마그네슘합금 미세조직과 기계적 특성에 미치는 응고속도와 알루미늄 함량의 영향

Abstract

Recently, development of magnesium alloys have been active in automobile industry due to their light weight. This study presents changes of microstructure and mechanical properties according to changes of aluminum content and solidification rate in magnesium alloy. The magnesium alloys used in the study had 3~9wt.% aluminum with 1wt.% zinc. The alloys were cast into a step-wide mold to investigate the effect of the cooling rate during solidification. Solidification rate increased with increasing of aluminum content and decreasing cast thickness. The major phases of the alloys were α-Mg, β-Mg17Al12, and eutectic α+β. The eutectic morphologies were consist of fully- or partially divorced eutectic phases. The hardness increased with increasing aluminum content and solidification rate. The effect of the aluminum content was more effective than solidification rate to increase the hardness. The changes of β-Mg17Al12 phase fraction was proportional to the hardness. The compressive strength was not affected by the solidification rate, but by the aluminum content. Heat treatment of the as-cast alloy was carried out to improve the mechanical properties of the as-cast billets. Hardness of billets changed continuously during the heat treatment. At the initial stage of the heat treatment, the hardness decreased due to the release of the residual stress of as-cast alloy. Then the hardness increased due to solid solution strengthening. After the peak, the hardness decreased again as the number of β-Mg17Al12 phases decreased in both continuous and gravity casting.