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Introduction

Zinc-coated steels provide excellentcorrosion resistance for automotivebody structural and closurecomponents. In addition, zinc-coatedsteels also enhance the productionstamping performance. These featuresgenuinely drive increased usage ofzinc-coated steels in the automotiveindustry. However, one of the mostsignificant challenges for implement-ing zinc-coated steels is to weld thezinc-coated steels, especially in a lap-joint configuration, without producing

weld discrepancies. Compared to themelting point of steel (over 1500°C),zinc has a significantly lower boilingpoint, 906°C. During the laser weldingprocess, a highly pressured zinc vaporis readily produced, which often leadsto the formation of various discrepan-cies, including weld spatter andporosities. Because of theaforementioned weld discrepancies,mechanical performance of the weldsis dramatically reduced. Therefore,steps must be taken with regard to thewelding process in order to suppressthe zinc vapor. Unfortunately, this in-

creases the manufacturing complexityand reduces productivity. With respect to productivity andflexibility, laser welding has many ad-vantages compared to other weldingprocesses. In the past several decades,different laser welding techniqueshave been proposed to weld zinc-coated steels in a lap-joint configura-tion. These laser welding techniquescan be categorized by the following: 1. Setting a prescribed root openingsize of 0.1–0.2 mm at the faying inter-face of metal sheets prior to the weld-ing process (Refs. 1, 2). The root opening can be created bya stamping process, mechanical meth-ods, as well as a laser beam. The rootopening created provides a lateralchannel for the highly pressurized zincvapor to escape from the interface be-fore the steel around the interface ismelted. Consequently, the formationof spatter can be avoided. However, aninconsistent root opening could leadto the formation of weld discrepanciessuch as undercutting and porosity. Re-cently, a remote laser-welding processhas been used to weld three zinc-coated steel sheets in a lap-joint con-figuration (Ref. 3). Experimentalresults demonstrated that with an op-timized root opening size, acceptablewelds could be achieved in a three-sheet stackup of zinc-coated steelsheets although porosity was still gen-erated within the welds (Ref. 3). 2. Enlarging the molten pool andthereby extending the solidificationtime with the use of a second heatsource, providing sufficient time forthe zinc vapor to escape from the

Semi­Cutting­Assisted Laser Welding ofZinc­Coated Steels in a Zero Root Opening,

Lap­Joint ConfigurationHigh­quality, zero root opening laser welds (4.8 m/min) in zinc­coated steels in a lap­joint

configuration were achieved

BY S. YANG, Z. CHEN, W. TAO, C. WANG, J. WANG, AND B. E. CARLSON

ABSTRACT Because of their excellent corrosion resistance, zinc­coated steels have beenwidely used in the automotive industry. However, the generation of highly pressur­ized zinc vapor during the laser beam welding process presents unique challengesfor body manufacturing. In this study, a semi­cutting­assisted laser welding processwas developed to weld zinc­coated steels in a zero root opening, lap­joint configu­ration using a specially designed nozzle for delivery of the shielding gas. Effects ofwelding speed on the weld quality were investigated, resulting in high­quality weldsbeing achieved at a relatively high welding speed of 4.8 m/min. The success inachieving high­quality, zinc­coated steel welds by semi­cutting laser welding isattributed to an improved drag force. This, in turn, is a result of the increasedshielding gas flow rate, which enlarges and stabilizes the keyhole, enabling the zincvapor to escape from the faying interface of the two metal sheets. Tensile shear andmicrohardness tests were conducted to evaluate mechanical properties of thewelds. Optical microscopy was also used to examine the microstructure of thewelds. It was demonstrated the weld strength was comparable to the base metal.

KEYWORDS• Zinc-Coated Steels • Lap-Joint Configuration with Zero Root Opening • High-Speed Welding • Shielding Gas Flow Rate • Weld Discrepancies• Keyhole

S. YANG (david.s.yang@gm.com) is a senior researcher, W. TAO is an associate researcher, and J. WANG is a lab group manager with China Science Lab, GeneralMotors Global Research & Development Center, China. Z. CHEN is a master student and C. WANG is a professor with Huazhong University of Science and Tech­nology, China. B. e. Carlson is lab group manager with General Motors research & Development Center, Warren, Mich.

