Research On Microstructure,Properties And Failure Mechanism Of High Strength Automobile Steel Resistance Spot Welded Joints

Facing the development of the automotive industry,energy conservation,environmental pollution and safety are the three major challenges.To solve this problem,the lightweight of automotive is an important issue.Lightweight requires supports of materials and connection technology.According to statistics,the number of resistance spot welding joints on the body-inwhite of a small car is 2000～5000,so the material support is mainly manifested in the application of advanced high-strength steel,and the support of connection technology is associated with the research of resistance spot welding technology.In this paper,Q&P 980,the high-strength automotive steel,was selected as the research object.From the perspective of improving the mechanical properties,the synergistic effect of single pulse resistance spot welding process,in-situ heat treatment process and post-weld heat treatment process were studied,based on process control.Then,the microstructure of the failure path,the failure location and the fusion zone were further analyzed.By establishing the relationship between microstructure and mechanical properties,the internal mechanism of failure mode improvement due to crack initiation and propagation was proposed,which provide theoretical basises for further expanding the application of quenching-partitioning steel in the automotive industry.(1)In order to explain the relationship between the microstructure and the mechanical properties of the welded joint in detail,the formation mechanism of the single-pulse welded joint in combination with the hardness distribution form was analyzed.The joint is divided into the fusion zone,the coarse grain heat affected zone,fine grain heat affected zone,intercritical heat affected zone,sub-critical heat affected zone and base metal.The formation of the bandlike bainite microstructure at the interface was found between the fusion zone and the heataffected zone.There is a softening phenomenon,making it become the preferred location for crack propagation.(2)Based on the analysis of Taguchi method,the relationship between microstructure and mechanical properties under different welding parameters(welding current,electrode force and welding time)was established by the single-pulse resistance spot welding experiment.The main factor affecting the tensile-shear properties of the welded joints is the nugget size which affected by welding thermal cycling.Finally,a preferred process parameters was obtained and welding process window of Q&P 980 steel was established.(3)Based on the single-pulse resistance spot welding process,the effect of the secondary pulse current on microstructure and mechanical properties of the welded joint was studied under the in-situ heat treatment process.the following mechanisms which had effects on the internal microstructural characteristics of the fusion zone was found as follow:tempering of martensite,re-austenitization and completely melting.Then,through the analysis of the mechanical properties,it has been found that the fundamental reason for the improvement of mechanical properties is the change of the failure mode.The internal mechanism is that the volume fraction of ferrite in the secondary heat affected zone were increased.Therefore,the fracture toughness of the welded joint was enhanced..(4)The mechanism of post-weld heat treatment temperature and time on the microstructure and the mechanical properties of spot welded joints was studied.The temper softening effect caused by post-weld heat treatment improves the fracture toughness of the fusion zone.Based on the analysis,it has been found that the post-weld heat treatment significantly improves the mechanical properties of the welded joint.The results showed its effect is significantly better than the in-situ heat treatment process.By analyzing its internal mechanism,the reduction of the residual strain and the increase of large-angle grain boundaries in the heat-affected zone forced the crack propagation path to change.Eventually,the fracture failure occurred in the softening zone of the initial heat affected zone.