6013铝合金微裂纹分析

Journal of Materials Processing Technology 231(2016)18–26Contents lists available at ScienceDirectJournal of Materials ProcessingTechnologyj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j m a t p r o t ecMicro-scale model based study of solidification cracking formation mechanism in Al fiber laser weldsXiaojie Wang a ,d ,Fenggui Lu a ,b ,∗,Hui-Ping Wang c ,∗,Zhaoxia Qu d ,Liqian Xia daShanghai Key Laboratory of Materials Laser Processing and Modification,School of Materials Science and Engineering,Shanghai Jiao Tong University,Shanghai 200240,PR China bCollaborative Innovation Center for Advanced Ship and Deep-Sea Exploration,Shanghai 200240,PR China cManufacturing System Research,GM Global R&D,Warren,MI 48090,USA dWelding and Corrosion Protection Technology Department Research Institute,Baoshan Iron &Steel Co.Ltd.,Shanghai 201900,PR Chinaa r t i c l ei n f oArticle history:Received 27September 2015Received in revised form 19November 2015Accepted 11December 2015Available online 15December 2015Keywords:Micro-scale model Solidification cracking Welding speed Pressure drop Micro structurea b s t r a c tEffect of welding speed on solidification cracking susceptibility in fiber laser welding of 6013aluminum alloy was investigated by considering both mechanical and metallurgical factors.A finite element model was developed at columnar grain scale to calculate the strain localization in the mushy zone.Based on the Rappaz-Drezet-Gremaud (RDG)criterion and study from the developed micro-scale model,the pressure drop in the inter-dendritic liquid film was used as cracking index to investigate the formation of solidification cracking.Cooling rate,solid fraction,local strain rate and microstructure characteristic resulted from different welding speeds were examined.Cracking sensitivity was shown to decrease with the increase of welding speed in the range between 2.5m/min and 3.5m/min in fiber laser welding of 6013aluminum alloy.Numerical calculations showed that mechanical tensile strain in the solid grain was in the order of magnitude of 10−4while the strain in the liquid film is in the order of magnitude of 10−2.The total pressure drop at the root of columnar grains was more than 150kPa,which was deemed as the critical pressure drop to form a crack in this study.The transverse solidification cracking was prone to initiate in the second half of mushy zone where the solid fraction was between 90%and 94%.©2015Elsevier B.V.All rights reserved.1.IntroductionAluminum alloys have been widely employed in body con-struction of automobiles and aircraft due to their outstanding mechanical properties and formability.Fusion-based welding pro-cesses such as arc welding and laser welding are commonly used in joining the Al materials.However,solidification crack,which is harmful to structural integrity,could easily exist in the welded Al joints (Kou 2003).The solidification cracking occurs in the mushy zone when insufficient amount of liquid flows back to heal the tear-ing of inter-dendritic liquid film during the material’s solidification processes (Hatami et al.,2008and Sheikhi et al.,2014).For high manufacturing productivity,welding speed is usually set to be as large as possible in practical applications.It is generally accepted in welding community that large welding speed would lead to hot cracking (Niel et al.,2013).Cicala et al.(2005),Fabregue∗Corresponding authors.E-mail addresses:Lfg119@ (F.Lu),hui-ping.wang@ (H.-P.Wang).et al.(2008a)and Tirand et al.(2013)independently investigated the effect of welding speed on hot cracking during Nd:YAG laser welding of AA6056aluminum alloy with welding speed in the range between 3m/min and 6m/min,4m/min and 6m/min,and 1.2m/min and 3.5m/min,respectively.They all observed that high welding speed resulted in high hot cracking susceptibility,which was explained by the resulted high weld pool instability,high cool-ing rate and high solidification rate.El-Batahgy and Kutsuna (2009)reported that the hot cracking started to occur in AA6061,AA5083and AA5052aluminum alloys during CO 2laser welding when the welding speed increased from 3m/min to 6m/min.Haboudou et al.