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molten pool and keyhole (Refs. 4 –7). The molten pool can be enlarged bycombining a laser beam with an arc orsecond laser beam that share acommon molten pool. With a longersolidification time, the zinc vapor hasa greater possibility to escape throughthe molten pool. The main constraintsof these techniques are imposed by thelimited space available in the produc-tion welding cell in addition to the rel-atively high capital investment neededfor two heat sources as well as complexoperation. 3. Cutting a slot by a laser beamalong the weld interface prior to laserwelding (Refs. 8–11). Another possibility is to cut a slotby a precursor laser beam in order toprovide an exit path for the zinc vaporand the welding process is carried outby a second laser beam (Ref. 9). Theslot size used in this method is a func-tion of the welding speed and themetal sheet stack-up thickness. Thenarrow bonded area at the fayinginterface of the metal sheets could re-duce the weld strength. 4. Modification of the zinc compo-sition by addition of a second alloyingelement such as copper or aluminum(Refs. 12, 13) or replacement of thezinc coating at the faying surface by anickel-based coating (Refs. 14, 15). The compounds of zinc-copper andzinc-aluminum as well as nickel havemelting points higher than the boilingpoint of zinc. Therefore, use of thesematerials would reduce or avoid thegeneration of zinc vapor resulting in amore stable welding process. The neg-

ative aspect of thesemethods is that they addextra process steps andthe materialcompositions could actto reduce the weldmechanical propertiescaused by excessive dis-solution of aluminum orcopper into the welds.

5. Laser welding as-sisted by arc or laser pre-heating (Refs. 16–18).

Use of a preheatsource causes the zinc

coating on the top surface of the work-pieces to be vaporized and a portion ofthe zinc coating at the faying interfaceis converted into zinc oxide. Theadvantage of zinc oxides is that theyhave a significantly higher meltingpoint than the boiling point of zinc(1975° vs. 906°C). Therefore, when thefollowing laser beam comes into posi-tion for welding, there is significantlyless zinc available to generate the dele-terious zinc vapor, stabilizing themolten pool and keyhole, whichfurther helps any remaining zinc vaporescape, thereby achieving defect-freewelds. 6. Optimization of shielding gas(Refs. 19, 20). The laser-induced plasmacan be suppressed and the interactionbetween zinc vapor and laser beam canbe reduced through optimization of theshielding gas for improved coupling ofthe laser beam energy. Furthermore,the addition of oxygen into the shield-ing gas can increase weld penetrationand stabilize the keyhole, which resultsin an improved weld quality of zinc-coated steels (Ref. 19). 7. Pulsed laser welding (Ref. 21). A stable keyhole can be achieved byoptimizing pulsed laser weldingparameters, including the peak power,duty cycle, travel speed, pulserepetition rate, and pulse energy. Theoptimization is focused on allowingthe zinc vapor to be mitigated andachieving visually sound welds. How-ever, typically a large amount of poros-ity is retained within the weld usingthis method. Another limitation for

this method is the relatively low weld-ing speed, which limits its application. 8. Laser welding assisted by vacuum(Ref. 22). This relatively new process uses asuction device to create a negative pres-sure zone (relative to ambient) directlyabove the molten pool. The purpose ofthis negative pressure zone is twofold.Firstly, a drag force is generated due tothe external suction device, which cancounterbalance the shear force inducedby the erupting zinc vapor. Secondly,the negative pressure zone facilitatesthe zinc vapor to escape along the suc-tion direction. As a result, the moltenpool becomes more stable and the key-hole will remain open and allow the es-cape of zinc vapor. Defect-free, zinc-coated welds were achieved using thismethod, but it requires ancillaryvacuum equipment to be introducedinto the production cell. Although the methods mentioned

above achieve technically acceptableweld quality, there are constraints asso-ciated with each of these methods,which inhibits the full implementationin production. Furthermore, laser weld-ing speeds of more than 3 m/min areneeded in order to meet typical produc-tivity requirements. Currently, there islimited published literature in the areaof laser welding of zinc-coated steels atspeeds greater than 3 m/min. In this study, a semi-cutting-assistedlaser welding process was developed(Patent Reference Number: GMC-338-A-CN) to weld zinc-coated steels in azero root opening, lap-joint configura-tion. Here, the semi-cutting-assistedlaser welding is referred to as a definedlaser welding process where a gas jetwith relatively higher gas flow velocitythan that in the conventional laserwelding process is used to increase thefluid flow transfer rate in the moltenpool. Instead of using a conventionalshielding gas with a relatively largediameter equal to or larger than 6 mm,a smaller shielding gas nozzle of 2 mmdiameter was used. To betterunderstand the effect of welding speedon the weld quality, experiments werecarried out at welding speeds of 3, 4.2,

Fig. 1 — Schematic of experimental setup with semi­cut­ting­assisted laser welding.

