Effects of fusion zone size and failure mode on peak load and energy absorption of advanced high-strength steel spot welds

This paper examines the effects of fusion zone size on failure modes, static strength, and energy absorption of resistance spot welds (RSW) of advanced high-strength steels (AHSS). DP800 and TRIP800 spot welds were considered. The main failure modes for spot welds are nugget pullout and interfacial fracture. Partial interfacial fracture is also observed. The critical fusion zone sizes to ensure nugget pullout failure mode are developed for both DP800 and TRIP800 using limit load based analytical model and microhardness measurements of the weld cross sections. Static weld strength tests using cross-tension samples were performed on the joint populations with controlled fusion zone sizes. The resulting peak load and energy absorption levels associated with each failure mode were studied for all the weld populations using statistical data analysis tools. The results in this study show that AHSS spot welds with fusion zone size of 4√t cannot produce nugget pullout mode for both the DP800 and TRIP800 materials examined. The critical fusion zone size for nugget pullout shall be derived for individual materials based on different base metal properties as well as different heat-affected zone (HAZ) and weld properties resulting from different welding parameters.