Probing the Effect of Pre- and Post-heating in Resistance Spot Welded Multi-material TWB system through Response Surface Methodology

This study investigates the optimization of the resistance spot welding (RSW) process in a three-sheet, multi-material configuration, utilizing steel alloys of varying thicknesses, namely, AISI 1035 (1.2 mm), ASTM A36 (2.0 mm), and IFHS 350 (2.5 mm). Advanced pulse modulation schemes, including single pulse (welding cycle), double pulse (preheating + welding and welding + post-heating cycle), and triple pulse (preheating + welding + post-heating cycle), were systematically analyzed. Response Surface Methodology (RSM) was employed to predict key weld outcomes, i.e., weld nugget diameter, elongation, and failure load; validated via Analysis of Variance (ANOVA). Microstructural analysis through Electron Backscatter Diffraction (EBSD) focused on kernel average misorientation (KAM) and grey-scale image quality (IQ) mapping to assess lattice distortion, dislocation density, and internal stresses. Results indicated that double pulse (welding + post-heating) and triple pulse cycle induce high dislocation density (~ 250–300 × 10¹² m⁻²) and non-diffusional martensitic transformation, leading to significant brittleness, reduced elongation, and failure load. In contrast, optimized preheating in the double pulse cycle (preheating + welding) increased weld nugget diameter from ~.5 → 10 mm, joint failure load from ~15 → 19.5 kN, and elongation from ~9 to 12.5 mm. This work provides critical insights into RSW process behaviour for multi-material, multi-thickness configurations, advancing the understanding of high-performance applications in complex welded structures.