It is well known that solidification of molten metals occurs under the influence of transient unsteady phenomena, which are difficult to control in actual casting operation. Examples include buoyancy flows, natural heat convection, repeated remelting and solidification in mushy zone and solid shell at the mold interface. This paper presents the results suggesting that ultrasound waves, irradiated into the hot-top mold offers an attractive way to control the transient phenomena in DC casting of aluminum alloy billets. A novel mathematical model was developed to simulate the DC casting with considering transient melt flow, heat transfer, ultrasound propagation, acoustic streaming and solidification. It was found that ultrasound irradiation alters the solidification behavior of melt and sump evolution in the earlier stage of casting, however as the billet length increases, the sump profile and average volume of mushy zone become almost the same for both the ultrasonic and conventional castings. In this condition, the casting speed becomes the key parameter influencing the sump evolution and mushy zone volume. The results reveal that ultrasound-driven acoustic streaming and turbulent oscillations result in a suppression of convective buoyancy flow in the sump and lead to an increase of frequency of the melt temperature oscillations at the solidification interface. One of the useful results of these phenomena can be a more frequent repetition of remelting/solidification cycle and improvement of the billet quality, particularly the billet surface morphology. The effect of ultrasound irradiation on the billet morphology was verified experimentally using a pilot DC caster.

众所周知，熔融金属的凝固是在瞬态非稳态现象的影响下发生的，该现象在实际的铸造操作中很难控制。实例包括浮力流动，自然对流，在糊状区和模具界面处的固体壳中的反复重熔和固化。本文提出的结果表明，照射到热顶模具中的超声波提供了一种有吸引力的方法来控制铝合金坯料直流铸造中的瞬态现象。考虑到瞬态熔体流动，传热，超声传播，声流和凝固，开发了一种新颖的数学模型来模拟直流铸造。发现在铸件的早期阶段，超声辐射会改变熔体的凝固行为和油底壳的演变，但是，随着钢坯长度的增加，超声波铸件和常规铸件的油槽轮廓和糊状区的平均体积几乎相同。在这种情况下，浇铸速度成为影响油底壳演变和糊状区体积的关键参数。结果表明，超声驱动的声流和湍流振荡导致集水槽中对流浮力流受到抑制，并导致凝固界面处熔体温度振荡的频率增加。这些现象的有用结果之一可以是更频繁地重复进行重熔/凝固周期并改善坯料质量，尤其是坯料表面形态。超声辐照对坯料形态的影响已通过试验DC连铸机进行了实验验证。