Characterizing the effect of GB misorientation on liquid metal embrittlement crack path of resistance spot welded TWIP steel

Abstract

Liquid metal embrittlement (LME) during resistance spot welding (RSW) of Zn-coated twinning-induced plasticity (TWIP) steel results in intergranular cracking driven by the interaction of liquid Zn, tensile stress, and the grain boundary (GB) network. This study investigates how GB misorientation and orientation relative to the electrode force loading axis influence the LME crack path across six weld times from 700 ms to 1700 ms. A comparative framework was developed in which, at each triple junction along the LME crack, the misorientation and angle relative to the loading axis of the chosen LME grain boundary are evaluated against those of the unchosen LME-free grain boundary. Σ3 coherent twin boundaries were LME-free at all weld times regardless of their orientation to the stress axis. Non-twin coincident site lattice (CSL) boundaries (Σ5 to Σ41) were predominantly LME-free at low weld times but progressively became LME GBs at high weld times, when their stress normalisation factor exceeded that of the competing boundary. The fraction of triple junctions where the LME grain boundary had the higher stress normalisation factor increased from 58% at 700 ms to 91% at 1700 ms, while the preference for the higher-misorientation-angle boundary declined from 73% to 56% over the same range. An LME susceptibility index incorporating grain boundary energy, temperature-dependent Zn diffusivity, and stress alignment is proposed as a framework for predicting crack path selection and guiding grain boundary engineering strategies (GBE) to reduce LME in resistance spot welded TWIP steel.