Study Of Microstructure And Properties For Laser Welding TRIP High Strength Sheet Steels

With the development of the automobile industry, the light weight, energy-efficient and environmentally friendly automobile production was strengthened. TRIP steel is a kind of important automobile high strength steel and use the TRIP590high strength steel could realize lightweight of the automobile on the basis of safety. Welding is an important process in automobile production, and welding quality directly affects the safety performance and overall appearance of the car. As an advanced welding method, laser welding has high welding speed and elegant shape. Laser welding process parameters has an important impact on weld quality.Using the optical microscopy (OM), scanning electron microscopy (SEM), electron microscopy (TEM), Electron Backscattering Diffraction (EBSD), Vickers hardness, tensile testing to study the effect of laser welding parameters on microstructure and properties of TRIP590steel. According to TRIP steel welding and laser welding equipment, This was the first time to design a comprehensive welding parameters. Selecting the change of laser welding process parameters were:laser welding power:1800W-2800W, laser welding speeds:2m/min-6m/min, defocus amount:1mm,-1mm, Omm and the formation mechanism of laser welding TRIP590steels was studied.Firstly, to study the three parameters including laser welding power, welding speed, defocus amount of impact on TRIP590steel welded joint properties that changing in a separate rule. The change of the joint organization and performance were studied when the welding power were2800w,2500w,2200w, the study found that laser welding power dramatically affected the organization and performance of the joint, the higher welding power, the more coarse microstructures in heat-affected zone (HAZ), the more martensitic transformation and the higher hardness and plasticity would be decreased. The change of the joint organization and performance were studied when the welding speeds changed among6m/min～3m/min, the study found that when the welding speed was higher, the HAZ microstructures was smaller, the bainite content was less and the more fully martensitic transformed in weld zone weld (WZ), the hardness was higher but the elongation unchanged. Laser welding joint of organization and performance were studied when the defocus amount changed in1mm,1mm,0mm, the study found that the HAZ and WZ microstructures transformed more fully with the reduce of defocus amount and the joint of the hardness was higher, the tensile strength slightly reduced. The elongation was lowest at defocus amount in Omm.Secondly, in the many changes of parameters designed a set of three factors and three levels orthogonal test, mainly to find the effects of various process parameters on the microstructure and properties of welded joints and determined the best process parameters, and then studied the two changes of process parameters interaction on the properties and organization of welding joint. By orthogonal experiments to determine the optimum parameters was the welding power2500W, welding speed4m/min, defocus lmm. The biggest impact on laser welding was the defocus amount, followed by the welding power, and finally was the welding speed. Analyzed complex interactions and relationships among welding speed, welding power, and the defocus impact on the properties and organization. The studies found that:the organization of laser welding HAZ was mainly affected by the welding heat input, between the welding power and welding speed, weld zone microstructure was mainly affected by the welding speed, because the welding speed determines the degree of supercooling, martensitic transformation was greatly impact on cooling degree, and the weld hardness was affected by martensite. The weld width was increased with the input increased, but this study found that the weld penetration and the joint width were not infinitely increased with the increase of the welding power and the decrease of welding speed, because the shielding effect of the plasma above the weld. Between the welding power and the defocus amount, the organization of the weld zone was mainly impacted by the defocus amount, the martensitic transformation was insufficient and weld joints hardness was low when the defocus amount was positive. Between the welding speed and the defocus amount, the amount of defocus determined the martensite content in the weld, when the defocus amount was positive, although the welding speed was improved, the degree of subcooling increased, but martensite was still insufficient, and the hardness of the joint was determined by the defocus amount.Finally, the interface TRIP590steel laser welded joint formation mechanism was studied, mainly to study the weld heat affected zone, fusion zone, melting zone tissue formation mechanism. By scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscattering diffraction (EBSD) observed the weld joint microstructure morphology, distribution and content of the retained austenite in each phase, the WZ grain orientation. Analysis the reason of the welded joint of different position of organizational change and microstructural evolution mechanism in different areas of the welded joint. The study found that:retained austenite distributed on martensite film as membrane or block, the carbides in the HAZ between the ferrite bar precipitated as a form of short rod. The base material grains distribution was uniform and had no preferred orientation, the residual austenite content was1.37%, average grain size was3.84μm. The weld zone grain size was larger than the base material, there were many sub-grain boundaries within the martensite lath and the grains had a preferred orientation, when the welding speed was5m/min, the grain size was about8μm, residual austenite content was about0.2%, when the welding speed was3m/min, the average grain size was about10μm, the content of residual austenite was about0.398%, and lath distribution were more cluttered.