Role of aluminum on liquid metal embrittlement susceptibility for Zn–Al–Mg/Sn coated hot-formed steels

In order to reduce the susceptibility of liquid metal embrittlement (LME) on advanced high strength steels, this paper proposes a strategy to increase the thickness of the inhibition layer and retard the interdiffusion of Fe and Zn by adjusting the Al content in the coating, so as to improve this key problem in industrial application. On the basis of it, phase transformation of Zn–Al–Mg/Sn coated hot-forming steel during austenitization is studied via CALPHAD method and results show that Fe–Al intermetallic layer may break down gradually with the increase of heating temperature, leading to the penetration of liquid zinc into steel substrate. Hence it is necessary to analyze the Fe–Zn interdiffusion behavior during heat treatment. And then, the formation of LME crack is investigated by austenitization treatment. It is shown that, when the aluminum content in Zn–xAl–1.8Mg coating increases from 0.2 wt.% to 3.5 wt.%, not only the thickness of inhibition layer increases, but the temperature for the inhibition layer breaking down increases from 420 °C to 750 °C as well. Moreover, after the addition of Sn element, the inhibition layer of Zn-3.5Al–1.8Mg–0.8Sn coating has better stability compared with that of Zn–3.5Al–1.8Mg coating at the same high temperature. Additionally, it is found that the Zn–3.5Al–1.8Mg coating/steel interface zone shows a better ductility property than that with that of lower Al content on the same deformation conditions during high temperature tensile process. It is illustrative that the presence of Sn can weaken the liquid Zn penetration into the steel, so as to control the LME phenomenon.