マグネシウム特集号 半溶融ＡＺ９１Ｄマグネシウム合金の組織およびみかけの粘性に及ぼすかくはん条件の影響

Abstract

AZ91D magnesium alloy was directly heated to mushy zone from room temperature without melting and was stirred at constant temperature, 858K, for 7.2ks. Effects of stirring conditions and the fineness of raw materials on microstruc- tural changes, behavior of apparent viscosity during isothermal stirring and macrostructures after the stirring were investigated. Microstructures of stir-cast AZ91D alloy consists of primary α phase and fine eutectic phase created by a rapid cooling after the stirring. Morphology of α phase changes from slender form to spheroid with the elapse of the stirring time. When raw material with large grains is stirred with a paddle-type stirrer, the spheroidization of α phase becomes faster and its size becomes smaller as stirring rate increases. But in the case of an attritor-type stirrer, the size of α phase increases with increase of stirring rate owing to acceleration of the coalescence of grains. The size of α phase decreases with decrease in fineness of microstructure of raw material. A rapid reduction of stirring torque is probably caused by decrease of solid fraction and the spheroidization of α phase at an initial stage of the stirring. Thereafter, stirring torque gradually decreases, corresponding approximately to the spheroidization of α phase. Based upon the results of a visualized experiment for fluid flow during the stirring by utilizing glycerin at 285K, it is found that the entrapping of gas into specimen can be prevented by increasing metal head.