LAUSR

Graphite powder was adopted to prevent AZ91D magnesium alloy from oxidizing during the melting and casting process. The microstructure of the resultant surface films formed on the molten alloy protected by 0, 2.7, 5.4, 8.1, and 10.8 g dm−2 graphite powder at 973 K (700 °C) for holding time of 30 minutes was investigated by scanning electron microscopy, energy dispersive spectrometer, X-ray diffraction, and the thermodynamic method. The results indicated that the surface films were composed of a protective layer and the underneath MgF2 particles with different morphology. The protective layer was continuous with a thickness range from 200 to 550 nm consisting of magnesium, oxygen, fluorine, carbon, and a small amount of aluminium, possibly existing in the form of MgO, MgF2, C, and MgAl2O4. The surface films were the result of the interaction between the graphite powder, the melt, and the ambient atmosphere. The unevenness of the micro surface morphology and the number and size of the underneath MgF2 particles increased with graphite powder amount. The mechanism of the effect of graphite powder amount on the resultant surface films was also discussed.