Abstract
Dynamic resistance analysis serves as a powerful tool for interpreting heat generation, current flow, and the evolution of nugget formation at the sheet/sheet interface during resistance spot welding (RSW). In conventional two-sheet stack-ups, the characteristic regions of the dynamic resistance curve and their corresponding physical events are well understood. However, this interpretation becomes less straightforward in multi-sheet, high stacking ratio configurations due to the presence of additional resistive interfaces. Even in complex multi-sheet RSW stack-ups, tracking of key features such as how the β-peak time and associated resistance drop rate shift with increasing thickness ratio can reveal systematic trends in nugget initiation and growth. By understanding these trends, welding schedules can be tailored to trigger earlier and more uniform nugget formation across all interfaces, thereby improving joint strength in high stack-up ratio configurations.
This study focuses on developing practical RSW strategies for high-thickness-ratio, multi-sheet automotive joints made from 3G-980, DP-980, and CR4 steels, configured in six two- and three-sheet stack-ups, covering thickness ratios from 2 to 6.3. Central-composite design experimental weld schedules based on AWS D8.1 were executed on a robotic MFDC welder while recording dynamic resistance; the signals were FFT-smoothed and differentiated to extract β-peak timing and post-peak resistance-drop rates as quantitative indicators of nugget initiation and growth. Experimental analysis in this study also included half-sectioned RSW coupled with infrared imaging and simufact simulations of all three grades of steel investigated.
Simufact and half-section experimental results revealed that in dissimilar stackups, the nugget front can advance and retreat dynamically during multi-pulse welding, with inter-sheet interactions reducing the effective resistance throughout the weld cycle. The pace of nugget growth, as indicated by the post-β peak slope of the dynamic resistance curve, was greatest at low electrode forces and high currents. Similar stackups showed the most aggressive resistance drops and more predictable heat generation at optimal conditions, while dissimilar or multi-thickness configurations exhibited more modest resistance changes and delayed nugget formation, reflecting complex heat flow paths and the influence of coatings or mismatched resistivities. Multi-pulse strategies with interpulse cooling were found to balance rapid nugget growth with process stability, enabling more robust welds in these challenging stackups. Ultimately, a robust optimization approach, centered on early β peak occurrence and steep post-β resistance drops, but constrained by expulsion risk, enabled the proposal of practical welding schedules across a range of advanced automotive stackups.