Contactless Ultrasonic Processing for Liquid Metals

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In the quest for lighter, stronger metals for the transport and aerospace industry we aim in this research to develop and evaluate a contactless electromagnetic excitation device producing vibration in liquid metal melts, for the purpose of Ultrasonic treatment (UST). UST of light alloy metals (aluminium or magnesium) at the liquid state, has been shown to improve their final mechanical properties, leading to finer microstructure, removal of dissolved gases, or dispersion of strengthening particles. A 'sonotrode' probe immersed in the melt is commonly used for this UST process, vibrating at ultrasonic frequency (~20-100 kHz) and generating intense sound waves in the molten metal. These pressure waves cause dissolved gas to come out of solution in the form of micro-bubbles (think of champagne), then oscillate in size and either implode generating high speed jets and shock waves or enlarge to the point where they float to the surface and out of the melt. This cavitation phenomenon assists crystal nucleation and/or breaks up emerging dendritic crystals to reduce grain size and so improve material properties. Strengthening particles in the form of oxides are used in metal composites to improve their properties. However, it is difficult to disperse these particles evenly, ensuring homogeneous performance in the final component. Furthermore, very small particles tend to cluster together becoming defects. The action of shock waves produced by collapsing bubbles is known to break up particle clusters, but then strong stirring is also needed to disperse them evenly. The same particle dispersion requirement is needed in aluminium recycling, to disperse unwanted inclusions, such as oxides or ferritic intermetallic particles. So UST is a very useful process but even in low temperature melts (Al, Mg), there are problems preventing widespread use by industry: the immersed probe is consumed contaminating the melt; the mass treated is restricted to a small volume surrounding the sonotrode horn, and further mechanical stirring is then necessary to spread the effect. Contactless UST as proposed here will avoid contamination and the cost of exotic probe materials, transferring the potential benefits of UST to a wider range of alloys, avoiding most of the drawbacks: (a) Bulk stirring is automatically generated by the electromagnetic 'Lorentz' force, (b) Scale-up is easy, since the induction coil can be designed to fit the application, provided the supply frequency is tuned to promote resonance, (c) Since there is no contact, there is no contamination of the melt, or need for a frequent probe replacement and finally (d) high temperature or reactive metals used in power or aeroengine applications can be treated in the same way (viz. nuclear steels, nickel and titanium alloy blades) The patented 'Contactless Sonotrode' concept originated from theoretical work and computer simulations carried out in Greenwich; but to translate its immense theoretical potential into a useful manufacturing technique, careful practical validation is needed through the proposed experimental programme at Birmingham and Oxford Universities. A prototype installation at Birmingham will investigate light alloys, steel and nickel in crucible melts, whilst Oxford will test the idea in the Direct Chill (DC) continuous casting process for aluminium ingot production.