Liquid metal embrittlement in Zn-coated steel resistance spot welding: Critical electrode-contact and nugget growth for stress development and cracking

Liquid metal embrittlement (LME) crack is a severe defect which can affect the joint integrity of Zn-coated advanced high-strength steel resistance spot welds (RSW). The tensile stress that causes LME cracking is a consequence of thermal contact at the electrode-sheet interface during welding. Currently, there is no systematic study to identify the critical stages of LME occurrence during welding with understanding the tensile stress development. This research aims to address this gap by combining in-situ observation, ex-situ verification, and finite element modeling to identify the critical stages in LME cracking. The study reveals that the large LME cracks form only after a critical welding time (tcrit) of 250 ms, when the growth of the weld nugget exceeds that of the electrode-sheet contact and electrode tip-face diameter. As tcrit was attained, the LME responsible thermal gradient increased from 2000 degrees C/mm to 4000 degrees C/mm, consequently tensile stress increased from 200 MPa to 300 MPa, resulting in large LME crack. A mechanistic model explaining the effect of nugget growth rate and electrode-contact growth on thermal gradient, stress development and LME cracking was then proposed and verified through cross-comparison of crack observations. This research provides valuable insights into the critical stages of LME cracking during RSW of Zn-coated steel and suggests enhancing the growth rate of electrode-contact as a solution to mitigate this issue without altering the nugget growth rate.