Zn-assisted Liquid Metal Embrittlement of High Mn Austenitic Steels

Abstract

The application of TWIP steel in the automotive industry is still limited due to many pending problems. Liquid metal embrittlement caused by Zn is one of the obstacles to be overcome, which is particularly accented when welding TWIP steels. The current study tackles the issue with experimental method as well as numerical one. LME resistivity of different high strength steel grades were investigated for comparative study. It is found that Zn embrittles all the Fe based alloy system regardless of constituent phases. The critical stress for LME has been found to be the one corresponding to strain of 0.4 %, which will cause LME at temperatures over 700 °C. TEM observation revealed that thermo-mechanically assisted diffusion of Zn can occur along grain boundaries of the substrate when strained at high temperatures, which might cause LME. A finite element model has been developed for stress and temperature analysis in welding conditions. It is suggested from FE simulation that tensile stress arises during the cooling stage of the welding, which might cause LME on the weld surface. The level of the tensile stress is dependent on the thermal expansion coefficient of the weld metal, explaining why TWIP steels are vulnerable to LME during resistance spot welding.