Abstract:The sulphur hexa-fluoride gas(SF₆), which is commonly used as the cover gas of molten magnesium alloys in the magnesium industry today, has an extremely high global warming potential(GWP). The protection mechanism of SF₆ containing cover gases on molten magnesium alloys was presented. The cover gas protects the melt by reacting with the melt to form a coherent protective film on the melt surface. The film contains MgO and MgF₂. Particles containing MgF₂ form on the interface between the oxide film and the bulk magnesium alloy, which correspond to the concave areas from the surface observation. These particles increase the Pilling and Bedworth ratio of surface film and enhance the protective capability of the films. Based on the understanding of the mechanism of SF₆, a melting technology in a sealed furnace was proposed, and the protection behavior of magnesium alloys in the sealed melting furnace was investigated under the protective atmosphere containing HFC-134a. The morphology and composition of the surface film were also studied. Experiments to evaluate the protective effect of two other fluorine containing gases with low GWPs on AZ91D alloy in the sealed furnace were also carried out, and the results show that the new gases are potential substitutes for SF₆.

Key words: magnesium alloy; melt protection; SF₆; surface film; protection mechanism