Effects of cover gases on melt protection of Mg alloys

Melt protection properties of inhibitor (SF6, HFC-134a and SO2)/air cover gas mixtures were investigated for different magnesium alloys, temperatures, gas concentrations and sealing conditions. Melt protection properties of the surface films formed in cover gas mixtures were also investigated for ambient atmosphere. The compositions of surface films formed in various cover gas mixtures were analyzed by X-ray photoelectron spectroscopy (XPS). For both under cover gas mixtures and ambient atmosphere, the surface film formed in HFC-134a with air cover gas mixtures provided effective melt protection properties for all test conditions. SO2 showed the same melt protection properties as SF6 for 680 °C whether the crucible is tightly sealed or not. XPS results revealed that the surface films formed under fluorine-bearing cover gas mixtures consist of MgO and MgF2. Magnesium fluoride which has high Pilling–Bedworth ratio (PBR) can act as a barrier to further oxidation and evaporation of magnesium due to its dense and continuous structure.