Effects of resistance spot welding parameters on microstructure and mechanical properties of the dissimilar welded joint of quenching and partitioning steels

In this study, the effect of welding currents and welding times on the microstructure and mechanical properties of the dissimilar welded joints of Q&P980 and Q&P1180 steel was investigated. The macrostructure and microstructure of the dissimilar welded joints were characterized and the relationship between the welding parameters and the mechanical performance was analyzed using confocal laser scanning microscope, scanning electron microscope, and mechanical properties testers. Results show that with the increasing welding current and welding time, the nugget diameter, indentation rate, and maximum shear force of the dissimilar joint increase. The absorption energy of the dissimilar joint increases when the welding current rises, while it increases first and then decreases with elevating welding time. All the hardness distributions of the dissimilar Q&P980/Q&P1180 joints exhibit the highest hardness value in the fusion zone and a gradually decreasing hardness value in the heat-affected zone. Moreover, with increasing current and time, much higher hardness occurs at the FZ/HAZ boundary. The microstructure characterization illustrates the martensite fraction in the intercritical heat-affected zone of the Q&P1180 side is higher than that of the Q&P980 side after the welding process. With the increase of welding current and time, the lath martensite in the fusion zone gradually coarsens. The coarsening martensite and the nugget diameter are responsible for the change in the shear force and energy absorption of the dissimilar Q&P980/Q&P1180 joints.