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.

中文翻译：

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直接冷硬铸造中的非接触式超声波处理

成分的均匀性和晶粒细化是有色合金铸锭直冷 (DC) 铸造的理想特性。超声波处理是一种经过验证的实现晶粒细化的方法，通过额外的熔体搅拌实现成分的均匀性。为此目的，浸入式超声波发生器技术已用于在直流铸造之前在流槽内和直接在贮槽中处理合金。在这两种情况下，混合都很弱，依赖于浮力驱动的流动，或者在后一种情况下依靠声流。在这项工作中，我们考虑了直接在连铸机中使用的替代电磁技术，诱导超声波振动与强烈的熔体搅拌相结合。这种“非接触式超声波发生器”技术依赖于朝向熔体降低的千赫兹频率感应线圈，调整频率以在熔池内达到声学共振。结合数值模型和物理实验开发的技术已成功用于批量细化微观结构和在坩埚中对铝进行脱气。在这项工作中，我们扩展了数值模型、耦合电磁学、流体流动、气体空化、传热和凝固，以检查在直流过程中使用的可行性。模拟表明，在剧烈混合的熔池内可以获得一致的共振模式，在 Blake 阈值处的高压区域是定位于液相线温度的空化所需的高压区域。据推测，由于空化导致的糊状区的极端条件会促进枝晶破碎，再加上强烈的搅拌，会导致细小的等轴晶粒。