The addition of manganese and zirconium is effective to remove iron from the melt of the recycled aluminum. In such a case, manganese and zirconium possibly remain in the melt. The influence of the remaining elements on various properties of aluminum alloys has not been clarified. In this study, effects of the remaining manganese and zirconium on the microstructures and mechanical properties of the 6061 aluminum alloy were investigated. The compound of Al8(Cr, Mn, Fe)2Si crystallizes if manganese is added to the 6061 aluminum alloy, resulting in the reduced solid solubility of silicon in the matrix. Since silicon contributes to age hardening of the 6061 aluminum alloy by forming metastable phases with magnesium the addition of manganese thus causes the decreases in the 0.2% proof stress and tensile strength of the 6061 alloy. Zirconium added to the 6061 aluminum alloy is partly retained as a solid solution in the matrix and the remainder forms Al–Zr or Al–Zr–Si compounds. The amounts of these compounds, however, are very small, and their effects on the tensile properties are negligible. The manganese content in the 6061 aluminum alloy with 0.3%Zr should be less than 0.4mass% to guarantee the tensile properties specified in JIS.
