Influence of calcium on ignition-proof mechanism of AM50 magnesium alloy

In order to improve the ignition proof of AM50 magnesium alloy, 1wt% Ca was added to AM50 magnesium alloy to prepare investigated alloy. The ignition temperature, ignition-proof holding time at the predetermined temperature, microstructure of oxide film and thermodynamics of oxide film formation were studied. The results show that after adding 1wt% Ca, the ignition temperature of AM50-1wt%Ca alloy reaches 832 °C, which is 248 °C higher than 584 °C of AM50 magnesium alloy. The holding time of AM50 alloy varies with temperature. The higher the temperature, the shorter the holding time. AM50 magnesium alloy can remain for about 720 s at 525 °C without ignition, while it only remains for about 50 s at 550 °C. The oxide film of AM50 magnesium alloy is mainly loose and porous MgO. The oxide film of AM50-1wt.%Ca alloy is a bilayer oxide film of MgO and CaO and CaO is mainly distributed in the outer layer of the oxide film. When the temperature is lower than 949 °C, the △ G of CaO formation is less than that of MgO formation during the oxidation process of AM50-1wt%Ca alloy, MgO formed in the early stage of oxidation can be reduced to [Mg] by [Ca] and the minimum [Ca] content required for the reaction is always less than 1wt%. The dense bilayer oxide film formed on the surface of AM50-1wt%Ca alloy can effectively protect alloy and its oxidation process conforms to the parabola variation law.