The effects of holding time and concentration of SF6 on the protection of AZ91D magnesium alloy were investigated in a sealed furnace with a cover gas mixture of N2 and SF6. The morphology

thickness

composition and structure of the surface films were also studied. The results show that the surface film is compact with a thickness of 0.6 to 1.4 μm

containing F and Mg elements with an atomic ratio of 2∶1

which forms MgF2.The surface film of MgF2 prevents magnesium alloy from evaporation and protects magnesium alloy melt. The concentration of SF6 in the gas mixture has a great influence on the protection of the magnesium melt

the magnesium melt was protected when the concentration of SF6 is greater than 0.03vol.%

while a protection failure occurred under a concentration of 0.01vol.% SF6. When the concentration of SF6 is 0.1vol.%

the magnesium alloy was protected for a holding time from 7.5 min to 120 min. A relatively thinner surface file was found when the holding time was 7.5 min

while no significant change in thickness of the surface film was found for a holding time from 15 min to 120 min.