Study On Microstructure And Properties Control And Mechanism Of Friction Stir Welded Joint Of Q&P1180 Steel

Quenching and Partitioning(Q&P)steel,as a typical representative of thirdgeneration high-strength steel,has excellent tensile strength,elongation,impact resistance and safety,and is mainly used in the manufacture of parts with high strength requirements and complex forming,such as automotive hinge plates and crash beams.These structural parts need to be connected by welding technology before forming,and the traditional welding technology for Q&P steel is prone to porosity defects,element burnout,thermal cracking and tissue segregation.Friction Stir Welding(FSW)as a highly efficient,green,environmentally friendly,low energy consumption of solid phase welding technology,with low welding temperature,refinement of grain,homogenization of the organization,can effectively avoid the melting defects in the traditional welding method to ensure excellent welding performance of the joint.Based on this background,this paper uses Q&P 1180 steel as the research object,using rapid cooling assisted and Friction Stir Mechanical Alloying(FSMA)combined with Heat Treatment(HT)two schemes to regulate the microstructure and mechanical properties of FSW joints,respectively.The effects of rapid cooling and carbon content on the structure and properties of FSW joints were investigated,as well as the effect of composition on the strength and plasticity of the joints.The addition process of powder in stirred friction mechanical alloying was investigated by combining layer misalignment energy and carbon diffusion theory;the strengthening mechanism of carbon in the FSW and HT processes was analyzed in depth.The evolution of the residual austenite and martensite in the tensile deformation process of the joint was explored in depth using quasi-in-situ tensile experiments;the strong plasticity mechanism of the FSW joint was verified.The experimental and theoretical basis is provided for the optimization of the performance of Q&P steel FSW joints and their applications.The main research contents and conclusions of this paper are as follows:(1)FSW and Rapid cooling friction stir welding(RCFSW)were used to weld Q&P 1180 steel respectively,and the mechanism of rapid cooling on the microstructure and deformation behavior of FSW joints was clarified.Under the same welding parameters,the tensile strength of the RCFSW joint is 1237 MPa,the elongation is 13.5%,and the strength and elongation reach 101%and 68%of the base metal,respectively.Compared to FSW,RCFSW fittings have a 30%higher elongation.This is because during the tensile deformation process,the plastic deformation of FSW joints is mainly concentrated in HAZ,while the plastic deformation of RCFSW is transferred from HAZ to the base metal,and finally fractured in the base metal.(2)Carbon powder was added to the FSW joints,and post-weld heat treatment was performed after FSW to regulate the joint organization and properties.The results showed that the elongation of the FSW joint remained unchanged and the tensile strength increased to 119%of that of the base material,reaching 1372 MPa.After post-weld heat treatment,the carbon element diffused into the residual austenite and the stability of the residual austenite in the Stir Zone(SZ)of the joint increased exponentially compared with that of the FSW joint(from 7%to 15%).The free carbon particles and carburized body are evenly distributed in the grain boundaries and grains,which increases the dislocation strengthening effect.The tensile strength and elongation of the heat-treated joints exceeded those of the base material,reaching 110%and 100%of the base material,respectively.(3)Based on the stable effect of carbon on residual austenite,the quasi-in-situ tensile method was used for in-depth research During the plastic deformation process,the evolutionary process of residual austenite and martensite revealed the strong plastic mechanism of joints.The results showed that the plasticization mechanisms of FSW joints were dislocation absorption(DARA)effect,TRIP effect and residual austenite impeded crack propagation(BCP)effect,respectively.After the addition of carbon-reinforced powder,the solid solution strengthening,dislocation strengthening,grain boundary strengthening and precipitation strengthening effects in the joint were improved,which was mainly due to the solid solution of carbon elements in the FSW process,and some of them were nailed to the grain boundaries and dislocations in the form of carbon elements.