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.
