Abstract
This study concerns the numerical simulation of two competing ultrasonic treatment (UST) strategies for microstructure refinement in the direct-chill (DC) casting of aluminium alloys. In the first, more conventional, case, the sonotrode vibrating at 17.3 kHz is immersed in the hop-top to treat the sump melt pool, in the second case, the sonotrode is inserted between baffles in the launder. It is known that microstructure refinement depends on the intensity of acoustic cavitation and the residence time of the treated fluid in the cavitation zone. The geometry, acoustic field intensity, induced flow velocities, and local temperature are factors which affect this treatment. The mathematical model developed in this work couples flow velocity, acoustics modified by cavitation, heat transfer, and solidification at the macroscale, with Lagrangian refiner particles, used to determine: (a) their residence time in the active zones, and (b) their eventual distribution in the sump as a function of the velocity field. This is the first attempt at using particle models as an efficient, though indirect, alternative to microstructure simulation, and the results indicate that UST in the launder, assisted with baffle separators, yields a more uniform distribution of refining particles, avoiding the strong acoustic streaming jet that, otherwise, accompanies hot-top treatment, and may lead to the strong segregation of refining particles. Experiments conducted in parallel to the numerical studies in this work appeared to support the results obtained in the simulation.
Highlights
This is a common approach in the DC casting process [9], a weakness of this approach is that with current implementations, effective treatment is restricted by the active zone being confined to a narrow region under the sonotrode
The addition of partitions, or baffles, to regulate the flow can be used to increase residence time and allow more of the aluminium alloy to be effectively processed. This approach was investigated in pilot scale experiments in the DC casting of an AA6XXX alloy with Zr addition [10], and we showed that grain refinement can be observed when processing is carried out in the launder, primarily driven by the activation/fragmentation of particles, which act as nucleation sites in the solidifying volume
Equations (1)–(3) are solved assuming a uniform initial distribution of 5 μm hydrogen bubbles in a periodic acoustic field operating at a frequency of 17.3 kHz
Summary
If UST is applied during directchill (DC) casting by immersing the mechanical sonotrode directly inside the hot-top, the acoustic jet can cause primary crystals of aluminium to be fragmented. This is a common approach in the DC casting process [9], a weakness of this approach is that with current implementations, effective treatment is restricted by the active zone being confined to a narrow region under the sonotrode. This approach was investigated in pilot scale experiments in the DC casting of an AA6XXX alloy with Zr addition [10], and we showed that grain refinement can be observed when processing is carried out in the launder, primarily driven by the activation/fragmentation of particles, which act as nucleation sites in the solidifying volume. A Lagrangian tracking algorithm is used to predict the distribution of the transported refining particles, and some experimental validation is provided to correlate this with the observed grain refinement from actual billets
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