(2003)showed that use of a second beam to decrease the cooling rate of molten pool would effectively prevent hot cracking.However,positive influence of welding speed on hot cracking of aluminum alloys was also reported.Chihoski (1972)indicated an improvement in weldability when increasing the welding speed to 0.8m/min in the gas tungsten arc (GTA)welding of AA2014alu-minum alloy.The high welding speed would induce a compressive loading zone at the tail of molten pool that helped suppress-ing hot cracking.Abbaschian and Lima (2003)studied the hot cracking sensitivity of Al–Cu alloy with various welding speeds/10.1016/j.jmatprotec.2015.12.0060924-0136/©2015Elsevier B.V.All rights reserved.X.Wang et al./Journal of Materials Processing Technology231(2016)18–2619 in continuous-wave CO2laser welding.With increasing weldingspeed from0.06m/min to3m/min,the cracking susceptibilityincreasedfirst and then dropped due to the refinement of porositiesand changes of liquid fraction.As discussed above,the influence of welding speed on hotcracking varies for different welding processes and at differentwelding speed.The cooling rate,solidification rate,stress state andmicrostructure characteristic all would influence the hot crack-ing susceptibility.Eskin and Katgerman(2004)stated that the hotcracking was a physical phenomenon affected jointly by thermal,mechanical and metallurgical features.Hence,a comprehensiveunderstanding of hot cracking sensitivity at different welding speedshould take into account all the factors mentioned above.Rappaz et al.(1999)proposed a hot cracking model,namedas RDG(Rappaz-Drezet-Gremaud)model,which considered theeffects of liquid feeding,permeability of microstructure,solid frac-tion and deformation of the grains on the hot cracking initiation.Wang et al.(2004)analyzed the effect of grain boundary misorien-tation on hot cracking tendency based on the RDG model.Drezetand Allehaux(2008)systematically analyzed the application ofRDG model in the prediction of hot cracking phenomenon in thewelds.Niel et al.(2013)extended the RDG model by introducingmicrostructure prediction and investigated the effect of weldingcurrent on hot cracking in the GTA welding.Sheikhi et al.(2014)used the RDG model to calculate the hot cracking sensitivity insuccessive laser weld spots considering preheating effect.In this study,the effect of welding speed on solidification crack-ing infiber laser welding of6013aluminum alloy was investigatedexperimentallyfirst.Next,a micro-scalefinite element model of acolumnar grain in mushy zone was developed which mainly inves-tigated the strain rate of grains with the existence of liquidfilm.Based on the RDG model and the corresponding pressure drop,theeffect of welding speed on hot cracking sensitivity was analyzed.Other important state variables such as cooling rate,solid fraction,stress state and microstructure characteristic were also examined.Also,the possible initiation site of solidification cracking infiberlaser welding of aluminum alloy was predicted.2.Weld solidification cracking model2.1.RDG criterionDue to large temperature gradient existing near fusion line of themolten pool in laser welding,columnar dendrite structure wouldappear near the fusion line while equiaxed grains exist in the centerof fusion zone where temperature gradient decreases(Niel et al.,2013).According to the study of Fabregue et al.(2008b),solid-ification cracking initiates near the fusion line where columnargrains exist.To investigate the formation of solidification crack-ing in columnar dendrite structure,Rappaz et al.(1999)proposedthe RDG model and deemed crack as a result betweenflow rate andthe deformation and shrinkage of columnar grains,as shown in Eq.(1):f l v l,x=−(1+ˇ)Lf s˙εdx−v Tˇf l(1)v T=V cos˛(2) where f l is the liquid fraction,v l,x is the velocity of thefluid along the x-axis which is parallel to columnar grain growth direction,ˇis the shrinkage factor,L is the mushy zone length along x-axis,f s is the solid fraction,˙εis the deformation rate of grain boundary per-pendicular to the grain growth direction,and v T is the solidification rate.Thefirst term on the right-hand side of Eq.(1)is theflow rate caused by the tensile deformation of mushy zone,and the second term on the right-hand side of Eq.(1)is theflow rate associated with the solidification shrinkage of mushy zone.Rai et al.(2008) provided the relationship between the solidification rate and the welding speed in Eq.