Table 1 — Chemical Composition of Low­Carbon Steel (wt­%)

Steel C Mn P Si S Al Cr Ca Ti≤0.006 ≤0.2 ≤0.025 _ ≤0.02 ≥0.015 _ _ 0.03/0.08

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and 5.4 m/min. High-quality welds wereachieved by using this new laser weldingtechnique. In addition, tensile shear andhardness tests were performed to evalu-ate mechanical properties of the welds.

Experimental Procedures

Materials

Zinc-coated, low-carbon steel waschosen for this research. The specimendimensions of the workpiece were 120× 85 × 0.8 mm. The zinc coating washot-dip galvanized at a level of 60g/m2 per side. Prior to welding, thesurfaces of the steel plates werecleaned with alcohol in order toremove any dirt on the surface of theworkpiece that may interfere withclamping of the workpieces. Table 1 lists the chemical composi-tion of the low-carbon steel used inthis study.

Laser Welding Procedure

The laser beam weldingexperiments were conducted using anIPG YLS-4000 fiber laser (wavelength:1070 nm; focal length: 250 mm; focalspot diameter: 0.3 mm). Pure argonwas used as the side shielding gas. Thetwo steel plates were tightly clampedtogether prior to laser welding, and it

is assumed that no root openingexisted between the two metal plates. A 2-mm-diameter shielding gas noz-zle of made of a copper alloy wasdesigned and applied in this study,schematically shown in Fig. 1. As shownin Fig. 1, the stand-off distance of theshielding gas nozzle was 10 mm, whichis an order of magnitude larger thanthat typically used for laser cutting. Forconventional laser cutting, the laser cut-ting header is perpendicular to theworkpiece, and the stand-off distancebetween the nozzle and the workpiece isselected in the range between 0.5 and1.5 mm (Ref. 23). Furthermore, thenozzle was set at an inclination angle of52 deg with respect to the upper work-piece top surface. During the laser weld-ing process, the shielding gas nozzle ispositioned such that it is in front of thelaser beam. The laser beam itself has aninclination angle of 5 deg with respectto the vertical direction, i.e., normal tothe upper workpiece surface, in order toavoid backreflection from the laserlight. The gas flow velocity at the exit ofthe shielding gas nozzle can becalculated by the equation: flowrate/cross-sectional area of the shield-ing gas nozzle. Assuming that theshielding gas flow rate from the storagecontainer to the exit of the shielding gasnozzle is kept constant, the gas-flow ve-locity used in this study is about 66m/s

.

During the laser welding process, thegas pressure is kept constant and thetemperature of the shielding gas is as-sumed to be at room temperature.

Metallography and MicrohardnessTest

The laser-welded lap joint wassectioned perpendicular to the weldjoint direction. This cross section wasthen cut, mounted, ground, polished,and etched as a precursor to microhard-ness measurements. Vickers microhard-ness tests were conducted using a loadof 100 g and a dwell time of 10 s. In ad-dition, optical microscopy was appliedfor microstructural examination.

Tensile Shear Test

Tensile shear test samples were pre-pared according to the sheet type ofASTM-E8/E8 M-08 standard (gaugelength: 50 mm). Tensile shear testswere conducted in uniaxial tensionusing a 3-kN load cell. The model

π

=−

=×

⎛

⎝

⎜⎜⎜⎜

⎞

⎠

⎟⎟⎟⎟

vflow rate

cross tional area of used nozzle

m h

sec

/0.75

1

3

2

Fig. 2 — Typical characteristics of the welds obtained by a conventional laser welding process. A — Top view; B — bottom view; C —cross­sectional view of porosity; D — porosity produced inside the weld.

A B

C D

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number of the tensile testing machineis QJ-211. All tests were run at ambi-ent temperature and a constantcrosshead velocity of 2 mm/min.Three replicates were conducted foreach welding condition, and the aver-age peak load was calculated.

