Characterizing the influence of weld time on the Fe-Zn interface during liquid metal embrittlement of resistance spot welded TWIP steel

Abstract

This study examines how weld time (WT) influences the Fe–Zn interface and liquid metal embrittlement (LME) during resistance spot welding (RSW) of electrogalvanized TWIP steel. Eight weld times ranging from 300 ms to 1700 ms were investigated under constant welding parameters. At 300 ms, the Zn coating remained intact with no intermetallic (IM) phases detected. At 500 ms, continuous layers of δ, Γ, and α-Fe(Zn) phases formed at the weld shoulder, confirmed by SEM-EDS, STEM-EDS, and EBSD, with α-Fe(Zn) showing Al enrichment and Mn depletion. At 700 ms, initial LME cracks appeared, accompanied by fragmented IM layers. For weld times between 900 ms and 1700 ms, crack width and depth increased significantly (from ∼21 μm to ∼490 μm), while IM phases were absent. Finite element analysis (FEA) simulations of temperature distribution correlated with experimental observations: IM formation occurred within the predicted stability range at 500 ms, became discontinuous at 700 ms, and disappeared at higher weld times as local temperatures exceeded 800 °C. These results demonstrate that IM formation precedes LME crack initiation and that prolonged weld time accelerates IM breakdown and crack propagation. The findings provide a mechanistic link between thermal conditions, interfacial reactions, and LME severity, offering guidance for optimizing RSW parameters in automotive applications.