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Effect of laser welding mode on the microstructure and mechanical performanceof dissimilar laser spot welds between low carbon and austenitic stainless steels

M.J. Torkamany ⇑, J. Sabbaghzadeh, M.J. HamediIranian National Centre for Laser Science and Technology (INLC), PO Box: 14665-576, Tehran, Iran

a r t i c l e i n f o

Article history:Received 8 March 2011Accepted 14 May 2011Available online 18 May 2011

Keywords:C. LasersD. WeldingE. Mechanical

a b s t r a c t

This paper aims at investigating metallurgical and mechanical characterization of dissimilar laser spotwelds between low carbon and austenitic stainless steel sheets. Microstructural examination, microhard-ness test and quasi-static tensile–shear test were performed. Mechanical properties of the welds weredescribed in terms of peak load. The effects of laser mean power on the performance of dissimilar laserspot welds have been studied. It was found that increasing laser mean power leads to the transition oflaser welding mode from conduction to keyhole. This transition causes a significant growth of the fusionzone size in the lower sheet, i.e. the low carbon steel sheet; since, the keyhole acts as an effective trap forthe laser beam and will greatly increase the energy absorption from the incident laser beam.

It is also shown that the fusion zone size in the weaker sheet, i.e. the low carbon steel sheet is the con-trolling factors in determination of the mechanical strength of dissimilar austenitic/ferritic laser spotwelds.

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1. Introduction

Laser spot welding (LSW), as an innovative technique, has thepotential to substitute the conventional resistance spot welding(RSW) method, since it possesses many advantages for instancebeing a none-contact single side method. Laser spot welds havebeen studied by some researchers [1–3]. Yan and Lee [1] have com-pared the LSWs with the RSWs and showed that the weld penetra-tion and the geometry of the weld cross section determine thestrength of the laser spot welds. Daneshpour et al. [2] have studiedlaser spot welded joints in transformation induced plasticity steelsheets under coach-peel loading condition and suggested laserspot welds as a viable alternative to the conventional resistancespot welds.

Dissimilar spot welding can be more complicated than similarwelding because of different thermo-physical properties of metalsand inevitably, various thermal cycle experienced by each metal.Dissimilar spot welded joints of low carbon and stainless steelsare currently used for car body assembling. The failure behaviorof dissimilar RSWs of low carbon steel and austenitic stainless steelwas studied by Marashi et al. [4]. They investigated the relation-ship between the failure behavior of the RSWs and fusion zonecharacteristics and concluded that the failure mode is controlledby the dilution between two base metals. The distributions of thealloying elements, which form at the welding interface of the

two base metals, would deteriorate the corrosion resistance andmechanical properties of the joint [5]. The interest of laser weldingis to limit the size of the fusion zone due to a much localized en-ergy input of the welding source, high cooling rates and short pro-cess time compared with other techniques [6]. Dissimilar andsimilar laser spot welds have been studied by Daneshpour et al.[3]. They joined dual phase advanced high strength and deep draw-ing steel using two joining processes; resistance and laser spotwelding and examined their monotonic performance under ten-sile–shear loading. They concluded that the dissimilar spot weld’sstrength is governed by the strength of the weaker sheet. Investi-gating the mechanical performance of dissimilar spot welds is ofparamount importance. However, reports in the literature dealingwith the effect of laser welding parameters on mechanical perfor-mance of laser spot welds are limited.

In the present paper, the effect of mean power on laser weldingmode transition from conduction to keyhole is studied. The effectof keyhole formation on the weld growth, the dilution betweentwo metals and the weld microstructure changes is discussed.The, dissimilar laser spot weld pullout failure mechanism is ana-lyzed and in the light of this mechanism, the effect of laser meanpower on joint strength was investigated.

2. Experimental procedure

The base materials used in this research were low carbon (DIN1.1010) and austenitic stainless steel (AISI 304L) sheets with athickness of 0.8 mm. The chemical compositions of the low carbon

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⇑ Corresponding author. Tel./fax: +98 9126873076.E-mail address: mjtorkamany@inlc.ir (M.J. Torkamany).

