Graduate School of Engineering
Department of Metallurgy

Ultrasonic-assisted Materials Processing: Fundamentals and Applications

The effects of ultrasound wave are associated with the ability of ultrasound to propagate through elastic mediums such as gas, liquids, and solids, and thus to transfer energy to places where it is needed. The purpose of our research is to investigate the ultrasound-related phenomena and to develop more efficient and sustainable processes for treatment of liquid metals and waste water. Particularly, we investigate such phenomena as cavitation and acoustic streaming through experiments and numerical simulation (Fig.1). These two phenomena arise when ultrasound waves propagate through liquid phases. Cavitation bubbles, when collapse, release huge amount of energy which is used for fragmentation and dispersion of solidifying crystals and solid particles in molten metals, as well as for formation of highly-reactive chemical radicals which help in decontamination of waste water. For example, irradiation of high-intense ultrasound waves into molten Al-Si alloy just before its solidification provides an effective way to refine the particles of primary silicon, as shown in Fig.2. This helps in creating a background for development of new high-silicon alloys with high wear resistance and low thermal expansion. Another example is the use of ultrasonic vibrations to create a new “frozen emulsion” type of composite materials.

Application of electromagnetic fields to environmental and materials processing

Our laboratory also hosts fundamental studies on the microwave processing of materials. Microwaves are electromagnetic waves with oscillating electric and magnetic field components at GHz frequencies. We separately investigate the irradiation effects of electric and magnetic fields. Notably, microwave magnetic-field irradiation with the imposition of an external magnetic field is of interest, not only because this raises magnetic resonance but also because it could be used in the development of new process for material fabrication. Moreover, microwave processing has a so-called nonthermal effect.

In Figure 3, part (a) shows XRD profiles obtained microwave-excited reaction of ZrO₂, B₂O₃, and C at 1300°C at various time. Part (b) is a SEM photograph of a ZrB₂ particle formed by the reaction. These results demonstrate that the fraction of ZrB₂ increased with time and was formed at an unexpectedly low temperature, based on the thermodynamic equilibrium. This feature could contribute to energy savings in industrial production.

In addition, we conduct fundamental studies on the high-frequency (kHz) induction heating and stirring of nonmetallic molten fluids. This includes experimental and simulation studies of molten vanadium oxide glass and molten salts.

Liquid Metals Processing

To improve recyclability of aluminum, there is a need to use more secondary aluminum and scrap, however these materials contain a lot of impurities. We are developing novel methods of mechanical stirring of molten aluminum, aiming at more efficient removal of impurities. To achieve these purposes, water model experiments are performed to investigate fluid flow and mass transfer during aluminum melt treatment and casting. Besides, numerical simulation is conducted to investigate transport phenomena in large-scale melting furnaces using super-computer. As shown in Fig. 4, the gas-liquid interface deformation during mechanical stirring was investigated by a water model experiment and numerical simulation, and the direction to reduce the entrainment of oxide film was developed.