Results and Discussion

Semi­Cutting­Assisted Laser Welding

A highly pressurized zinc vapor isreadily produced at the faying surfaceduring laser welding of zinc-coatedsteels in a zero root opening, lap-jointconfiguration. The zinc vapor quicklyexpands inside the molten pool anddisrupts both the molten pool andkeyhole — Fig. 3A. This disruption is,in part, rippling of the molten poolsurface where different types ofrippling having various magnitudesare observed during the laser weldingprocess. The rippling fluctuates in theform of waves at high frequency andmagnitude, and reflects most of thelaser beam energy from the surface ofthe molten pool (Ref. 22). As a conse-quence, the coupling of the laser-beamenergy into the workpiece is dramati-cally reduced and the weld penetrationdepth becomes shallow. Furthermore,in the worse situation, the liquid metalin the molten pool moves along thewelding direction and collapses thekeyhole. The collapsed keyhole entrapsthe zinc vapor, which quickly expandsinside the molten pool. Under thesewelding conditions, liquid metal isejected out of the molten pool in therear of the keyhole as the zinc vapor

reaches the molten pool surface. The ejected liquid metal condensesin the air and deposits on the top sur-face of the workpieces, resulting in theformation of spatter — Fig. 2A. If theliquid metal cannot fill in the cavitycaused by the loss of the ejected liquidmetal in a timely manner, then poros-ity is produced in the welds (Ref. 7) —Fig. 2A, C. This phenomenon was ob-served to occur intermittently. Afterthe highly pressurized zinc vapor is re-leased, the molten pool becomes rela-tively stable. However, when the zincvapor pressure level builds to somethreshold, the molten pool againbecomes unstable. The presence ofthese weld discrepancies dramaticallyreduces the weld strength. Figure 2A shows typical weldfeatures obtained by conventionallaser welding. As shown in Fig. 2B, in-consistent weld penetration is usuallyobserved in the welds. In order to suppress the zinc vapor,a new laser welding process wasproposed to weld zinc-coated steels ina zero root opening, lap-joint configu-ration where an innovative nozzle wasdesigned to simultaneously create asemi-cutting action in addition to alaser welding process. This newlydeveloped method is hence forthreferred to as the semi-cutting-assisted laser welding process. Asshown in Fig. 3B, a much smaller 2-mm-diameter shielding gas nozzle wasused instead of the regular shieldinggas nozzle with a diameter with equalto or greater than 8 mm. Compared toconventional laser welding, thenarrower nozzle size used in the semi-cutting-assisted laser welding process

reduces the coverage area of shieldinggas on the workpiece, thus increasingthe possibility of the weld oxidation.However, oxidation of a laser weldwith a width of about 1–6 mm can beprevented by the divergence of shield-ing gas even when using a smaller noz-zle with a diameter of about 2–6 mm. In this study, the weld size is about1.0 mm in Fig. 4 and can be fullycovered by the smaller nozzle. Becauseof the reduced cross-sectional area ofthe nozzle, the velocity of the shieldinggas was dramatically increased. Whenthe nozzle diameter is reduced from 8 to2 mm, the velocity of the shielding gasis increased by 16 times since the veloc-ity of shielding gas is inversely propor-tional to the square of the nozzle diame-ter. This way, a straight and nonturbu-lent gas jet having greater momentum iscreated that when applied during thelaser welding process creates a higherdrag force exerting on the molten pooland keyhole. As a consequence, themass transfer rate of the molten liquidfrom the front of the keyhole to themolten pool behind the keyhole isincreased due to the increased pressurefrom the higher velocity gas jet used inthe semi-cutting-assisted laser welding.Furthermore, the surface tension forcethat tends to close the keyhole isencountered.

The drag force (Fd) exerted on top ofthe molten pool and the keyhole canbe determined by Newton’s equationfor drag force:

where r is the density of the gas (kg

π ρ= ⎛

⎝⎜⎞⎠⎟

⎛

⎝⎜⎜

⎞

⎠⎟⎟

F Cd v

d dg

4 2(1)

2

Fig. 3 — Schematic comparison. A — Conventional laser welding; B — semi­cutting­assisted laser welding.

