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

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.

Melt treatment is the primary stage of any metal production followed by casting (solidification) of ingots or final shapes. 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. This project aims at fundamental understanding of phenomena underlying this potentially revolutionary materials processing technology, paving the way to its industrial implementation, and educating specialists in this area.
The results of this project will enable much-awaited industrial application of environmentally friendly ultrasonic melt processing to degassing and structure modification of advanced light-alloy materials and their composites for modern applications driven by sustainability and environment. The ultrasonic melt processing will result in reduction or elimination of cleaning gases (green-house gases, Cl, and F) and expensive cleaning and grain-refining additions (Ar, Zr, Ti, and B) while allowing the production of high-quality light-weight materials, not only from primary but also from recycled stock. This will additionally increase sustainability of light-weight materials. For example, the replacement of primary aluminium with recycled materials will have the immediate effect of decreasing by 95% energy consumption and reducing by 0.3 tonne/per tonne Al CO2-equivalent emissions typically spent on the primary electrolytic reduction and refining of aluminium.
Furthermore, the developed new knowledge and models can be used for the development of light-metal composite materials and applied to other physical means of melt processing (e.g. electro-magnetic vibrations), which is outside the scope of the current project. In addition, the knowledge obtained and models can be used in other research and industrial fields where cavitation is induced in a physico-chemical system. Examples of such systems can be found in pharmaceutical, biotechnological, chemical, and food industries.
The results of the research will be actively disseminated in the academic and technological communities through international and national conferences and seminars; publications in peer-reviewed journals; and through existing academic and industrial networks of the applying research centres. The results can be directly exploited by R&D centres and industry for the up-scaling of laboratory results to the industrial processing of light alloy melts (degassing, casting); and the models and fundamental knowledge developed can be used for the advances in new technologies in metallurgical and other industries. The results will be of interest for both the academic community (sonochemistry, acoustics, physical chemistry, materials science, hydraulics) and the industry (metal production, chemical and food industry).
All in all (as illustrated above) the cavitation-aided melt processing technologies enabled by the fundamental knowledge, numerical models and up-scaling methodology gained in this project will contribute to the improvement of the quality of life through environmental sustainability, reduced green-house effect, and the development of new products, and also strengthen the international competitiveness of UK industry.
Key beneficiaries include the two young researchers educated and experienced in this project, and producers of aluminium alloys (Alcoa, Rio-Tinto-Alcan, Novelis), master alloys (Foseco, LSM), automobiles (JLR, Vauxhall), and airspace technique (Airbus-EADS, ESA).