Contactless Ultrasonic Treatment in Direct Chill Casting
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https://doi.org/10.1007/s11837-020-04370-7 © 2020 The Author(s). This article is an open access publication
SOLIDIFICATION BEHAVIOR IN THE PRESENCE OF EXTERNAL FIELDS
Contactless Ultrasonic Treatment in Direct Chill Casting CATHERINE E. H. TONRY ,1,2 VALDIS BOJAREVICS ,1 GEORGI DJAMBAZOV ,1 and KOULIS PERICLEOUS 1 1.—Computational Science and Engineering Group, University of Greenwich, Park Row, London SE10 9LS, UK. 2.–e-mail: [email protected]
Uniformity of composition and grain refinement are desirable traits in the direct chill (DC) casting of non-ferrous alloy ingots. Ultrasonic treatment is a proven method for achieving grain refinement, with uniformity of composition achieved by additional melt stirring. The immersed sonotrode technique has been employed for this purpose to treat alloys both within the launder prior to DC casting and directly in the sump. In both cases, mixing is weak, relying on buoyancy-driven flow or in the latter case on acoustic streaming. In this work, we consider an alternative electromagnetic technique used directly in the caster, inducing ultrasonic vibrations coupled to strong melt stirring. This ‘contactless sonotrode’ technique relies on a kilohertz-frequency induction coil lowered towards the melt, with the frequency tuned to reach acoustic resonance within the melt pool. The technique developed with a combination of numerical models and physical experiments has been successfully used in batch to refine the microstructure and to degas aluminum in a crucible. In this work, we extend the numerical model, coupling electromagnetics, fluid flow, gas cavitation, heat transfer, and solidification to examine the feasibility of use in the DC process. Simulations show that a consistent resonant mode is obtainable within a vigorously mixed melt pool, with high-pressure regions at the Blake threshold required for cavitation localized to the liquidus temperature. It is assumed that extreme conditions in the mushy zone due to cavitation would promote dendrite fragmentation and coupled with strong stirring, would lead to fine equiaxed grains.
INTRODUCTION Ultrasonic treatment (UST) of melts has been shown to lead to degassing and grain refinement.1–3 Grain refinement is necessary in direct chill (DC) casting to prevent hot tearing,4 shrinkage porosity,5 and cold cracking.6 A fine grain structure gives improved material properties to the metals, and also facilitates subsequent mechanical working. Columnar to equiaxed transition (CET)7,8 is required to achieve a uniform fine grain structure in the core of the ingot. There are many methods to achieve CET, with the most common practice relying on grain refiners.9 Alternative methods used to promote CET include melt conditioning,10 electric current pulse,11 pulse magneto-oscillation,12 pulse magnetic field,13 electromagnetic (EM) stirring,14 and, as discussed here, UST.
There have been many investigations of UST of alloys in DC casting,1,15–18 where the focus is typically on treating the melt prior to casting, either in a crucible15 or in
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