A B

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m–3), d is the keyhole diameter, is thegas flow velocity, and Cd is the drag co-efficient. The drag coefficient dependson Reynolds number. Here, the slot Reynolds number is de-termined by the following equation(Refs. 24, 25):

where m is the dynamic viscosity of thegases (kg ms–1) and is the gas flowvelocity. The relationship between thedrag coefficient and Reynolds numbercan be found in the handbooks of FluidMechanics for objects of several differ-ent shapes. In addition, the drag coef-ficient also can be experimentallymeasured. Here, we will not give a de-tailed discussion. For a given flowrate, the welding process using asmaller shielding gas nozzle cross sec-tion has a higher shielding gasvelocity. According to Equation 1, ahigher drag force is created by increas-ing the shielding gas flow rate, whichnot only drives the molten metal inthe front of the keyhole to quicklyflow back to the rear of the keyholebut also enables a portion of the zincvapor to quickly escape from the bot-tom of the keyhole. In addition, thelaser-induced plasma plume withinand above the keyhole is suppressedby the side shielding gas. This factorhelps to provide a consistent couplingbetween the laser-beam energy andthe workpieces. As a consequence, thekeyhole is enlarged and stabilized,through which the zinc vapor iscontinuously released withoutinterrupting the molten pool. Figure 4 shows the experimental re-sults obtained by the recentlydeveloped method, which shows that adefect-free, lap joint with completepenetration was obtained. No spatteror porosity was observed in the welds.The surface of the weld is very smoothand complete joint penetration wasachieved. As can be seen in thepolished cross section of the weld pre-

sented in Fig. 4C,the grain sizewithin both theweld zone andheat-affected zone(HAZ) is largerthan those in thebase material, aswould beexpected. Compared to

the stand-off distance of traditionallaser cutting, up to 1.0 mm, the 10mm stand-off distance of the nozzle inthe cutting-assisted laser weldingprocess is relatively large but is neces-sary in order to avoid direct removal ofthe liquid metal by the high pressureflow of the gas. An important point isthat the gas flow rate should becontrolled depending on the size ofthe nozzle, welding speed, and laserpower. For the nozzle used in thisstudy, the preferred shielding gas flowrate is in the range of 0.5 to 1.5 m3/h,which is dramatically lower than thatused in conventional laser welding.When the flow rate of shielding gas isgreater than 1.5 m3/h, the 0.8-mm-thick workpieces are typically cut intotwo. However, if the flow rate of theshielding gas is lower than 0.5 m3/h,the drag force from the nozzle is notsufficient to enlarge and stabilize thekeyhole resulting in the weldingprocess once again becoming unstable.

Effect of Welding Speed

Welding speed plays a significantrole on the weld quality of zinc-coatedsteels. There is very little published onsuccessfully achieving high-qualitywelds in zinc-coated steels at weldingspeeds greater than 4 m/min.Therefore, in order to study the effectof welding speed upon weld quality,welds were made using the semi-cutting-assisted laser welding processwith a range of welding speeds (3.0 to5.4 m/min) and variable laser power

settings (3.0, 3.6, and 4.0 kW). Figure 5 is a collection of photosshowing the quality of the experimen-tal results. As can be seen, defect-freewelds were obtained at the weldingspeeds of 3 and 4.8 m/min. However,when the welding speed was increasedto 5.4 m/min, spatter adjacent to theweld and porosity in the weld were ob-served. In addition, the weld surfacewas irregular. At the low weldingspeed, the molten pool is relativelylarge and the solidification time is longallowing sufficient time for the zincvapor to escape. As the welding speedis increased, the molten pool becomessmaller and a shorter solidificationtime is available for the zinc vapor toescape. Furthermore, at the higherwelding speed, the direction of laser-induced plasma and plume, and theswelling in the molten pool fluctuatetemporally and spatially at higher fre-quency than that at the lower weldingspeed (Ref. 22). Therefore, the zincvapor has a greater probability to dis-turb the molten pool and the keyholehas a higher probability of collapsing.Furthermore, increasing the weldingspeed decreases the collapse time ofthe keyhole and transforms thekeyhole from a cylindrical shape to atapered shape (Ref. 26). The reducedsize of the keyhole around theinterface of the two metal sheets facil-itates the entrapment and expansionof the zinc vapor inside the moltenpool. Therefore, the molten poolbecomes turbulent and results in theformation of severe spatter and poros-ity in the welds.

μ( )= (2)pvdRe /

Fig. 4 — Typical welds obtained by the semi­cutting­assisted laserwelding process (laser power 3.6 kW; welding speed 4.8 m/min).A — Top view; B — bottom view; C — cross­sectional view.