(2),where V is the welding speed and␣is the angle between the normal of solid-liquid interface and the direction of welding velocity.The permeability of microstructure reflects the degree of microstructure in beingfilled back by liquidflow.Niel et al.(2013) defined the permeability of mushy zone in Al welds as follows:K=22180(1−f s)3f2s(3)where 2is the secondary dendrite arm spacing.Theflow rate is related to the pressure gradient in the liquid,the permeabil-ity of the mushy zone and viscosity of the liquid( )by f l v l,x=−(K/ )(dp/dx),which cited from the study of Kubo and Pehlke (1985).Substituting thisflow rate calculation into Eq.(1),the pres-sure drop in the interdendritic liquid could be calculated as follows (Rappaz et al.,1999):P=18022(1+ˇ)LE(x)f2s(1−f s)3dx+18022v Tˇ×Lf s1−f s2dx(4)In the RDG model,the interdendritic liquid pressure drop between the tip and root of the dentrite is required to ensure that adequate liquid feeding back occurs to compensate the deforma-tion and shrinkage of mushy zone.If the pressure drop reaches the critical pressure drop that a cavity forms,hot cracking initiates.Eq.(4)tells us that the solidification rate at columnar grain tip,solid fraction,deformation rate of the columnar grain,length of mushy zone and secondary dendrite arm spacing all would affect the pres-sure drop,hence the susceptibility of solidification cracking.Based on the work of Rappaz et al.(1999),shrinkage factor and viscosity were taken as0.06and0.001Pa s for Al welds,respec-tively.The secondary dendrite arm spacing 2could be obtained by experimental observation.The others,such as mushy zone length, solid fraction,solidification rate and strain rate,are taken from calculated results.2.2.Thermal-mechanical simulation of welding processWe now know that hot cracking susceptibility is affected by many factors and these factors are determined by the thermal field and mechanical deformation in the process.In order to obtain temperature and strain/stress distribution in the weld,a thermal-mechanical simulation of the laser welding process is performed.The heat conduction and the thermal-elastic-plastic deformation are considered in the simulation to calculate the tem-perature and strain/stress.To increase calculation precision t,a three dimension’s(3D)thermal-mechanical model is developed with the model size of60mm×35mm×2.5mm.In addition,the model considers temperature-dependent material physical param-eters,a volumetric heat source model,strain/stress relaxation in the weld molten pool and solidification shrinkage of the weld metal. The temperature-dependent material physical parameters of6013 aluminum alloy used in the model are given in Fig.1.Thefiber laser heat input was modeled by a cylindrical Gaussian distribution heat model,which was proposed by Kong et al.(2011). It is described by Feng(1994)to represent the stress/strain relax-ation of liquid metal in weld pool and the element rebirth method was adopted.Elements whose temperatures are over the liquidus point would be killed(zero stress)in the model,and re-birthed again as their temperatures drop to be lower than the liquidus20X.Wang et al./Journal of Materials Processing Technology231(2016)18–26Fig.1.Temperature-dependent material physical parameters of6013aluminum alloy:(a)physical parameters and(b)mechanical properties.point.The solidification shrinkage of weld metal was considered in the model by setting the reference temperature of solidified metal to liquidus point and adding the volume shrinkage of solidified metal to thermal expansion.2.3.Micro-scale model for local strain in the mushy zoneAccording to the solidification theory,David and Vitek(1989) indicated that the columnar to equiaxed transition(CET)meant that the rapid growth of equiaxed grains in the center of fusion zone inhibited the columnar grain growth.For weld microstruc-ture,the possible location of crack initiation,which is surrounded by liquidus line(T L),solidus line(T coalescence)and CET line,would be subdivided into two regions,as shown in Fig.2.Region A is defined as location where columnar grain tip connects with molten pool while Region B is defined as location where columnar grain growth is prevented by equiaxed grain.Microstructure in Region A has a similar columnar morphology as one discussed in RDG model.How-ever,the solidification rate at the columnar grain tip is zero due to the columnar to equiaxed transition in Region B,and the required