding

conditions that give smooth weld beadsare controlled mainly by the averagepeak power density, followed by themean power and the travel speed. Rec-tangular power pulses always producegood quality welds in lap joints over anoperating range of the dominant pro-cessing parameters. The operating rangesfor the parameters are found to be aver-age peak power density (APPD) between3.73 x 109 and 5.47 x 109 W/m2, meanpowers between 297 and 396 W, andtravel speeds between 1 and 3 mm/s, re-spectively. The variations in the pulse du-ration over the operating window did notappreciably affect the seam weld qualityexcept of leading to a small difference inthe welding speeds.

Acknowledgment

The author would like to give his deepappreciation to Professor W. M. Steen,Dr. Ken Walkins, and Dr. Zhu Liu of LaserGroup at the University of Liverpool,U.K., for their substantial assistance indoing this work.

References

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To Order Custom Reprints ofArticles in the

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