Influence of High-Power Ultrasonic Irradiation on Primary Nucleation Process during Solidification

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In this paper, we describe the effects of ultrasonically preformed cavitation multibubbles in molten metal on both the onset of nucleation and the grain refinement of the solidified metal. Wood's alloy, that is, a bismuth-based quaternary eutectic alloy having a low melting point, was employed in the experiments. It was found that the irradiation of ultrasound at a frequency of 20 kHz and a power of 800 W for a prescribed time before solidification can affect the solidification microstructures of the alloy even if no ultrasound is additionally irradiated during solidification. Scanning electron microscope (SEM) image analysis showed that the microstructures were refined by irradiating ultrasound before solidification. This result suggests that numerous ultrasonically induced nanobubbles remained in the molten metal and provided many sites of nucleation. The size of a stably remaining cavitation bubble is estimated to be about 0.2 μm. The grain size of the metal after solidification decreased with an increase in the time elapsed between ultrasound irradiation and solidification. It is very interesting to note that the refinement of solidification microstructures by irradiating ultrasound occurs not during, but before solidification.
Published by the Japan Society of Applied Physics through the Institute of Pure and Applied Physicsの論文
2007-07-30