flow rate in Part B is only to compensate the deformation of mushy zone if no crack forms.Prediction of the cracking susceptibility and possible initiation location requires knowledge of microstructure characteristic and local strain rate perpendicular to columnar grain.The microstruc-ture characteristic,which includes the columnar grain length, primary dendrite arm spacing of columnar grain and orientation of the grain boundary in the macroscopic frame,can be obtainedbyFig.2.The schematic of weldmicrostructure.Fig.3.Size and boundary condition of the FEM model on columnar grains. experimental observations.The local strain rate on columnar grain is calculated by a FEM model on columnar grains in this study.As shown in Fig.3,the dimensions of FEM model on columnar grains are determined by mushy zone length(L),primary den-drite arm spacing( )and thickness of liquidfilm(d).Based on the temperature distribution obtained from the thermal-mechanical welding process simulation,the mushy zone length is taken as dis-tance between liquidus point line(921K)and fusion line in Region A,and the columnar grain length is deemed as mushy zone length in Region B.Thickness of liquidfilm between columnar grains is cal-culated by Eq.(5)where is primary dendrite arm spacing and f s is the solid fraction,which is calculated with the help of Thermo-Calc software.d= (1−f s)(5) The FEM model on columnar grains consists of solid and liq-uid phases at the same time,and a continuum-level modeling is employed to calculate the local strain rate perpendicular to colum-nar grain by special treatment of liquid physical parameters.The solid and liquid physical parameters used in the model are given in Fig.1(b)and Fig.4,respectively.The mechanical properties of liquid phase were treated as one-twentieth of the solid by a trial-and-error iterative process based on the reportedfindings by Coniglio and Cross(2009)and Bordreuil and Niel(2014).Their work showed that the local strain in liquid was about102-103times of local strain in solid.The boundary condition of temperature and displacement were obtained from process simulation and linearly interpolated from the process domain to the columnar grain domain.Temper-ature along the columnar grain is used to calculate solid fraction,X.Wang et al./Journal of Materials Processing Technology231(2016)18–2621Fig.4.Temperature-dependent material physical parameters used in the FEM model on columnar grains.and the calculated local strain rate of the solid-liquid interface is adopted as the deformation rate of solids in Eq.(4).The RDG criterion simplifies the feeding back of liquidflows only along the growth direction of columnar grain.Therefore,only strain on grain boundary perpendicular to the growth direction of grain is required.Considering the orientation of columnar grain boundary in macroscopic frame,Bordreuil and Niel(2014)proposed the for-mulation for the strain perpendicular to columnar grain as follows:ε=εx sin2Â+εy cos2Â+2 xy sinÂcosÂ(6)whereεx is mechanical strain in horizontal direction,εy is mechan-ical strain in vertical direction andÂis the angle between the columnar grain direction and the welding velocity.Once the strain was obtained,the strain rate can be easily calculated and it varies from location to location in the mushy zone.3.Welding experimentAA6013aluminum alloy specimens with the dimension of 150mm×125mm×2.5mm were used in the welding test.The chemical composition of AA6013is0.95Mg,0.62Si,0.95Cu,0.22 Fe,0.37Mn(wt.%),balanced by Al.YLS-10000fiber laser system was taken as heating source with spot diameter being0.8mm.To investigate the effect of welding speed on solidification cracking in fiber laser welding of AA6013aluminum alloy,we tested with the welding velocity being2.5m/min,2.7m/min,3.0m/min,3.3m/min and3.5m/min.The laser power was3.3kW,and argon shielding gasflow rate was15L/min.Experimental test system was set up referring to the test of Ploshikhin et al.