Fundamental Study of Cavitation Melt Processing: Opening the Way to Treating Large Volumes (UltraMelt)

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Ultrasonic cavitation treatment offers sustainable, economical and pollution-free solutions to melt processing of conventional and advanced metallic materials with resulting significant improvement of quality and properties. However, the transfer of this advanced and promising technology to industry has been hindered by difficulties in treating large volumes of liquid metal as required by processes such as continuous casting. The time is right to tackle the problem as the industry is looking for new advanced technologies for sustainable manufacturing and our economic competitors, e.g. USA and China, are performing extensive scientific research in this area. The selection of this topic is both appropriate and ambitious. Current knowledge cannot answer a seemingly simple question: how long does it take to treat a certain volume of liquid with an ultrasonic source for minimum energy input, cost and complexity? This research aims to answer this question paving the way to extensive industrial use of ultrasonic melt processing with the benefit of improving the properties of lightweight structural alloys, simultaneously reducing the need for degassing and grain refinement additives - polluting (Cl, F) and expensive (Zr, T, B, Ar) - and eliminating complicated processing steps such as fluxing and rotary degassing. Before technological advances can be made, scientific understanding of the underlying phenomena, causes and effects, is essential. This project aims to respond to the challenge of efficiently treating large liquid volumes by: (1) developing a comprehensive numerical model that couples various multi-scale and multiphysics phenomena occurring inside and outside the cavitation region and by (2) changing emphasis from conventional static batch treatment to processing in continuous liquid flow. The novelty of the suggested approach lies in a fully three-dimensional, quantified experimental characterization and numerical description of (i) the primary and extended cavitation region, (ii) acoustic and secondary flows, (iii) mass-transfer through the boundary of the cavitation region, and (iv) processing in a moving volume (flow). The results of this research open the way to treating large volumes of melt with fewer ultrasonic sources and in a shorter time. To achieve the aims, dedicated, quantifiable experiments with three-dimensional characterization plus the use of responsive indicators of treatment efficiency will be combined with advanced modelling; bridging and coupling different length and time-scales and physical phenomena, ranging from the vibrational growth and collapse of individual cavitation bubbles, to the transport of bubble clusters in the bulk fluid and possible re-entrainment in the intense acoustic zone, and to the mass transfer throughout the treated volume. The influence of collapsing bubbles on flow momentum, energy and turbulence will be included in a fully coupled system. The experimental results will be used both as input and for validation of the model throughout its development.