Study on Weldability and Mechanical Properties of RSW Annular-Nugget Joint: Part 2 — Mechanical Properties Characterization

The mechanical properties of DP780 steel welded using resistance spot welding (RSW) were evaluated by comparing a new ring conductive electrode (RSW-R) with a spherical electrode (RSW-S). Part 1 of this study demonstrated that RSW-R optimizes heat distribution and current density, significantly broadening the weldability range of DP780 steel. Building on this foundation, the present study further investigated the impact of RSW-R on mechanical properties. Compared to RSW-S, RSW-R increased the upper limit of peak tensile-shear loads by 15.9% and failure energy by 91.5%. Moreover, at similar peak tensile-shear loads (11 kA for RSW-S, 10 kA for RSW-R), RSW-R reduced joule heat input by 26.8% and surface indentation by 87.3% compared to RSW-S. The peak tensile-shear load was found to be related to the nugget area when interfacial fracture occurred, while nugget diameter played a more significant role in pullout fracture (PF) mode. Notably, in PF mode, the unconnected region at the inner diameter did not substantially compromise the tensile-shear mechanical properties. Electrode life tests further confirmed that RSW-R achieved stable joint formation for up to 140 consecutive welds, demonstrating durability comparable to RSW-S.