Process window expansion with transferable applicability in three-layer dissimilar steel resistance spot welding via expulsion prevention

This study addresses the challenge of insufficient weld penetration in the outer thin low-carbon steel during resistance spot welding of three-layer dissimilar stack-ups containing advanced high-strength steels. To overcome thermal imbalance constraints, an innovative strategy leveraging plastic shell containment is proposed to elevate the expulsion-free heat input threshold. By applying a combined preheating and ramping current profile, a coordinated “shell-first, nugget-second” sequence is achieved. This mechanism creates a solid-state barrier prior to rapid fusion, effectively preventing expulsion. Experimental results demonstrate that while the reference constant-current schedule fails to maintain a process window under a 2 mm initial gap (IG) disturbance, the proposed strategy significantly enhances process stability. It increases the maximum expulsion-free heat input by 24 % (to 6338 J) under normal conditions and by 77 % (to 6482 J) under the IG condition. Crucially, the increased heat input drives nugget growth across all interfaces, achieving a penetration depth of 0.38 mm (48 % penetration ratio) in the low-carbon steel sheet under the gap condition. These findings validate the strategy's effectiveness in ensuring weld quality and robustness, which is further confirmed by its transferability to a lower-resistivity DX54D cover sheet.