IF steel shim-assisted process to improve mechanical behaviour of Sn-containing hot stamping steel resistance spot weld joint

The steel industry’s shift towards scrap-based, low-carbon steelmaking introduces residual Sn, which compromises the resistance spot weldability of advanced hot-stamping steels essential for automotive lightweighting. This study investigates the impact of Sn on the mechanical properties of such welds and evaluates a shim-assisted RSW (SA-RSW) process using an interstitial-free steel shim for mitigation. Increasing Sn content degraded the cross-tension performance, with a critical content of 0.05% causing a transition from the desirable pull-out fracture to partial interface fracture and a 20% reduction in peak load, primarily due to Sn segregation. The SA-RSW process effectively diluted Sn concentration in the fusion zone by ∼66%, restoring the pull-out fracture mode. However, it concentrated residual stress near the fusion line, leading to cleavage brittle fracture and reduced displacement, which diminished energy absorption despite Sn-induced strengthening increasing the peak load. Based on these results, a practical selection strategy is proposed. Using conventional RSW for Sn content below 0.05%, and adopting the SA-RSW process for Sn content at or above 0.05%. This work provides insights into Sn-induced embrittlement and a viable joining strategy for sustainable manufacturing.