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steel (CS) and the stainless steel (SS) are given in Table 1. The yieldstrength and ultimate tensile strength of CS sheet were determinedaccording to ASTM E8m [7] at room temperature as 180 and370 MPa, respectively. The corresponding values for SS sheets were330 and 730 MPa, respectively.

Laser welding was performed using a pulsed Nd:YAG laser mod-el IQL-10 with a maximum mean laser power of 400 W. Pure argongas (99%) with a coaxial nozzle at 5–10 l/min flow rate was usedfor shielding. The optical set up was set in a way to obtain a laserbeam diameter of 0.3 mm on the workpiece. The pulse frequencyand pulse duration (on time) were fixed at 14 Hz and 8.5 ms,respectively. The laser mean power is a very effective factor amongother laser parameters; therefore the experiments are done withvarying the laser mean power from 180 to 270 W at an incrementof 15 W through increasing laser pulse energy. The welding speedwas made constant at 3 mm/s to obtain an adequate pulse overlap-ping value. The experiments were carried out in a random order toavoid any systematic error.

Weld configuration was arranged in a way that SS sheet was ontop. Laser welding was conducted from one side (the upper sheet)of the overlapped joints. A fixture was used to prevent the air gapand provide a good contact between upper and lower sheets. Thetensile–shear test samples were prepared similar to specimensused for RSWs, according to ANSI/AWS/SAE/D8.9-97 standard [8].As it is shown in Fig. 1, a circular welding path with a diameterof 6 mm was chosen to obtain a ring shaped weld metal (similarto the resistance spot weld nugget). As can be seen in Fig. 1, thestart and end point of the weld circle path were located in such away to avoid high stress level during tensile–shear loading condi-tion. The sample dimensions are given in Fig. 1. Tensile–shear testswere performed at a cross head of 2 mm/min with an Instron uni-versal testing machine. Peak loads were extracted from the load-displacement curve. Failure mode was determined from the failedsamples.

The welds were cut at the cross section as is shown in Fig. 1.Macroscopic pictures were taken after the appropriate initial stepsof cutting, sanding, polishing, and etching. Marble (4 g CuSO4,20 ml HCl, 20 ml H2O) and Kalling’s No. 1 (5 g CuCl2, 100 ml HCl,100 ml ethanol) etching reagent were used to reveal the macro-structure and microstructure of the samples, respectively. Opticalmicroscopy was used to examine the microstructures and to mea-sure the physical attributes of the weld. Microhardness test, a tech-nique that has proven to be useful in quantifying microstructure–mechanical property relationships, was used to determine thehardness profile in the horizontal directions (100 lm away fromweld centerline for low carbon steel side and stainless steel side),using a 100 g load on a Shimadzu microhardness tester.

3. Results and discussion

3.1. Shape and dimensions of the fusion zone

The mechanical performance of spot welds is mainly governedby the shape and dimensions of the fusion zone. Laser spot weldmacrographs for different laser mean powers are illustrated inFig. 2 showing the evolution of the fusion zone by increasing thewelding power. Owing to the one-side nature of the laser weldingtechnique, the fusion zone has an asymmetric shape such that the

volume of the weld metal in the upper sheet side is larger than thatof the lower sheet side. Despite the asymmetrical shape of the fu-sion zone (FZ), by reason of conducting laser welding from theupper sheet, different laser absorptivity and thermal conductanceof the two base metals lead to the differences in the heat absorp-tion and dissipation that in turn, affect weld metal formation andits growth. Absorptivity of the stainless steel at 1.06 lm Nd:YAGlaser radiation is more than three times greater than that of thecarbon steel [9]. As a consequence, larger fraction of the incidentlaser radiation is initially absorbed, when stainless steel is usedas the upper sheet in the overlapped laser spot welded joints.Moreover, the lower thermal conductivity of the stainless steelcompared to the low carbon steel, leads to larger fusion zone andnarrower heat affected zone (HAZ) in the former.

