Microstructural Evolution and Mechanical Performance of Plasma Arc Welded Dissimilar DP590-DP780 Dual-Phase Steels

Dual-phase (DP) steels have attracted significant interest in the automotive industry due to their exceptional strength and formability. However, welding of automotive components can alter the microstructure, influencing the mechanical performance. This study investigates the microstructural evolution and mechanical performance of dissimilar DP590-DP780 steels welded using plasma arc welding (PAW). Detailed microstructural analysis revealed that the fusion zone (FZ) was dominated by lath martensite, whereas the near heat-affected zone (HAZ) exhibited coarsened prior austenite grains, and the far-HAZ showed extensive martensite tempering. X-ray diffraction (XRD) revealed ferrite and martensite in the base metal (BM)s, while the weld metal (WM) showed dominant, broadened martensite peaks, indicating finer lath structures. Electron Backscatter Diffraction (EBSD) revealed moderate textures in the BMs, weak dispersed orientations in the FZ from rapid solidification, and strong 111/110 textures in the HAZ due to grain coarsening and partial recrystallization. Hardness mapping revealed significant softening in the far-HAZ, dropping below both BMs, which directly influenced joint performance. Tensile testing showed that the WM achieved an ultimate tensile strength (UTS) of 625 ± 7 MPa, comparable to that of the DP590-BM. However, WM exhibited significantly lower elongation (19%) compared to DP590-BM (30%) and DP780-BM (25%), indicating a loss of ductility due to strain localization in the softened HAZ. Fractography showed uniformly fine dimples in DP590-BM, shallower dimples in DP780-BM, and coarse irregular dimples with bigger voids in the WM, consistent with the ductility trend. The results highlight that although PAW yields defect-free DP590-DP780 welds with adequate joint strength, far-HAZ softening in the WM compromises structural performance.