Quantitative analysis of liquid metal embrittlement crack initiation using an in-situ monitoring approach

Liquid metal embrittlement (LME) in the Fe–Zn system is a challenge faced by the automotive industry. LME may degrade weld strength as it leads to surface cracks at the periphery of the weld nugget. Numerous studies have strived to understand the conditions under which LME cracking occurs using the finite element method (FEM) model or physical simulation, but these conditions have not yet been validated, especially in actual resistance spot welding (RSW) environments. This study observed the critical conditions under which LME crack was initiated using a synchronized camera system consisting of a high-speed camera and an infrared camera. High-speed camera (HS) observations show that liquid metal embrittlement cracking behavior can be categorized into two stages by crack growth rate: initiation and propagation. LME cracks occur at the weld periphery when a critical condition is met, where there is a high thermal gradient generated by electrode collapse. The material responds to the thermal gradient by creating thermal stresses and strains. These stresses also affect LME crack speed, where crack growth rate increased with increased thermal stress. The measurement within the current study established the boundary of safe temperature and temperature gradients that will lead to a minimization of LME cracks.