Characterizing zinc diffusion during liquid metal embrittlement of resistance spot welded TWIP steel

Liquid metal embrittlement (LME) during resistance spot welding (RSW) of twinning induced plasticity (TWIP) steel is primarily driven by stress-assisted grain boundary (GB) diffusion of zinc (Zn). Although GB diffusion is widely recognized as the dominant LME mechanism, experimental quantification is challenging due to resolution limitations. This study characterizes Zn diffusion in TWIP steel during RSW by conducting energy dispersive X-ray spectroscopy (EDS) line scans ahead of LME cracks in both the rolling direction (RD) and normal direction (ND) over weld times from 700 to 1700 ms. Results reveal that Zn diffusion distance increases with weld time, with consistently higher diffusion in the ND. To compare experimental measurements with diffusion theory, an FEA simulation based on Fick's law was employed to approximate bulk Zn diffusion under varying temperatures. The model predicts Zn diffusion trends consistent with experimental observations. Although the diffusion distance predicted in the simulation exceeds measured values, directional trends are accurately captured. A theoretical framework to compare GB and bulk diffusion was proposed. GB diffusion distance of Zn is estimated to be approximately 30 times greater than bulk diffusion, establishing a quantitative link between weld time and Zn diffusion during RSW of TWIP steel.