Abstract: The microstructure evolution and partially remelting processing of two-phase-region casting AZ91D magnesium alloy ingots were investigated. The result indicates that the semisolid feedstock cast by held at two-phase-region temperature (585℃) for 30min and then poured in water-cooled-iron mould, as is remelted at 575℃ for 30～45min, refining and homogeneous solid grains with higher spheroidization degree and higher effective liquid volume fraction can be obtained. The microstructure evolution stages of the feedstock include initial stage, spheroidizing stage, and grouping and merging stage, which partially overlap with each other. For the special original semisolid ingot, the spheroidizing stage can be enlarged and the overlap bands can be narrowed through varying remelting temperature, therefore, the optimum microstructure feature and remelting processing will be realized, which will be beneficial to improve the process controllability and the thixoforming property of feedstock.
Microstructure evolution and partially remelting processing of two-phase-region casting AZ91D semisolid slurry ingot
Abstract：
The microstructure evolution and partially remelting processing of two-phase-region casting AZ91D magnesium alloy ingots were investigated. The result indicates that the semisolid feedstock cast by held at two-phase-region temperature (585 ℃) for 30 min and then poured in water-cooled-iron mould, as is remelted at 575 ℃ for 30～45 min, refining and homogeneous solid grains with higher spheroidization degree and higher effective liquid volume fraction can be obtained. The microstructure evolution stages of the feedstock include initial stage, spheroidizing stage, and grouping and merging stage, which partially overlap with each other. For the special original semisolid ingot, the spheroidizing stage can be enlarged and the overlap bands can be narrowed through varying remelting temperature, therefore, the optimum microstructure feature and remelting processing will be realized, which will be beneficial to improve the process controllability and the thixoforming property of feedstock.
图2 坯料原始组织及在575 ℃保持不同时间的组织 Fig.2 Microstructures of as-cast and as-remelted at 575 ℃ for various times (a)—Microstructure of as-cast; (b)—5 min; (c)—15 min; (d)—30 min; (e)—45 min; (f)—60 min
图3 575 ℃下保温不同时间的固相颗粒等积圆直径分布 Fig.3 Distribution maps of grain equal-area-circle diameter at 575℃ holding for different times
图6 固相颗粒的平均等积圆直径, 平均圆度和固相颗粒数密度 随重熔温度的变化(保温时间: 30 min) Fig.6 Variations of average equal-area-circle diameter, average roundness and number density vs remelting temperature (holding for 30 min)
图7 坯料部分重熔组织演变阶段示意图 Fig.7 Diagrammatic sketch of microstructure evolution stages of ingot during remelting tib, tie—Start and end time of i zone; θm, θs—Liquidus and solidus temperature; θop——Optimum temperature; θl, θh—Remelting temperature lower or higher than θop
