Research On Optimization Of Resistance Spot Welding Process And Numerical Simulation Of Q&P980CR Steel

Automobile light-weight has become an important goal of automobile research and manufacture.The application of high strength steel can ensure that the thickness of the steel plate can be reduced on the premise of meeting the requirements of automobile body stiffness and strength,so as to realize the light-weight of automobile body.Q&P steel（Quenching and Partitioning）is an excellent choice for automotive structural parts because of its high strength and good formability.In the automobile manufacturing industry,resistance spot welding has become the most widely used process method in the connection of automobile body structure parts due to its advantages of high efficiency and automation.However,in the practical application of resistance spot welding of Q&P steel,due to the high Carbon equivalent number（Ceq）of the base material,the welding joints contain much hardening microstructure,which leads to poor plasticity,and seriously affects the mechanical properties of the welding joints and restricts its practical application in auto-body manufacturing.Therefore,the research on the optimization of process and performance of Q&P steel resistance spot welding is of great significance for further promotion of the wide application of Q&P steel.In this paper,Q&P980CR steel plate（thickness:1.2mm）was taken as the research object,and two welding modes of power-frequency AC and mid-frequency inverter were adopted to carry out multiple groups of spot welding tests,so as to study the influence of Q&P spot welding process parameters on the joint performance.Within the range of better welding parameters obtained by the experiment,the welding current has the greatest influence on the mechanical properties and nugget size of the joint,while the influence of the electrode force is the least.In general,under the same welding current,the tensile-shear strength and ductility ratio（CTS/TSS）of the inverter mode are slightly higher than that of the AC mode.The microstructure and tensile-shear fracture mode of the joint under the two welding modes are similar.The microstructure of the nugget zone is strip martensite in columnar crystal forms along the heat conduction direction（i.e.the vertical direction of the plate bonding surface）.The heat affected zone includes superheating zone,fine grain zone and incomplete quenching zone.The hardness of the nugget zone is much higher than that of base material.With the conventional spot welding process,the interface fracture of the spot welding joints occurred in the tensile-shear tests,and the button fracture pattern could not be obtained.The finite element analysis software SORPAS is used in this paper to establish the finite element model of Q&P 980CR steel spot welding process and simulate the formation of temperature field under typical welding parameters in the mid-frequency inverter mode.Distribution of temperature field,thermal cycle curve,peak temperature of the nugget center and dynamic change of contact resistance,etc.during the welding process are obtained,which explains the formation of the nugget and final microstructure of different zone of the joint.According to the characteristics of temperature field and thermal cycle during the spot welding process of Q&P 980 steel,that the joint is prone to interface fracture is due to the high carbon equivalent of Q&P 980 steel and the fast cooling rate after welding,which causes more formation of hardening microstructure and poor plasticity of the joint.In this paper two approaches to optimization of the Q&P 980 spot welding joint performance are put forward.Based on the numerical simulation,combined with the continuous cooling transformation curve（CCT curve）,the optimization scheme of Q&P 980 steel spot welding process by adding the postheat current pulse is proposed,and the influence of the postheat current pulse on the thermal cycle curve and cooling rate in section-t_8/5 is obtained,and the suitable postheat current pulse welding parameters are determined.In addition,another process optimization idea by applying forge force in the post-welding electrode force maintaining stage of Q&P980 steel spot welding process is proposed in this article,and the microstructure of the joint is denser through applying the forge force after welding.In this paper,the experimental methods are carried out to verify two of the optimization methods proposed above.The results indicate that,by adding the postheat current pulse,the tensile-shear strength and the cross-tensile strength of the joints increase by 18.2%and 38.5%,respectively.Additional forge force after welding not only densifies the joint microstructure,but also changes the stress/strain during the martensite phase transition,and attain the effect of refined grain strengthening,thus affecting the properties of the martensite and the failure mode of the spot welding joint.By applying the forge force,the tensile-shear strength increases by10.1%,with both optimization methods above,the fracture mode of the joints changes from interface fracture under conventional process parameters to button fracture.The conclusion in this paper provides a certain theoretical basis and technical support for the practical application of Q&P steel and performance optimization of the Q&P steel spot welding joint.