Influence of additive element on surface oxide film of A356 alloy

The influences of RE-modification and Sr-modification on the hydrogen content and surface oxide film of A356 aluminum alloy melt were investigated. The hydrogen content of the melt was measured by reduce pressure test. The phases in the surface oxide film were analyzed by X-ray diffractometry (XRD), and the morphology of the surface oxide film was observed by scanning electronic microscopy (SEM). The results show that RE-modification reduces the hydrogen content of A356 aluminum alloy greatly. Contrarily, Sr-modification increases the hydrogen content remarkably. After being treated with RE, a large number of LaAlliOis consisting of Al2O3 and La2O3, are generated in the surface oxide film of A356 alloy. The surface oxide film of Sr-modification is almost composed of Al2Sr04. According to the results of SEM, the surface oxide film of Sr-modification is very easy to crack, destroy the continuity and compactness of surface oxide film, accelerate the vapor diffusing into the melt, consequently, increase the hydrogen content of A356 alloy melt significantly. But RE-modification makes the surface oxide film compact, and restrains the aluminum exposed to water, so reduces the hydrogen content of A356 alloy melt.