During the last decade, Mg alloys have attracted much attention for their application in various fields as a light-weight structural material. The widespread application of Mg alloys is due to the fact that Mg is the eighth most abundant element in the earth's crust and the third most plentiful element dissolved in seawater, with a relatively low cost, low density, good machinability, high specific strength and easy recycling capability. All these properties make Mg alloys excellent candidates as a structural material. The specific gravity of the magnesium is 1.74 that is about 2/5 of the titanium or about 2/3 of aluminium. The magnesium is the most lightweight in metal structure materials, and the strength is at the same level as aluminium. Lightweight structure is very useful in aviation, aerospace, automotive, transportation and other fields. Magnesium alloy has an obvious advantage in these fields because of its low density and high specific strength. Magnesium-alloy components are generally produced by casting processes. Mg-Al based alloys, such as AZ and AM series magnesium alloys, are the most commonly used commercial materials in automotive fields.