(2007)that the mechanical clamping was applied on one side of the weld line while the other side is free.In this study,the distance between weld centerline and closest free edge was7.5mm,and the distance from the weld cen-terline to the clamping edge was20mm.Bead-on-plate weld was adopted in the experiment to avoid the influence of gap between two overlap sheets on the cracking susceptibility.The Al specimen surfaces were cleaned prior to welding test to remove any oils and contaminations.The welding test showed that transverse crack appeared when the welding speeds were2.5m/min,2.7m/min and3.0m/min. That is,the transverse solidification cracking is prone to initiate when the welding speed is relatively low forfiber laser welding of6013aluminum alloy.Fig.5gives pictures of the weld’s top surface.Fig.5(a)shows the weld surface for V=2.7m/min while Fig.5(b)gives the weld surface for V=3.3m/min.At the low welding speed of V=2.7m/min,the weld beam is wide and the transverse solidification cracking was observed to initiate near the fusion line,as shown in Fig.5(a).When the welding speed increased to3.3m/min,the transverse solidification cracking disappeared. Therefore,the experimental results indicated that the low welding speed increased solidification cracking susceptibility in thefiber laser welding of6013aluminum alloys.4.Results and discussion4.1.Metallographic analysisGrain structure characteristic plays a significant role in solidification cracking behavior.The metallographic pictures of microstructure for two different welding speeds are given in Fig.6(a–d).The pictures show that the columnar dendrite grains grow near two sides of fusion zone and the equiaxed dendrite grains appear in the middle of fusion zone for both cases.Since the weld-ing speed determines the temperature gradient and solidification rate,the resultant microstructure in Fig.6(a,b)exhibit obvious dis-tinction.The columnar to equiaxed transition was advanced when the welding speed increased from2.7m/min to3.3m/min.The length of columnar grain was about500␮m with welding speed of2.7m/min and300␮m with welding speed of3.3m/min.Fig.6 (c)and(d)shows the local amplification of columnar grains for two cases.The columnar grain boundaries are plotted by black dotted lines.As a simple estimation,the primary dendrite arm spacing is about51␮m for the speed of2.7m/min and60␮m for the speed of3.3m/min.Based on the study of Kimura et al.(2009)that afiner grain size led to lower susceptibility to cracking,grain morphol-ogy induced by the lower welding speed in this study had less equiaxed grains and more columnar grains,hence a higher solidifi-cation cracking susceptibility.From the metallurgical point ofview, Fig.5.The top surfaces of weld generated with welding speed of:(a)V=2.7m/min and(b)V=3.3m/min.22X.Wang et al./Journal of Materials Processing Technology231(2016)18–26Fig.6.The microstructure characteristics of weld for different welding speeds:(a)V=2.7m/min;(b)V=3.3m/min;(c)and(d)are local amplification of columnar grains.Fig.7.Calculated temperature evolution versus the welding time at a point of the weld centerline.Fig.8.The calculated longitudinal mechanical strain distribution on top surface of test plates for two cases:(a)V=2.7m/min and(b)V=3.3m/min.X.Wang et al./Journal of Materials Processing Technology231(2016)18–2623Fig.9.The local mechanical strain perpendicular to columnar grains on top surface:(a)V=2.7m/min and(b)V=3.3m/min.24X.Wang et al./Journal of Materials Processing Technology231(2016)18–26the long mushy zone increases the difficulty of liquid metal toflow back,and small primary dendrite arm spacing increases the degree of stress concentration.4.2.Thermal-mechanicalfield during welding processFig.7illustrates the temperature evolution versus the welding time at a point of the weld centerline.The cooling rate between the liquidus and solidus points is investigated.Due to the fast welding speed in laser welding,the cooling rates for both cases are very high with3380K/s for2.7m/min and4770K/s for3.3m/min.The higher the welding speed,the higher the cooling rate.Solidification cracking occurs in mushy zone where liquid and solid phases coexist.In this study,the temperature range between liquidus point and coalescence point is defined as mushy zone. The longitudinal mechanical strain,which is related to transverse solidification cracking,is shown in Fig.8.The tensile longitudi-nal strain appears in later part of mushy zone for both cases, which promotes cracking.The maximum longitudinal mechanical strain in mushy zone is1.4%for2.7m/min and0.9%for3.3m/min, respectively.Based on the results of welding process simulation, relatively small cooling rate and large deformation are induced at low welding speed.In order to determine