Verification of the Mechanism of Grain Refinement by Ultrasonic Treatment of Aluminum-4 Wt Pct Silicon Molten Alloy

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THE ultrasonic treatment to molten metals (UST) produces a fine grain microstructure. Several mechanisms of the grain refining by UST have been suggested. For example, Eskin et al. suggested that cavitation generated by UST promotes the wetting of nonmetallic inclusions and particles by liquid aluminum and involves them in the solidification process.[1] Ramirez et al. suggested that the pressure pulse-melting point mechanism and cavitation-induced wetting contributes to nucleation.[2] However, the true mechanism of this refining has not been elucidated to date. By verifying the mechanisms, a fine grain microstructure without a grain refiner is expected by direct chill casters and shape casters. The mechanisms should be classified into three mechanisms. The first one is the nonequilibrium nucleation mechanism. Fukui et al. suggested that the nonequilibrium nucleus was generated by the collapse of acoustic cavitations induced by ultrasound irradiation.[3] KAZUKI MATSUDA, TSUYOSHI TAKEHARA, MINGHAN YANG, and HIKARU UNO, Students, are with the Department of Modern Mechanical Engineering, Graduate School of Waseda University, 3-4-1 Shinjuku-ku Okubo, Tokyo 169-8555, Japan. Contact e-mail: [email protected] TAKASHI KUBO and GAKU MIYANO, Employees, are with UACJ Corporation, 1-7-2 Chiyoda-ku Otemachi, Tokyo 101-8970, Japan. MAKOTO YOSHIDA, Professor, is with the Department of Modern Mechanical Engineering, Graduate School of Waseda University, and also with the Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Shinjuku-ku Nishiwaseda, Tokyo 169-0051, Japan. Manuscript submitted December 25, 2014. Article published online March 2, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A

The second one is the nucleation hastening mechanism, which has been studied extensively. Hori and Uezawa found that primary crystals increase remarkably by UST.[4] They suggested that this will not be caused by mechanical breaking of primaries but by promotion of nucleation. Araki Takashi proposed that nucleation was hastened by UST[5] because extraordinary pressure created by the cavities would stabilize the nuclei. Abramov indicated that the occurrence of shock waves, when cavitation bubbles collapse and melt agitation takes place as acoustic streaming progresses, was likely to have an influence on the kinetics of crystal nucleation.[6] Eskin reported that the crystal nucleation process had been shown to be a major factor contributing to the formation of nondendritic grains.[7] Liu et al. described that properly applying a supersonic vibration to a cooling alloy melt could effectively decrease the dynamic nucleation barrier for the a-Al phases.[8] Osawa et al. pointed out that acoustic stream played a role in prompting the nucleation of primary crystals.[9] Osawa et al. also reported that crystals were suddenly generated with recalescence by UST.[10] Jian et al. suggested that cavitations-induced heterogeneous nucleation played a more important role than dendrite fragmentation in the formation of glo