The characterization of surface films formed on molten magnesium and AZ91D alloy in air/1,1,1,2-tetrafluoroethane atmospheres

Abstract: During the melting and casting of magnesium and its alloys, it must be taken appropriate protective measures to prevent the melt from oxidizing and burning, because magnesium is a very active and readily ignitable element. In this paper, the morphologies and compositions of the surface films formed on magnesium and AZ91D alloy in air/1,1,1,2-tetrafluoroethane gas mixture at different conditions were investigated and analyzed by means of SEM, XRD and XPS. The experimental results showed that the morphologies and compositions of the surface films formed on molten magnesium and AZ91D alloy were different under the atmosphere of air containing different contents of 1,1,1,2-tetrafluoroethane. When the concentration of 1,1,1,2-tetrafluoroethane was less than 0.1vol.%, the surface films were mainly composed of MgO, and MgF2 for magnesium, and MgO, Mg3N2, AlN and MgAl2O4 for AZ91D alloy, which presented very coarse and incompact surfaces. When the concentration of 1,1,1,2-tetrafluoroethane was more than 1vol.%, the surface film on molten magnesium was not uniform, which was mainly composed of MgF2. The surface formed on molten AZ91D alloy was relatively smooth and compact, which was mainly composed of MgF2 and C. [Copyright &y& Elsevier]