Decoding the Microstructural Architecture and Phase Transformations of Dissimilar RSW Joints Between HSLA440 and DP590 Automotive Steels

This study aims to investigate the microstructural evolution and hardness behavior of dissimilar resistance spot-welded (RSW) joints between HSLA440 and DP590 automotive steels. Welds were produced at welding currents ranging from 6 to 11 kA in accordance with ISO 18278-2, and the resulting fusion zones (FZ), heat-affected zones (HAZ), and base metals were characterized using optical and scanning electron microscopy. The fusion zone showed a cast structure dominated by martensite and ferrite, formed under high cooling rate induced by RSW process. Microstructural gradients were observed across the HAZ in both two steels. Including the development of coarse-grained and fine-grained martensite in the upper-critical heat-affected zone (UCHAZ), mixed martensite + ferrite + retained austenite in the inter-critical heat-affected zone (ICHAZ), (in the both steel) and tempered martensite (DP590) or coarse grain ferrite (in HSLA440) in the sub-critical heat-affected zone (SCHAZ). Increasing the welding current intensified heat input, enlarged the weld nugget, and promoted higher martensite volume fractions, resulting in increased FZ hardness but more softening in the SCHAZ—particularly on the HSLA440 side. The minimum hardness in the SCHAZ corresponded to the microstructure at elevated currents. The results show the critical coupling between heat flow and phase transformation in the integrity of dissimilar RSW joints.