Abstract:

The quality of numerous resistance spot welded connections that assemble automotive structural components influences vehicle crashworthiness and occupant safety. In this study, the sequential failure of resistance spot welds in multi-weld components was examined using the Caiman geometry comprised of two partially joined hat channel rails. Caiman components made of third-generation advanced high-strength steels (3G-980 and 3G-1180) were loaded under quasi-static tensile conditions. Metrics such as peak load, absorbed energy, failure plug ratio, and crack propagation lengths were used to quantify the differences in terms of the mechanical performance, and failure behavior of the investigated joints. Electron backscatter diffraction (EBSD) analysis revealed two distinct failure paths: first, failure through the martensitic microstructure within the prior austenite grains (PAG) of the upper-critical heat-affected zone (UCHAZ) in the case of 3G-980 joints. Second, failure initiation from the UCHAZ and propagation along the epitaxial structure of the fusion zone (FZ) for 3G-1180 spot welds. Although micro-segregation of alloying elements within the inter-dendritic microstructure of FZ was recorded via electron probe microanalysis (EPMA), no segregation of alloying elements was observed in PAG boundaries of the failure initiation sites (UCHAZ). The discrepancies in failure paths were further investigated by uniaxial testing of simulated UCHAZ. Results from tensile testing revealed an 8% lower plastic work value for the recreated 3G-1180 UCHAZ compared to that of 3G-980. The discrepancies in plastic work values were correlated to the observed failure behaviors. Results from this study will facilitate the application of 3G-AHSS in automotive vehicle assemblies by providing optimized weld parameters and properties of resistance spot welded joints in tensile loaded multi-weld components.

Keywords: Resistance spot welding, advanced high strength steels, mechanical properties, failure behavior

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Dr. Michael Worswick