Abstract

This paper presents an investigation of the evolution of flow structures and cavitation intensity in water as an analogue for a liquid metal under ultrasonic excitation. Results are presented for 20 kHz high-power ultrasound. The input power ranged from 50% (8.5 μm p-p) to 100% (17 μm p-p). To identify the streaming structures and understand the recirculation flows for different vibrational amplitudes of the sonotrode, particle image velocimetry (PIV) measured the velocity field. Simultaneously, a calibrated cavitometer probe measured acoustic intensity in the fluid. The cavitation intensity away from the acoustic source decreased with increasing input acoustic power, but was relatively constant inside the cavitation zone (irrespective of the input power). PIV measurements showed that the direction of the flow pattern was strongly related to the vibrational amplitude of the sonotrode. These results are compared with the predictions of an acoustic cavitation model. The outcome of the present work will help to determine the efficient optimization of ultrasonic processing of liquid metals that is of increasing technological importance.

Highlights

  • Ultrasonic melt treatment (UST) is an environment friendly and economical alternative to many conventional melt processes used to control liquid metal quality, such as fluxing, modification, and gas lancing

  • The cavitation spectrum consisted of acoustic emissions from the fundamental frequency and emissions from the collapsing bubbles that were strongly dependent on the acoustic power and the distance from the source

  • The cavitation intensity gradually decreased with acoustic power up to a point where it rebounded in magnitude

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Summary

Introduction

Ultrasonic melt treatment (UST) is an environment friendly and economical alternative to many conventional melt processes used to control liquid metal quality, such as fluxing, modification, and gas lancing. UST involves introducing high-intensity ultrasonic waves into liquid metal to induce acoustic cavitation [1]. The understanding and quantification of recirculation patterns within the acoustic cavitation zone and the mass exchange between the cavitation zone and the surrounding fluid are important for optimizing UST e.g. for the grain refinement of the as-cast structure. Wang et al [11] showed that a continuous recirculation pattern created a low temperature gradient throughout the melt: this favours an equiaxed grain structure, which is usually preferred by industry. In direct-chill casting, induced-acoustic streaming promotes forced convection that is opposed and much stronger to natural convection in the melt, facilitating further activation (wetting) of extrinsic particles, solid fragmentation and self-grain refining, mixing and elemental homogenisation within the bulk melt [5]

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