Protection behavior of SO2-containing cover gases to molten magnesium alloy

Sulphur dioxide (SO2) mixed with carrier gases was used as an alternative to SF6 to protect molten magnesium alloys. The protection behavior of AZ91D alloy in a sealed melting furnace was investigated under the atmosphere containing SO2 with different mixed gases. The morphology and composition of the surface film were studied. The melt was well protected in an atmosphere of SO2 and a proper amount of air, and was not protected properly in the other atmospheres. Based on the understanding of the protective effects of SO2 in the sealed furnace, the protection mechanism of SO2-containing cover gases on molten AZ91D alloy was studied in an open melting furnace. The cover gas protected the melt by reacting with the melt to form a coherent protective film with a network structure on the melt surface. The film contains MgO and MgS. MgS increases the Pilling and Bedworth ratio of the surface film and enhances the protective capability of the films.