Microstructural characterization and mechanical properties in resistance spot welding of Q&P980 steel involving "effective softening" at the fusion boundary

This study proposes a welding process for Q & P980 steel that involves prolonged post-weld pulses. The process effectively softens the nuggets and results in welded joints with excellent comprehensive mechanical properties. The relationship between microstructural characteristics, element distribution and mechanical properties of weldments was elucidated for the samples subjected to different heat input histories. The microstructure evolution was analyzed using electron probe micro-analyzer (EPMA), electron backscatter diffraction (EBSD), and the mechanical properties of the spot welds were evaluated at room temperature by tensile shear (TS) and cross tension (CT) tests. The findings indicate that the softened area expands from the fusion boundary towards the opposite direction of the nugget as the heat input increases. A mixed microstructure of martensite and retained austenite, characterized by a larger size of prior austenite grains and lower kernel average misorientation, is formed in the "effective softening" region. This region provides serves as the primary location for crack nucleation and governs the propagation path of cracks within the halo ring-like softening zone, which has a significant impact on the fracture mode of the weld. The energy absorption in the TS test was increased by 95.40% compared to the single pulse weld (SPW), while the best combination of strength and energy absorption of the weldment in CT test was 7.2 kN and 64.24 J, respectively.