Effect of Melt Holding Time on the As-cast Microstructure and Hardness of Aluminum Alloy 2618

Abstract

The effects of the melt holding time on the type, quantity and morphological distribution of aluminum alloy 2618 were investigated by means of OM, XRD and SEM. And the influence of the melt holding time on the hardness of alloy was explored. The results show that the heredity of the microstructure of the master alloy is not completely eliminated when the melt holding time is less than 10 min at 740 ℃. And it will make Al9FeNi phase coarse needle-like, and segregate with Al7Cu2Fe phase at the grain boundary or inside the grain. Properly prolonging the melt holding time can change the shape of Al9FeNi phase from coarse needle-like to fine needle-like and make the distribution of Al9FeNi phase and Al7Cu2Fe phase more uniform. Aluminum alloy 2618 was melted at 740 ℃ for 30 min and then poured into graphite molds at room temperature, which can make Al9FeNi phase fine and uniformly distributed in the matrix. With the extension of the melt holding time, the size of the nonequilibrium crystalline phase is basically unchanged, but the grain size will be slightly coarsened. The hardness of the alloy showed a trend of first increasing and then decreasing with the prolongation of the melt holing time within 10~60min, and when the melt was held for 30min, the hardness of the alloy reached the maximum HRB value of Rockwell hardness within the range of this test. The maximum value is 48.62HRB, and the hardness distribution of the alloy is relatively uniform. Therefore, it is beneficial to refine the precipitated phase, reduce the macro segregation, and refine the grain and improve the hardness of the alloy, while alloy 2618 was poured into the graphite casting at room temperature with melting at 740℃ and held for 30min.