Research On Microstructure And Stress Field Of Spot Welding Joint For Dual Phase Steel Based On Temperature Field

The dual phase steel has high strength and good corrosion resistance as well as goodpunching performance. It has become the main steel used in the automobiles of lightquality and high safety in the future and been widely used in the resistance spot weldingmanufacture of automobiles body. Therefore, the study on resistance spot welding processof dual phase steel,which includes formation mechanism, microstructure transformationand deformation control so as to get high quality welding joint will have an importantimpact on its wide use.In this thesis, the microstructure transformation and stress field of resistance spotwelding joint for DP600steel with a thickness of1.7mm were researched by usingsequential coupling method and thermal elastic-plastic theory based on the temperaturefield.First, the temperature field calculation results were obtained based ontwo-dimensional axisymmetric model, which fully considered the effect of contactresistance, latent heat, thermal boundary conditions and thermal-electric parameterschanging with different temperature. Then, the nugget formation process and temperaturevariation during cooling process were analyzed. After that, the accuracy of this simulationwas verified by comparing simulation and experimental results under different weldingparameters.Secondly, the microstructure transformation of welding joint was researched withtemperature field simulation and microscopic experimental results. It shows that more heatinput will make austenization more homogeneous and the spot welding joint will be morehomogeneous. During cooling process, martensitic transformation will happen in the HAZzone. Meanwhile, the more close to the nugget zone, the larger the grain size of martensitewill be.Finally, this thesis studied the distribution and variation of stress and strain field during such three main welding process including pre-pressure，heating by current inputand cooling under electrode pressure. In this analysis, the effect of phase transformationon thermal expansion coefficient was considered by linear rule of mixture. Then, theresearch of workpiece deformation show that the electrode indentation is obviouslyformed in the mid of heating process and will keep increasing until the end of coolingprocess. The wedge is obviously formed in the beginning of heating process and keepincreasing until the end of heating process,and then will turn to decreasing in the coolingprocess. The longer cooling time, the deeper electrode indentation will be, whereassmaller wedge will be. By contrast, the variation of electrode pressure has no crucialimpact on deformation.