Contactless Ultrasonic Treatment in Direct Chill Casting

• Published in: JOM (1989) 2020-11, Vol.72 (11), p.4082-4091
• Main Authors: Tonry, Catherine E. H., Bojarevics, Valdis, Djambazov, Georgi, Pericleous, Koulis
• Format: Article
• Language: English
• Subjects: Acoustic resonance; Acoustic streaming; Acoustics; Aluminum; Boundary conditions; Casting; Casting alloys; Cavitation; Chemistry/Food Science; Composition; Computational fluid dynamics; Crucibles; Dendritic structure; Direct chill casting; Earth Sciences; Electric currents; Electromagnetic induction; Engineering; Environment; Ferrous alloys; Fluid flow; Grain refinement; Heat transfer; Induction coils; Ingot casting; Liquidus; Magnetic fields; Mathematical models; Mushy zones; Numerical models; Physics; Radiation; Reynolds number; Solidification; Solidification Behavior in The Presence of External Fields; Solids; Stirring; Turbulence models; Ultrasonic processing; Ultrasonic vibration; Velocity; Viscosity
• ISSN: 1047-4838; 1543-1851
• DOI: 10.1007/s11837-020-04370-7

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.