Contactless Ultrasonic Treatment in Direct Chill Casting

Catherine E H Tonry,Georgi Djambazov,Koulis Pericleous,Valdis Bojarevics

doi:10.1007/s11837-020-04370-7

Abstract

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.

Highlights

Ultrasonic treatment (UST) of melts has been shown to lead to degassing and grain refinement.[1,2,3] Grain refinement is necessary in direct chill (DC) casting to prevent hot tearing,[4] shrinkage porosity,[5] and cold cracking.[6]
This paper focuses on numerical modeling efforts to test the feasibility of using the top-coil in vertical DC casting, taking into consideration the geometry and the continuous nature of this process
The pulsating hydrogen bubbles represented in the time-dependent cavitation model cause attenuation of the ultrasound by relieving the overall tension in the liquid during the rarefaction stage of the cycle, further weakening the non-resonant modes. This initial modeling work has highlighted the potential of using a contactless ultrasonic device for continuous UST in DC casting

Summary

Introduction

Ultrasonic treatment (UST) of melts has been shown to lead to degassing and grain refinement.[1,2,3] 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 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

Methods

Results

Conclusion