Microstructural and mechanical characterization of robotic resistance spot welds in dissimilar Al-SS-Al three-sheet stacks

This study investigates the robotic resistance spot welding (RRSW) behavior of a dissimilar three-layer aluminum–steel–aluminum stack, focusing on how variations in current, weld time, and secondary pulse modulation influence weld penetration, interfacial microstructure, and mechanical performance. Taguchi L9 experimental matrix was employed to plan experimentations under controlled electrode force and pulse conditions. Weld penetration depth was found to increase consistently with thermal input, transitioning from sub-critical interfacial fusion to full-thickness nugget formation. However, excessive energy input led to over-penetration, weld asymmetry, and expulsion, highlighting the need for thermal input optimization. Microhardness profiling across weld zones revealed an inverse correlation between overall fusion zone hardness and increasing heat input, although localized hardness enhancements were observed under specific pulse configurations. Optimal mechanical performance was achieved under mid-range energy inputs, where thin and uniform IMC layers (1–3 µm) provided sufficient interfacial strength without inducing brittleness. The findings underscore the importance of pulse shaping and thermal control in managing interfacial reactions and solidification kinetics during dissimilar metal welding. The study offers insights for optimizing RRSW in multi-material stacks, emphasizing that joint reliability depends not solely on weld size or hardness, but on achieving a precise balance between metallurgical bonding and thermal moderation.