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Accepted: 2004-04-05, 資料番号: SA1005121000

The mixing-crystallization model has been used to evaluate the effects ofthe convective mixing on the undercooling and solidification behavior of a hypereutectic Al–Si alloy during the mixing process of two different molten alloys: the first melt (Al–12 mass%Si alloy) and the second melt (Al–32 mass%Si alloy). This model treats the mixing of these alloys in stretching lamellar configurations to consider the interactions between the fluid clumps ofthe second melt. The obtained results are summarized as follows: (1) The undercooling of the second melt in the lamellar configurations are smaller than that in the single slab configuration because of the interactions between the second melt lamellar. (2) The number of primary crystals is maximized at a certain strain rate of stretching lamellar depending on the initial melt-distribution. Therefore, excessive agitation during the initial stage of the solidification may depress the refinement of the primary crystals. (3) Nonuniform distribution of lamellar thickness is effective for the crystal refinement compared with the uniform one.

金属の半凝固加工プロセスでは、粒状の固相と液相とが混在したスラリー状の半凝固金属を製造し、これを直接加工して成形体を得る。その基幹技術となる半凝固金属製造法の原理は、固液共存状態の合金を攪拌することにより初晶粒子の粒径を制御するプロセス変数として、攪拌の剪断歪速度と冷却速度とが重要である。前者は攪拌速度の指標であり、後者はデンドライトアームおよびその断片の粗大化程度に関係する。一方、過共晶Al-Si合金のように硬質のファセット状初晶粒子を晶出する合金系では、攪拌の破砕効果のみにより初晶を実用レベルまで微細化することは困難であり、急冷凝固より初晶の核生成数そのものを増加させる方策が必要である。過初晶Al-Si合金は、鋳造用Al合金の中で最も優れた耐熱摩耗性と低い熱膨張率を有する。この特性は、共晶組織中に分散した初晶Si粒子によって担われており、この初晶粒子のサイズの制御が実用上不可欠となっている。例えば、エンジンのシリンダボア部の初晶サイズは、30～50μｍが適当とされている。本稿で紹介する「スラリー・溶湯混合法」（SMM法：Slurry-Melt Mixing process）は、微細な初晶Siを含有する過共晶Al-Si合金スラリーを徐冷環境下で製造するプロセスである。本プロセスの特徴は組成と温度の異なる2種類の金属流体の混合により急冷凝固を実現する点にあり、その原理は従来の半凝固金属製造プロセスとは大きく異なっている。

A simplified numerical model is presented to investigate the undercooling and solidification behavior of a hypereutectic Al–Si alloy during the mixing process of two kinds of molten alloys with different temperature and composition. This model involves a single slab of the second melt (Al–32 mass%Si alloy) stretching in the infinite bulk of the first melt (Al–12 mass%Si alloy). The second melt and, in some cases, a part of the first melt near the second melt are undercooled and solidified in the course of the mixing. The number of primary crystals is maximized at a certain strain rate of the stretching slab, depending on the initial thickness of the slab, while the maximum undercooling of the second melt increases as the strain rate increases. These results suggest that excessive agitation during the mixing may depress the refinement of the primary crystals.

NASDAではTR-1A(宇宙実験用小型ロケット)ミッションの一環として、微小重力下での高温加熱炉(HTF)を用いたガラスの溶融・凝固実験を計画している。HTFミッションの予備研究として、1991年10月、MU-300型航空機による実験を行った。本実験は、微小重力環境下におけるガラス材料の溶融状態把握、ガラス材料のシリンダへの支持方法、およびフォトダイオードを用いる温度計測技術の確立を目的として実施した。ここでは、ガラス材料の溶融・凝固の状態、CCD(電荷結合素子)カメラによる過冷却現象観測、熱電対とフォトダイオードによる温度および光度計測の実験結果について述べた。, The experiment of melt and solidification of glass using HTF (High Temperature Furnace) under microgravity environment is planned by NASDA as a part of TR-1A (Test Rocket-1A) sounding rocket missions. This experiment using MU-300 aircraft in October 1991 was carried out as the preliminary study of HTF mission. The purpose of this experiment is to understand melting condition of glass materials under microgravity environment and to establish how to support a glass material on the cylinder and how to measure the temperature using photodiode. This paper describes the result of melting and solidifying condition of glass materials, as well as observing undercooling phenomena using CCD camera and measuring the temperature and luminosity using thermocouple and photodiode., 資料番号: AA0000739003, レポート番号: NASDA-TMR-960022

The primary silicon crystals of hypereutectic Al-Si alloys are reported to be refined by the Duplex Casting process (a two-step casting of two molten alloys with different composition; i. e., "the first alloy" with lower liquidus temperature and "the second alloy" with higher liquidus temperature). Recently, concerning the refining mechanism, the authors have proposed that copious nucleation of primary silicon crystals takes place in the second alloy which is rapidly cooled and highly undercooled during mixing with the first alloy. The purpose of this study is to investigate the degree of under-cooling and the solidification behavior of the fluid clumps of the second alloy which are thought to form the clustering structures of fine primary crystals. The relationships between the degree of undercooling and the primary silicon size of Al-32 mass%Si and Al-22 mass%Si alloys were obtained by use of three solidification techniques: (1) air-cooling or immersion into a metal bath of melt-filled graphite crucible, (2) casting in a wedge-shaped copper mold, and (3) melt-in-jection into a lead bath. The primary silicon size of the finest cluster observed in the ingot produced by the Duplex Casting process with the combination of Al-12 mass%Si alloy and Al-32 mass%Si alloy is 15μm, and the corresponding undercooling is estimated at 243K.