Microstructural evolution and mechanical performance of plasma arc welded dissimilar DP590–DP780 dual-phase steels

This study examines the microstructural evolution and mechanical properties of dissimilar DP590–DP780 steels joined by plasma arc welding (PAW). Characterization techniques included optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), hardness mapping, tensile testing, and fractographic analysis. Welding was performed at 150 A current, 25–28 V arc voltage, 50 mm/min speed, 10 mm nozzle-to-work distance, and 2.5 mm nozzle diameter. The fusion zone (FZ) mainly consists of lath martensite with an average thickness of (1.0 ± 0.4) µm. Near the heat-affected zone (HAZ), martensite laths coarsen to about (1.7 ± 0.5) µm and are entrapped within prior austenite grain boundaries. The far HAZ shows tempered martensite, indicating thermal-induced microstructural changes. XRD confirmed ferrite and martensite phases in the base metals (BMs), while the weld metal (WM) exhibited broadened martensitic peaks consistent with its blocky and fresh lath structure. EBSD orientation distribution function (ODF) analysis revealed moderate texture in the BMs, moderate-to-strong texture in the FZ due to rapid solidification, and strong texture in the HAZ caused by grain coarsening. Hardness mapping identified significant softening in the far HAZ, with values dropping to HV 186 (DP590 side) and HV 216 (DP780 side), representing reductions of 11% and 23% relative to their respective BMs. Tensile testing showed the WM achieved an ultimate tensile strength (UTS) of 625 MPa, comparable to DP590-BM, but with lower elongation (19%) compared to DP590 (30%) and DP780 (25%) BMs, indicating ductility loss linked to strain localization in the softened HAZ. Fractography showed fine dimples in DP590-BM, shallower dimples in DP780-BM, and larger, irregular dimples in the WM, correlating with ductility trends. 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.