A B

C

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Microhardness Test

The microhardness was analyzedalong a profile from base metal throughthe weld back to base metal atincrements of 0.1 mm and at a depth of0.3 and 1.3 mm below the top surface ofthe weld. Figure 6A, B are graphical rep-resentations of typical microhardnessdistributions through the weldsproduced at welding speeds of 3 and 4.8m/min, respectively. As can be seen inFig. 6A, B, the base metal had ahardenss of approximately 100 HV anda maximum hardness of approximately180 HV, which is located in the weld andis explained by the formation ofmartensite within the weld caused bythe melting and rapid cooling after laserwelding. No softening was found in theHAZ, and no effect of increasing weldspeed was observed on the peakhardness value. For low-carbon steels used in thisstudy, there exist different types of fer-rite in base metal, HAZ, and the weldzone, which have various morphologies.The variation in the ferrite’s morphol-ogy results in different hardnesses atdifferent zones. Base metal mainly con-tains the equiaxed ferrite. However,weld zones and HAZ mainly containpolygonal ferrite and the elongatedcolumnar ferrite, respectively.Compared to equiaxed ferrite, there ex-ists higher dislocation density and alarge amount of subboundaries withinthe polygonal ferrite, thus exhibiting

higher hardness value in the weld zonethan that in the base metal (Refs. 27,28). Also, the higher residual stress inHAZ leads to higher hardness (Ref. 28).

Tensile Test

Figure 7A is a plot of tensile proper-ties from representative welds obtainedat a welding speed of 3 and 4.8 m/min,respectively. It can be observed fromFig. 7C that the semi-cutting-assistedlaser welded tensile samples fracture inthe base metal and not in the weld zoneor HAZ. However, the weld itself is veryhard and constrains the total amount ofdeformation along the gauge length, re-sulting in comparable fracture strengthsbut lower total strains — Fig. 7A.

Conclusions and Future Work

The ability to laser weld zinc-coatedsteels in a zero root opening, lap-jointconfiguration with the assistance of asemi-cutting jet of shielding gas wasstudied in this work. Furthermore, theeffects of welding speed on weld qualitywere investigated. Under theexperimental conditions used, thefollowing conclusions can be drawn: 1. The shielding gas nozzle applieddirectly to the top of the keyhole andthe molten pool is redesigned. The criti-cal modification is a reduced shieldinggas nozzle cross-sectional area in orderto increase the shielding gas velocity. Itis found that for a gas at a given

pressure level and temperature, the re-sulting drag force exerted on the moltenpool and the keyhole stability is signifi-cantly enhanced by increasing the flowrate of the shielding gas used. 2. When the keyhole is enlargedand stabilized due to the increaseddrag force, the release of zinc vapor isimproved. This improvement led todefect-free welds being achieved in azero root opening, lap-joint configura-tion by use of the semi-cutting-assisted laser welding process at awelding speed of up to 4.8 m/min anda shielding gas nozzle cross section of2 mm diameter. 3. The welding speed plays a signifi-cant role on the weldability of zinc-coated steels. The higher the weldingspeed, the more the instability of thewelding process. It is demonstratedthat the laser-induced plasma andplume direction changes significantlywith the increased welding speed. Thedepth of penetration decreased withan increase in travel speed.

Fig. 5 — Comparison of weld quality at different welding speeds (top views of the ob­tained welds). A — 3.0 m/min; B — 4.8 m/min; C — 5.4 m/min.

Fig. 6 — Microhardness profile of weldsobtained at the following welding speed:A — 3 m/min; B — 4.8 m/min. (16­U: thehardness profile at the 0.3 mm depthfrom the top surface of sample 16; 16­D:the hardness profile at the 1.3 mm depthfrom the top surface of sample 16; 16­U:the hardness profile at the 0.3 mm depthfrom the top surface of sample 8; 16­D:the hardness profile at the 1.3 mm depthfrom the top surface of sample 8.)

A A

B

B

C

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4. The welds obtained by semi-cutting-assisted laser welding exhibit agreater weld strength, i.e. hardness, andnegligible HAZ as compared to the basemetal. Thus, under tensile shearloading, the base metal fractured beforeany fracture of the weld or HAZ. 5. The semi-cutting-assisted laserwelding process is easy to operate in theproduction environment and meets theproductivity requirements of the auto-motive industry. As for future work, a correlation isneeded between the laser power and thenozzle angle as well as the workpiecethickness. Furthermore, a high-speedcamera with an illumination light willbe used to monitor in real-time the dy-namic behavior of the molten pool, thekeyhole, and the laser-induced plasmaand plume.

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Fig. 7 — Tensile test results. A — Relationship between engineer­ing stresses and engineering strain; B — fracture location ofbase materials; C — fracture location of typical welds.

References

A B

C

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