Fig. 3 presents the area of the weld metal at the cross section ofthe LSWs in both SS and CS sides against laser mean power. Thisgraph shows a general upward trend in the fusion zone size with

Table 1Chemical composition (wt.%) of low carbon and austenitic stainless steel.

Element C Mn P S Si Cr Ni Mo Cu Nb Fe

Austenitic stainless steel 0.028 1.19 0.032 0.002 0.246 18.62 8.02 0.322 1.61 0.023 BaseLow carbon steel 0.042 0.235 0.011 0.006 0.011 0.008 0.035 0.003 0.034 0.001 Base

Fig. 1. Tensile–shear test samples were made according to ANSI/AWS/SAE/D8.9-97.

Fig. 2. Evolution of fusion zone by increasing laser mean power (Q) from a to d; (a)Q = 180 W, (b) Q = 210 W, (c) Q = 240 W, (d) Q = 270 W.

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increasing laser mean power. This indicates that increasing the la-ser power increases the heat input and therefore, increases the fu-sion zone size.

To assess the variations of the fusion zone size, the appliedpower density to the upper and the lower sheets is taken into ac-count, which is mentioned in Table 2 for each sample. The lasermean power (Q), peak power (M) and power density (P) are ex-pressed by the following relations:

Q ¼ E� f ð1Þ

M ¼ E=t ð2Þ

P ¼ 4M=ðpd2Þ ð3Þ

where E is the pulse energy, f is the pulse frequency, t is the pulseduration and d is the laser beam diameter (0.3 mm in our setup).

According to the energy transfer mode and penetration depth,there are two kinds of laser welding. When the laser beam radiateson metal surface, the metal is heated to the melting point and con-sequently the weld penetration is determined by the rate of intakeof energy from the laser and outtake of energy through the thicknessof material by conduction. Welding in such condition is generallyknown as conduction welding. The second kind of laser weldingmode occurs when the laser beam power density is increased to a le-vel sufficient to evaporate a thin layer of material. Subsequently adeep hole is created inside the weld pool. This keyhole is an effectivetrap for the laser beam [10]. The keyhole welding in a 0.8 mm thickAISI 304 stainless steel sheet can only occur for the power densitiesgreater than 2 � 105 W/cm2 [11]. From Fig. 2, it is conspicuous thatin all samples, the keyhole is formed in the upper (SS) sheet, full

penetration into the sheet thickness is reached and the laser beamexits the stainless steel sheet. After crossing the narrow air gapbetween the two overlapped sheets, the laser beam hits the carbonsteel sheet while have lost a fraction of its energy. The remainedenergy of the laser beam governs the welding mode in the lowersheet.

The fusion zone size (FZS) on the lower sheet (CS) is consideredas the basis for defining the transition from conduction to keyholewelding mode. From 210 W to 240 W laser mean power, in Fig. 3, asharp increase in the FZ area of the low carbon steel side can beobserved. This is attributed to the keyhole formation in the CSsheet at the mean power of approximately 210 W. The keyhole actsas an effective trap for the laser beam and will greatly increase theenergy absorption from the incident laser beam. This is because thelaser beam undergoes multiple reflection and absorption in thekeyhole (Fresnel absorption). In other words the change in modeof absorption from conduction to keyhole provides a 4–5 timesincrease in laser power coupling efficiency [10].

When laser mean power increased to approximately 240 W, fullpenetration is reached through the thickness of two overlappedsheets. After full penetration, with increasing heat input throughincreasing laser mean power, the FZ size in low carbon steel sideincreases more gradually. In practice, penetration of the weldthrough the whole thickness allows the laser beam and the plasmato escape out of the molten pool from the weld root and therefore,lead to a decline in the absorbed energy into the material [10].

The fusion zone size in the SS side increases steadily withincreasing laser mean power; however, at laser mean powersgreater than 255 W, a decrease of the FZ area in the upper sheet(SS sheet) is observed. This reduction is due to the considerableweld spattering on the upper sheet. After full penetration, furtherincrease in energy transfer rate and too much power causes vapor-ization and material ejection in the upper sheet, as in drilling,which is known as a weld defect called weld spattering.

