Grain Refinement in Continuously Cast Magnesium Alloy Billets by Ultrasonic Radiation

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  • 超音波照射による連続鋳造マグネシウム合金ビレットの結晶粒微細化
  • チョウオンパ ショウシャ ニ ヨル レンゾク チュウゾウ マグネシウム ゴウキン ビレット ノ ケッショウリュウ ビサイカ

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Abstract

Grain refinement of magnesium alloys is required for improving mechanical properties to expand their application fields. Ultrasonic vibration has been applied to a variety of liquid phase processes such as grain refinement and degassing in aluminum alloys. In the present study, ultrasonic radiation was applied to the continuous casting of magnesium alloy to refine its grain size. The microstructures of continuously cast billets was observed as a function of immersing depth of an ultrasonic horn into molten magnesium alloy, which is the distance between the solidification front and ultrasonic horn tip. Microstructures with grain size of 22μm were attained in the area with a small distance between the solidification front and horn tip. However, well refined regions were limited to the central area in the horizontal cross-section of the billets. In order to obtain refined grains over the whole billet, it was proposed that the ultrasonic horn be rotated around the center line to expand the area with a small distance between the solidification front and horn tip over the whole cross-section of the billet. By providing additional rotating motion to the ultrasonic horn, average grain size of 50μm was achieved in sono-solidified continuously cast billets. Furthermore, the effects of immersing depth of the ultrasonic horn on the microstructure were also examined from the viewpoint of acoustic cavitation by using luminol solution. The luminescent area was found to become much wider due to acoustic cavitation in continuous casting with a rotating ultrasonic horn compared to that with a fixed horn. Pressure measurements below the ultrasonic horn showed that the peak to peak pressure amplitude is strongly attenuated at one tenth of wave length, thereby limiting the generation of acoustic cavitation to the area with a short distance between the solidification front and horn tip.

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