Liquid Metal Embrittlement Susceptibility of Hot Formed Zn-Al-Mg Coated Steel with Eutectic Coating Microstructure

Abstract

Liquid metal embrittlement (LME) in Zn-based coating plates during hot stamping is an abnormal phenomenon where intimate contact between liquid Zn and a steel matrix results in the penetration of liquid Zn into the matrix, causing ruptures. In order to alleviate LME phenomenon, this paper designed a series of eutectic Zn-Al-Mg coating alloys to improve the uniformity of the Zn element distribution in the coating during heat treatment and inhibit the reaction between Fe and Zn. The high temperature mechanical properties of the coated steels are determined using thermal simulation to calculate the relative reduction in fracture energy, which is used to evaluate the LME level of the different composition coatings. It is suggested that the Zn-4.5Al-3.0Mg coating shows the highest LME resistance at 920 °C. The microstructure of these Zn-Al-Mg coated steels is observed after austenitization at 850 °C~920 °C for 3 min, which shows that the uniformity of the microstructure after austenitizing is affected by the composition of the coating and the austenitizing temperature. The higher temperature benefits the homogenization of the coating and steel and inhibits LME. The findings of this study provide valuable insights for the development of ternary Zn based LME resistant coatings.