Study On Microstructure,mechanical Properties And Fatigue Crack Growth Behaviour Of Laser Welded Dissimilar DP980/QP980 Steel

Advanced high strength steel is an important way to realize automobile lightweight.Laser tailor welding of dissimilar steel is conducive to the combination of excellent mechanical properties of different advanced high strength steel.Fatigue failure is one of the main failure forms of body welding parts.However,the microstructure evolution and fracture mechanism of heterogeneous joints are complex,and the law of fatigue crack propagation is not clear.Therefore,this paper takes the laser tailor welded joints of the similar and dissimilar steels DP980/QP980 as research object,and the microstructure evolution,mechanical properties and fracture toughness of the welded joints was analyzed.The fatigue crack growth behavior and fracture mechanism at different notch positions of the joints was studied.The main conclusions are as follows:(1)The microstructure of fusion zone and upper critical heat affected zone of dissimilar steel and the similar steel joint is lath martensite,the critical heat affected zone composed of martensite and ferrite,and the microstructure of subcritical critical heat affected zone is tempered martensite;The microhardness of the fusion zone of dissimilar steel joints is between that of the similar steel joints;The subcritical heat affected zone of dissimilar steel joint forms a softening zone due to the decrease of hardness caused by martensite tempered.The hardness of DP980 side and DP980 side is reduced by 26 HV and 37 HV respectively.(2)Dissimilar steel and similar steel joints failure in the softening zone during tensile process(dissimilar joint failure in the softening zone on DP980 side),and shows continuous yield.The joint efficiency is between 96% and 98%.In the tearing process of dissimilar steel joints,when the notch is located in the heat affected zone,the crack propagates along the softening zone,and the fracture toughness decrease about 21.8% and 17.8% than that of DP980 and QP980 base metals,respectively;When the notch is located in the fusion zone,the crack tends to the softening zone due to the asymmetric plastic zone at the crack tip.At this time,the fracture toughness does not decrease significantly while the fracture strength increases by about 20% ～24%.(3)The overmatching effect of joint caused by laser welding hinders the fatigue crack propagation.When the notch of dissimilar steel joint is located in heat affected zone and fusion zone,the fatigue crack growth life is higher than that of base metal and the fatigue crack growth rate is lower than that of the base metal when stress intensity factor(ΔK)is low,while its rate exceeds that of the corresponding base metal when ΔK is high.The fracture surface in the stable fatigue growth region is mainly characterized by typical fatigue striations and secondary cracks.When the notch of dissimilar steel joint is located in the fusion zone,the crack closure caused by roughness is also observed;With the increase of ΔK,the fatigue fracture mode is dominant by ductile fracture mechanism,and the fatigue fracture is completely plastic dimple after unstable fracture.