3.2. Joint structure

A typical macrostructure of an austenitic/ferritic dissimilar LSWcan be seen in Fig. 4. Left hand side of the picture shows the heter-ogeneous structure produced at the joint region. This region con-sists of three distinct zones; weld fusion zone (FZ), heat affectedzone (HAZ), surrounding the weld metal in each metal sheet, andthe base metal (BM).

As can be seen in the right hand side of Fig. 4, new equiaxedgrains were nucleated at the fusion boundary. However, the solid-ification structure of the bulk FZ was predominantly fine columnardendrites which had been nucleated at the fusion line and growntoward the center of the molten pool. Dendritic structure is attrib-uted to the very high cooling rate, which is roughly 104–106 �C/s inthe fusion zone of the laser keyhole welds [12,13]. The fusion linesof the successive pulses are obvious in the FZ, Fig. 4. This is due tothe fact that at higher overlapping values of pulsed laser welding,the volume fraction of material that re-melts and re-solidifiesincreases and the weld pool produced by each pulse does notcompletely solidify, individually [10,14].

It is worth mentioning that no phase transformation occurs inthe HAZ of stainless steel side since austenitic stainless steel basemetal is not transformable. However, grain structure of this regionis affected by the welding process and some grain growth andrecrystallization is obvious in the HAZ of SS and CS side, respec-tively. As the result of the low rate of heat input, the HAZ adjacentto the fusion zone is approximately 20 lm on the stainless steelside. The extent of the HAZ for low carbon steel is however ofthe order of 80 lm wide. The extreme narrowness of the HAZ inlaser welding is attributed to the short duration of the thermalcycle.

Fig. 3. Fusion zone size in low carbon and stainless steel sides of the laser spot weldas a function of the laser mean power.

Table 2Laser mean power, laser energy, laser peak power and laser power density applied toeach sample.

Sample Mean power(W)

Energy(J)

Peak power(W)

Power density (W/cm2)

1 180 12.86 1513 2.55 � 105

2 195 13.93 1639 2.76 � 105

3 210 15.00 1765 2.97 � 105

4 225 16.07 1891 3.18 � 105

5 240 17.14 2017 3.40 � 105

6 255 18.21 2143 3.61 � 105

7 270 19.29 2269 3.82 � 105

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When joining two different materials, the weld metal composi-tion is different from the base metal composition. Dilution is ex-pressed as the contribution percentage of low carbon steel in theweld fusion zone (defined as low carbon to the whole weld metalvolume ratio in the weld nugget). A dilution of 10% means that theweld metal contains 10% low carbon steel and 90% stainless steel.Dilution for each sample is calculated based on the fusion zone size

Fig. 4. Macroscopic pictures showing the weld structure. Right picture demonstrates the growth of the columnar grains perpendicular to the fusion line.

Table 3Variation of the dilution with increasing laser mean power, the average microhard-ness value in fusion zone of each FZ and the predicted microstructure (according toSchaeffler diagram).

Mean power Dilution Av. FZ hardness (Hv) Predicted microstructure

180 2.64 230 A + M + F195 15.85 298 A + M + F210 16.50 328 A + M + F225 29.87 436 M240 38.04 418 M255 40.69 397 M270 47.24 378 M

A: Austenite, M: Martensite, F: Ferrite.

Fig. 5. Shaeffler diagram predicting the microstructure in the fusion zone.

Fig. 6. Triple microstructure in the fusion zone of 16.5% dilution (sample 3),containing martensite (M), ferrite (F) and austenite (A).

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measurements on the weld cross section (Fig. 3). The effect of thewelding power on the dilution of two base metals is presented inTable 3. The microstructure of the fusion zone of dissimilar metaljoints between low carbon and stainless steel can be predictedusing constitution diagrams like Schaeffler diagram [15].

