Understanding Zn-assisted liquid metal embrittlement in resistance spot welding of Zn-coated TRIP steel sheets: Effects of Si and Al on Fe–Zn reactions

High-strength transformation-induced plasticity (TRIP) steels have been widely used in the automotive industry owing to their excellent mechanical properties and superior weldability. However, during hot forming or resistance spot welding of Zn-coated TRIP steels, the liquid Zn can penetrate to the substrate along grain boundaries, leading to the occurrence of liquid metal embrittlement (LME). In this study, effects of key alloying elements, Si and Al, on LME susceptibility of Zn-coated TRIP steels were investigated. The reference (Ref) steel, containing 1.4 wt.% Si, exhibited delayed Fe-Zn alloying reactions, which suppressed the formation of interfacial α-Fe(Zn) phases. This allowed the liquid Zn to persist at the coating/substrate interface, thereby promoting Zn penetration into the substrate and LME crack initiation. In contrast, the 0.7Al steel, in which a portion of Si was replaced by 0.7 wt.% Al, exhibited continuous formation of α-Fe(Zn) at the interface, effectively preventing Zn penetration and significantly mitigating LME susceptibility. Resistance spot welding and high-temperature tensile testing at 800 °C revealed that the Ref steel exhibited a greater number and length of LME cracks than the 0.7Al steel since the presence of Γ-Fe 3 Zn 10 phases within the cracks of the Ref steel indicated infiltration of liquid Zn. In the 0.7Al steel, the formation of a continuous α-Fe(Zn) interfacial layer acted as an effective barrier to Zn diffusion and crack propagation. These results demonstrate that Si promotes LME by suppressing Fe-Zn reactions, whereas Al alleviates LME through interfacial α-Fe(Zn) formation. The findings provide the importance of optimizing the Si/Al ratio to enhance microstructural stability and weld integrity in Zn-coated TRIP steels.