Grain Refinement of Primary Silicon in Hypereutectic Al-Si Alloy by Ultrasonic Radiation

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  • 超音波照射による過共晶Al-Si合金の初晶けい素微細化
  • チョウオンパ ショウシャ ニ ヨル カキョウショウ Al Si ゴウキン ノ ショショウ ケイソ ビサイカ

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Abstract

Ultrasonic radiation during solidification is known to be an effective tool for achieving grain refinement in various kinds of alloys. Since the mechanism of grain refinement by ultrasonic vibration is not fully understood yet, it is not widely applied to practical casting processes. Through the observation of non-equilibrium α-Al grains appearing in sono-solidified hypereutectic Al-Si alloys, the authors have proposed the crystallizing mechanism of α-Al grains, in which the nucleation of α-Al grains arises from extremely high pressure generated by the collapse of acoustic cavitation in molten Al-Si alloys. However, the mechanism cannot explain the refinement of primary silicon in hypereutectic Al-Si alloys, because the liquidus temperature of primary silicon becomes lower with increasing pressure. In the present study, the refining mechanism of primary silicon was examined through their morphology observations after applying ultrasonic radiation to molten Al-Si alloy, during solidification and at constant temperatures. Fine primary silicon was recognized in Al-18mass%Si alloys quenched from above the liquidus temperature after ultrasonic radiation. It was also found that acoustic cavitation during the sono-solidification leads to the generation of inhomogeneous nuclei, that is, cavitation bubbles, so that a large number of inhomogeneous nuclei of cavitation bubbles causes the grain refinement of primary silicon even above the liquidus temperature.

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