Abstract:The paper addresses the microstructure evolution of dissimilar resistance spot welding between martensitic steel and high-strength low-alloy steel. Resistance spot welding tests were carried out on MS1400 and HSLA420 with a thickness of 1.2 mm. Shear samples were obtained by changing the welding time, current and pressure. Shear tests were carried out by a tensile testing machine. The better welding process parameters were determined by generating the pull-out failure mode. By analyzing the microstructure of fusion zone and heat-affected zone on both sides under better parameters, it is found that the fusion zone is completely martensitized. The heat-affected zone is mainly composed of mixed ferrite, martensite and granular bainite structure. The microhardness test of the joint shows that the microstructure changes in each region are basically consistent with the microhardness distribution curve.