In-situ dual force: A novel pathway to improving the mechanical properties of resistance spot welds

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Although in-situ post-weld heat treatment (PWHT) has been a viable method to modify resistance spot weld microstructure and improve joint mechanical properties, prevailing methodologies only employ post-weld current to initiate microstructural transformations. This study uses in-situ dual force (DF) and a PWHT current pulse to induce microstructural changes specifically at the edge of the fusion zone (FZ), a region prone to crack propagation. After a short cooling period following the welding cycle, the application of strain energy from the DF and thermal energy from the PWHT current resulted in the formation of new equiaxed grains via austenite recrystallization. The energy absorption capability of the weld improved by 39% after the DF schedule and 85% when DF was combined with a PWHT current. The changes in mechanical properties resulted from strain hardening induced by the DF schedule, while grain refinement from the combined DF and PWHT current schedule led to the deviation of cracks at the edge of the FZ. In contrast, the crack propagated directly into the FZ along the columnar structure in the as-welded condition. The novel application of in-situ DF extends beyond the conventional PWHT and offers a promising avenue to trigger microstructural changes in the weld which can improve mechanical performance and overall crashworthiness.