Demands for aluminum alloy are increasing in various fields because it is light and strength. Especially, JIS AC7A represented by Al-Mg alloy in aluminum casting has high corrosion resistance and strength. On the other hand, casting factories prepare casting materials (JIS AC7A) by mixing ingot and returned materials. In some cases, silicon is found contained when the chemical composition of aluminum alloy is checked. The increase in silicon in the Al-Mg alloy decreases mechanical properties. In this study, in order to control the quality of the molten metal, we investigated aluminum alloy material with changed silicon content by the vacuum gas test and the K-mold method, which are melt quality evaluation methods. As a result, it was difficult to distinguish the difference in silicon content in the observation results of the vacuum gas test. But as the amount of silicon increased, the breaking angle of the K-mold specimen changed. As the amount of silicon increased, the breaking angle decreased and when the amount of silicon in the alloy was 0.17% or more, the specimen broke easily. Furthermore, according to the observation of metallic microscope, we found that Mg2Si forms at the grain boundary, and as the content of silicon in the alloy increases, Mg2Si also tends to increase. There was a correlation between the amount of silicon and the breaking angle. The breaking angle evaluation method using the K-mold specimen can be used for screening silicon contamination in AC7A alloy.