the cracking sensitivity, we still need to know local strain rate in addition to the cooling rate and deformation of the mushy zone we have calculated from the thermal-mechanical analysis.4.3.Distribution of local strain rateFig.9gives the contour of local mechanical strain perpendic-ular to columnar grains at different positions in mushy zone.The liquidfilm sits in the middle of columnar grains.The differences of columnar grains are the thickness of liquidfilm,which presents the different solid-liquid fraction during the solidification process.For positions a and b,which sits in the early part of mushy zone,the compressive mechanical strain occurs.From position c to position e,the tensile mechanical strain distributes in the whole columnar grain.It is noted that the local mechanical strain in solid columnar grain is much smaller than mechanical strain in liquid film,and the extent of difference increases with the decreasing of liquidfilm thickness.For the case with welding speed of2.7m/min, the magnitude order of local mechanical strain is about10−4in solid grain and the order of local mechanical strain in liquidfilm is10−2, which is about102–103times than the mechanical strain in solid. The same characteristic is also found in case with welding speed of 3.3m/min.In addition,the local mechanical strain of solid–liquid interface is relatively large in the case of low welding speed.For the test case with welding speed being2.7m/min,the rate of mechanical strain perpendicular to the solid-liquid interface along columnar grain is shown in Fig.10.The black dotted line represents initiation of the columnar grain and the x-coordinate represents the location along the length of grain.In the early part of mushy zone such as positions a,b and c,the mechanical strain rate on the solid-liquid interface is lower than the mechanical strain rate in the latter part of mushy zone such as positions d and e.The maximum local mechanical strain rate occurs in the latter part of mushy zone with the magnitude order of10−1under the test condition.4.4.Analysis of solidification cracking susceptibilityThe feeding back of liquidflow plays a significant role in the solidification cracking behavior.Fig.11shows the requiredflow rate to compensate deformation and shrinkage of columnar grains in the mushy zone.The requiredflow rate is same at position a and position b since the solidification shrinkage is the dominant effect in this region,whereas the contribution of strain rate isvery Fig.10.The perpendicular strain rates on the solid-liquid interface along columnargrain.Fig.11.The requiredflow rate to compensate deformation and shrinkage of colum-nar grain in mushy zone.little.At position c,the effect of solidification shrinkage disappears because the columnar grains stop growth already and the effect of strain rate is still small at this moment.Hence,the requiredflow rate reaches the smallest values at position c.With the increase of solid fraction,the strain rate grows significantly,as shown in Fig.10. As a result,the requiredflow rate is high at position d.The columnar grain length decreases from position d to position e while the strain rate does not increase too much,and the competing results is that the requiredflow rate decreases again from position d to position paring two different welding speeds,we note that the low welding speed needs relatively highflow rate to compensate grain deformation and shrinkage,which is mainly induced by the high strain rate and large columnar grain length at low welding speed.Fig.12shows the shrinkage and deformation contributions to the pressure drop in the interdendritic liquid.Under the condition of this test,the magnitude of pressure drop induced by deforma-tion is obviously higher than pressure drop induced by shrinkage, which indicates that the strain rate is more significant than solid-ification rate in controlling the solidification cracking initiation in this test.Fig.13shows the solidification cracking susceptibility for different welding speeds based on the pressure drop at the root of columnar grains.Drezet et al.(2008)used150kPa as the criti-cal pressure drop for cracking initiation in Al welds and this value is also adopted in this study.The calculated result indicates that the test case with welding speed being2.7m/min is susceptible toX.Wang et al./Journal of Materials Processing Technology231(2016)18–2625Fig.12.