The predicted microstructure of the weld metal for each lasermean power using Schaeffler diagram (see Fig. 5) and the corre-sponding dilution value, are mentioned and compared with themicohardness measurements in FZ in Table 3. The application ofthis diagram might be inaccurate because of the very high coolingrates encounters in the laser welding process.

As can be seen in Table 3, the Schaeffler diagram predicts thatthe welds produced using mean powers less than 210 W possessa triple microstructure containing austenite, martensite and ferrite.However, other welds are predicted to compose of a fully martens-itic microstructure. Results from hardness measurements supportthis prediction (see Table 3) since there is a sharp increase in theaverage hardness of the welds resulted from mean powers greaterthan 210 W. The existence of the martensite results in high hard-ness values in the fusion zone. In addition, the results of the optical

metallography agree qualitatively with the predictions based onthe Schaeffler diagram. Typical triplex and fully martensitic weldmetal microstructures are shown in Figs. 6 and 7, respectively.The observed microstructure is consistent with that reported bySun [16]. His study on dissimilar laser weld joining AISI 347 and13CrMo44 steels revealed that the weld metal has a mixed micro-structure containing austenite and martensite. Anawa and Olabi[12] reported a complex austenitic–ferritic structure in the fusionzone of a dissimilar laser weld between AISI 316 and AISI 1008steels. However, their microhardness measurements contrarilyshow the evidence of the martensite formation in the fusion zonesince the average hardness is around 460 Hv in this region.Although the microstructure of laser weld was predictable in thisstudy, some inhomogeneous regions were observed. This is possi-bly related to inhomogeneous mixing of the two base metals as aconsequence of the fast cooling rate of the laser welding process.

3.3. Hardness profiles

The microhardness measurements indicated that the basematerials hardness was about 170 and 270 Hv, for the low carbonand austenitic stainless steel, respectively. These values are inagreement with the microstructure and chemistry of the base met-als. The horizontal hardness profile for sample 5 obtained using240 W mean power is demonstrated in Fig. 8. As it can be observed,the hardness of the FZ is 2.3 times greater than that of the CS and1.5 times more than that of the SS. The average hardness of theweld metal is much higher than the hardness of both base metals.Higher hardness values in the weld nugget can be attributed to themartensite formation in this zone. The hardness value in the FZ inboth upper and lower sheets is at a peak. However, the peak hard-ness value for the stainless steel side is slightly higher than this va-lue for low carbon steel side. This is probably due to theinhomogeneous mixing of the two base metals in the fusion zoneand the rapid solidification of the weld pool.

4. Load carrying capacity and failure behavior

The experiments revealed that the laser mean power signifi-cantly affected the load carrying capacity of the laser spot weldsunder the tensile–shear monotonic loading. As can be seen inFig. 9, increasing the laser mean power resulted in increasing thepeak load of dissimilar joints. The photographs representing

Fig. 7. Fully martensitic microstructure in the fusion zone of the welds produced bymean power higher than 210 W; this picture shows the weld metal micrograph ofsample 5.

Fig. 8. Microhardness profile across the fusion zone in both base metal sheets, 100 lm away from the interface of the overlapped sheets. Dash lines indicate the fusion line.

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fractured samples of LSWs are also shown in Fig. 9 which demon-strate pullout failure mode. All specimens in this study failed underpullout failure mode which is the preferred mode to provide opti-mum structural performance.

Observations during the tensile test of the lap shear samples re-vealed that as the sample is pulled, initially, the weld ring experi-ences a rotation which causes bending in the base metal in thevicinity of the weld ring in both joined sheets. Bending moment in-duces tensile–stress around the weld nugget which is the drivingforce for the pullout failure mode [17–19]. As is conspicuous inFig. 10, large amount of bending took place in low carbon steelsheet because of its lower stiffness. In fact, the sheet with the lowerstrength tends to rotate more than the stronger one because in thelatter, the material around the weld ring is more constrained ondeformation.

