| Complex Phase steel(CP)has the advantages of high strength,good energy absorption,bending,tensile and flanging properties,and Transformation Induced Plasticity(TRIP)steel has the advantages of high strong Plasticity volume and strain hardening index.It is widely used in automobile structural parts and strengthening parts.Laser welding technology has the characteristics of high energy density,small welding stress and deformation,suitable for welding a variety of heterogeneous materials,and no heat treatment after welding.It can realize the combination of heterogeneous high strength steel,improve the material utilization rate,and expand the application range and potential of advanced high strength steel.Through the welding technology of automobile steel components,on the one hand to improve the mechanical properties of automobile components,on the other hand to improve the production efficiency and safety.In addition,there are few researches on the microstructure transformation and properties of laser welded joint of CP steel and TRIP steel dissimilar high strength steel at home and abroad,and there may be softening phenomenon in the joint microstructure.Therefore,this experimental study was conducted.The microstructure and properties of laser welded joint of CP980,CP980 and TRIP980 steel were studied.The microstructure and properties of welded joints were studied under different heat inputs,and the forming mechanism of welded joints was studied.Firstly,the effects of laser power and welding speed on the macroscopic morphology,microstructure and properties of welded joints of 1.5 mm thick CP steel were studied.The control variable method is used to optimize the welding parameters of laser welding to obtain better welding heat input and good welding joints.The welding heat input is 340 J/cm?960 J/cm.With the increase of the welding heat input,the weld weld width becomes wider and the overall morphology of the weld cross section remains unchanged.The weld microstructure is mainly lath martensite.The microstructure of the heat affected zone is mainly lath martensite,ferrite and tempered martensite.The softening phenomenon appears in the tempering zone,which is mainly due to the precipitation of tempered martensite and polygonal ferrite,leading to the decrease of hardness in the zone.The welding heat input has little effect on the tensile and formability of the welded joint.Secondly,the influence of welding heat input on the microstructure and properties of laser welded joints of 1.5 mm thick CP980 steel and TRIP980 steel was studied.Through three factors and three levels orthogonal test of laser power,welding speed and defocus,the welding parameters of laser welding are optimized.It is concluded that the welding speed has little effect on the joint performance,and the welding heat input is 330J/cm?900 J/cm.The microstructure and properties of the joint were analyzed after laser welding with better welding parameters.The quality of welded joint was observed from the macroscopic morphology,and the microstructure of welded joint was analyzed.The microstructure of welded joint was mainly martensite,ferrite and tempered martensite.Secondly,the tensile test shows that the welded joint has good tensile properties.The results of XRD phase analysis and EDS linear scanning show that the chemical elements in the center of the weld are diffused.Finally,EBSD analysis is carried out on the dissimilar steel welded joint,through the analysis of the weld zone and heat affected zone on both sides of the weld center.The results show that the grain size in the base metal area is uniform and the grain orientation is in the [101] direction.Weld grain coarse,large Angle grain boundary,low plastic toughness,high strength.There are a few stress concentration areas in the weld zone and partial recrystallization occurs.In the heat affected zone,the grain size is larger only in the coarse grain zone,and there is no stress concentration zone. |
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