Improving the mechanical performance of press-hardened steel resistance spot welds via in-situ grain refinement

This study investigates the effect of in-situ post-weld heat treatment (PWHT) parameters on the microstructural changes at the edge of the fusion zone (FZ) of a resistance spot weld, which is known to be susceptible to crack propagation. The in-situ PWHT parameters (cool time and post-weld time) were varied to determine the temperature regime that induces in-situ grain refinement at the edge of the FZ. Findings revealed that applying the post-weld current when the edge of the FZ is austenitic (after a short cool time) and below the melting point prompts in-situ grain refinement from columnar to a recrystallized equiaxed structure. Meanwhile, after a longer cool time, when the edge of the FZ cools below the Ms temperature, reheating the edge of the FZ above the austenite start temperature results in martensite reversion to austenite with the grain structure similar to the initial epitaxial structure after cooling. The result indicates that in-situ grain refinement can be achieved through a PWHT schedule that involves reheating the edge of the FZ to initiate austenite recrystallization. Moreover, the formation of new equiaxed grains from the austenite recrystallization process effectively retards crack propagation during cross-tension test resulting in 89 % improvement in the energy absorption capability compared to the baseline condition. Therefore, optimizing in-situ PWHT parameters to initiate austenite recrystallization at the edge of the FZ is critical to enhancing joint performance and overall crashworthiness.