Generally, in the laser spot welds which fail under pull out fail-ure mode, the crack initiates and grows at the lower sheet. This isprobably due to the fact that there is smaller volume of the weldmetal in the lower sheet. This produces lower stiffness and there-fore, gives rise to the higher levels of strain concentration in thematerial adjacent to the weld ring in the lower sheet. In dissimilarLSWs, this together with the higher strength of SS steel sheet gen-erates higher level of strain concentration in the CS sheet. As load-ing continues, strain localization at the base metal of the lowcarbon steel side leads to localized necking at this region. Fig. 11depicts the cross section of a failed LSW indicating that the failureis advanced by necking in the CS sheet.

Attentive inspection of the failed samples exhibits that thecrack initiates in the CS sheet. Firstly, initiated crack follows theperimeter of the weld ring. However, after a while, crack deviatesfrom its main root and propagates obliquely into the base metalsheet (see Fig. 12). In brief, the failure mechanism in the dissimilarlaser spot welds is the necking in the BM of the weaker sheet. Theinitiated crack in the CS sheet follows the circumference of theweld ring and then deviates toward the base metal.

To sum up, it is deduced that the strength and the fusion zonesize of the low carbon steel side control the joint strength in thepullout failure mode. Fig. 13 shows the effect of the fusion zonesize of CS side on the fracture load of LSWs. As a whole, there isan upward trend in the load carrying capacity with increasingthe weld size. This is because of the increase in the weld ring resis-tance against the rotation and therefore increasing the requiredforce for necking at the failure location. Further thought shows thatload carrying capacity initially increases steeply with increasingFZS possibly because of the transition in the laser welding modeto the keyhole welding. However, after the keyhole formation inthe CS sheet (see Figs. 2 and 3), the increasing trend becomes moregradual.

When the full penetration through CS sheet is achieved, furtherincrease in the laser mean power (more than 250 W) does not

Fig. 9. Load carrying capacity versus laser mean power; corresponding failedsampled are given to show the pullout failure type.

Fig. 10. Tensile–shear standard specimen under loading; the low carbon steel sheetunderwent a higher level of deflection because of less weld metal content and lowerstiffness of the base metal.

Fig. 11. Macrograph of the cross-section of a failed sample; this section demonstrates the pull out type of failure mode and occurrence of the necking. ‘‘T’’ represents tensilenormal stress induced in the material surrounding the weld fusion zone.

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considerably increase the fusion zone size in the lower (CS) sheetand consequently it would not lead to an effective increase in theload carrying capacity of the laser spot welded joints.

5. Conclusions

In this research, laser spot welding of low carbon to austeniticstainless steel sheets was studied. Effect of the increasing lasermean power on the shape, dimension and structure of the weldfusion zone was investigated. The mechanical performance of dis-similar laser spot welded joints was analyzed under lap-shearloading. The following conclusions can be drawn from thisresearch:

(1) Laser spot welding is a very successful process for joiningaustenitic stainless steel and low carbon steel in an over-lapped configuration.

(2) The fusion zone has an asymmetric shape in dissimilar LSWbetween low carbon steel and austenitic stainless steelbecause of the one-side nature of the laser welding processin addition to the different laser beam absorption and thethermal conductivity of the base metals.

(3) Transition of the laser welding mode from conduction tokeyhole welding via increasing the laser mean power signif-icantly affects the fusion zone size in the lower (CS) sheetand in view of that the dilution between two base metalsstrongly depends on the laser welding mode.

(4) Increasing the laser mean power leads to an increase in theload carrying capacity of the LSW primarily due to theincreasing fusion zone size in the CS sheet. Larger volumeof the high strength weld metal in the CS sheet lessens therotation of the weld ring and therefore delays the strainsconcentration in the base materials around the weld ringand the subsequent necking.

Acknowledgement

The authors would like to acknowledge the Iranian NationalCentre for Laser Science and Technology (INLC), for providing foun-dations (Project Number 9557001) for this research.

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Fig. 12. A failed sample showing the crack in the low carbon steel sheet; crackfirstly follows the circumference of the weld ring and then propagates obliquelyinto the base metal sheet.

Fig. 13. Load carrying capacity as a function of the size of the fusion zone in the lowcarbon steel sheet.

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