Abstract
Modification of Al-22mass%Si and Al-22mass%Si-2.5mass%Mg alloys was examined by combined use of the Duplex Casting process and addition of Na and P. In the Duplex Casting process, two kinds of alloy melts with different compositions were cast in the mold at a given interval. In this study, the combinations of Na-treated Al-12mass%Si alloy or Na-treated Al-12mass%Si-5mass%Mg alloy as the first melt and P-treated Al-32mass%Si alloy as the second melt were used. In the conventional casting, simultaneous addition of Na and P tends to coarsen the primary silicon crystal. On the other hand, refined primary silicon crystals and modified eutectic silicon crystals are observed in the ingots obtained by combined use of the Duplex Casting process and addition of Na and P. In the vicinity of the side surface of the ingots, markedly refined primary silicon crystals are observed, which show the synergistic effect of the Duplex Casting process and P addition to the second melt. Origin of Primary Silicon Free Zone in the ingots is the first melt incompletely mixed. Acceleration of circulation flow of the second melt in the mold alleviates the macrosegregation of primary silicon.
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