(a)Mechanical contribution to the pressure drop and(b)shrinkage contribution to the pressure drop at the root of columnargrains.Fig.13.The total pressure drop at the root of columnar grains and the potential location of solidification cracking in mushy zone.solidification cracking,which agrees well with the experimental tests.The susceptible location of cracking initiation is also revealed in Fig.13where thefirst half of mushy zone was not sensitive to cracking and the risky region of cracking initiation mainly con-centrated on the second half of mushy zone near position d.In this study,the different positions could be deemed as different solid fractions during solidification process.Based on the calculated results,solid fraction of90%could be deemed as the critical solid fraction at which pressure drop exceeds the critical value for crack formation.5.ConclusionsIn this study,effect of welding speed on solidification cracking susceptibility duringfiber laser welding of6013aluminum alloy was investigated considering both the mechanical and metallurgi-cal factors.First,afinite element model was developed at columnar grain scale to calculate the strain localization in the mushy zone. The RDG criterion was used for the study of effect of welding parameters on cracking susceptibility.The main conclusions are as follows:1.During thefiber laser welding of6013aluminum alloy,increas-ing welding speed from2.5m/min to3.5m/min would increase the susceptibility of transverse solidification cracking.2.Over the range of experiments considered,the order of mag-nitude of local tensile mechanical strain is about10−4in solid grain while the order of magnitude of strain in liquidfilm is10−2,which is almost102–103times of the local mechanical strain in solid.3.Pressure drop exceeding the critical value for crack formationmainly occurs in the second half of mushy zone where the solid fraction is between90%and94%.The solid fraction of90%could be deemed as the critical solid fraction at which crack initiates.4.The model developed in this study deems the crack as a result ofthe fracture by liquidfilm among columnar grains.So,the pro-posed microscale model could be extended to other Al-based alloys when solidification cracking initiation relates with the liquidfilm.AcknowledgmentsThis work was carried out in the framework of the project “Investigation on laser welding of aluminum alloys through pro-cess simulation”sponsored by General Motors Global Research& Development.The authors also gratefully acknowledge thefinan-cial support by the National Natural Science Foundation of China (Grant No.51204109).ReferencesAbbaschian,L.,Lima,M.S.F.,2003.Cracking susceptibility of aluminum alloys during laser welding.Mater.Res.6,273–278.Bordreuil,C.,Niel,A.,2014.Modeling of hot cracking in welding with a cellular automaton combined with an intergranularfluidflow p.Mater.Sci.82,442–450.Cicala,E.,Duffet,G.,Andrzejewski,H.,Grevey,D.,Ignat,S.,2005.Hot cracking in Al–Mg–Si alloy laser welding–operating parameters and their effects.Mater.Sci.Eng.A395,1–9.Chihoski,R.A.,1972.The character of stressfields around a weld arc moving on aluminum sheet.Weld.J.51,9–18.Coniglio,N.,Cross,C.E.,2009.Mechanisms for solidification crack initiation and growth in aluminum welding.Metal.Mater.Trans.A40,2718–2728.David,S.,Vitek,J.,1989.Correlation between solidification parameters and weld microstructures.Int.Mater.Rev.34,213–245.Drezet,J.,Lima,M.,Wagniere,J.,Rappaz,M.,Kurz,W.,2008.Crack-free aluminum alloy welds using a twin laser process.Proceedings of International Conference on Safety and Reliability of Welded Components in Energy and ProcessingIndustry,87–94.Drezet,J.M.,Allehaux,D.,2008.Application of the Rappaz-Drezet-Gremaud Hot Tearing Criterion to Welding of Aluminium Alloys Hot Cracking Phenomena in Welds II.Springer,pp.27–45.Eskin,D.,Katgerman,L.,2004.Mechanical properties in the semi-solid state and hot tearing of aluminium alloys.Prog.Mater.Sci.49,629–711.El-Batahgy,A.,Kutsuna,M.,ser beam welding of AA5052,AA5083,and AA6061aluminum alloys.Adv.Mater.Sci.Eng.2009,1–9.Fabregue,D.,Deschamps,A.,Suery,M.,Proudhon,H.,2008a.Hot Tearing During Laser Butt Welding of6xxx Aluminium Alloys:Process Optimisation and2D/3D Characterisation of Hot Tears Hot Cracking Phenomena in Welds II.Springer,pp.241–255.。