Study of Morphology of Oxide Film Formed on Magnesium Alloys in Casting Conditions (AZ91)

Morphology of surface oxide film formed during pouring of molten magnesium alloy has been investigated. Due to surface turbulence during casting, the oxide film necessarily makes folded cause in a dry surface to dry surface mode creating a double oxide film with the volume of air that can be encapsulated between folds of the film and this led to make gas bubbles or shrinkage cavities form. These kinds of oxides called new oxide films that form in a very short time during pouring. It seems to be one of the main reasons for dross-like defects. However, study of characterization and features of oxide film affected on prediction of final mechanical properties. The inner, unwetted surfaces of the doubled film representing an unbounded interface in the liquid and therefore, effectively constitute a crack. Samples for the study were prepared based on a technique in which an oxide metal sandwich was made by the bubble impingement technique, after impingement the contact areas of two adjacent and entrapped bubbles oxide-metal-oxide layer (OMO) were selected for the study. Features such as thickness, size, morphology and chemical composition of the oxide film were studied using a scanning electron microscope (SEM). Energy dispersive X-ray (EDX) microanalysis was performed for detection of the composition of the oxide layers. Results showed that the morphology of the oxide film in molten of magnesium alloys is folded and quite rough included globally phases of magnesium oxide. Recently, researches showed that the morphology of the oxide film in aluminum alloys is different due to composition of base alloy. Magnesium alloys in liquid state due to high oxidation rate is sensitive to formation of oxide film. Thickness of the oxide film folds in magnesium alloys is 2-4 µm that in comparison to aluminum alloys are ten times higher. However, potential of casting defects is higher in Mg alloys. The contacting interface between impinged bubbles (OMO) represents an elegant and powerful means for studying surface films of liquid metals and study for solidification structure