Effect of Cooling Rate and Remelting Temperature on the Solidification Structure of Al-5Zr Master Alloy

Abstract

Zr is an important element to improve the heat resistance of aluminum alloys, which is usually added to alloys using the Al-Zr master alloys. The microstructure of Al-Zr master alloys has a significant impact on the properties of Zr-bearing aluminum alloys. In this paper, the microstructure of commercial Al-5Zr master alloys was examined, and the effect of the remelting temperature and cooling rate on the solidification structure of the remelted Al-5Zr master alloys was investigated, aiming to develop a feasible way for quality improvement of Al-5Zr master alloys. The results showed that the microstructure of the remelted Al-5Zr master alloy could be regulated effectively by controlling the remelting temperature and cooling rate. When the remelting temperature was 1320 °C, the primary Al3Zr phase in the remelted Al-5Zr master alloy was mainly precipitated as coarse plate-like or fine long needle-like. Higher cooling rate increased nucleation density and refined microstructure. The average length of the primary Al3Zr phase was 178.2, 87.4, and 61.3 μm when the cooling rate was 4.6, 30.8, and 43.9 °C/s, respectively. Lower remelting temperature was generally conducive to refinement of primary Al3Zr phase. When the remelting temperature was 920 °C, the primary Al3Zr phase in the remelted Al-5Zr master alloy was mainly precipitated as block-like, fine needle-like, and petal-like. When the cooling rate was 4.6 °C, coarse plate-like Al3Zr phase precipitated. With increasing cooling rate to 25.3 °C, the coarse plate-like Al3Zr phase disappeared and the block-like and fine needle-like Al3Zr phase precipitated followed by a large number of fine petal-like Al3Zr phase precipitated after cooling rate to 45.6 °C. The optimized remelting process can improve the microstructure of the commercial Al-5Zr master alloy.