Multiphysics Modelling of Ultrasonic Melt Treatment in the Hot-Top and Launder during Direct-Chill Casting: Path to Indirect Microstructure Simulation

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.

In-situ observations and acoustic measurements upon fragmentation of free-floating intermetallics under ultrasonic cavitation in water

“…4 a (vii). This is in very good agreement with recent investigation by Beckwith et al [3] , where it was shown that the effective residence time in a real case scenario of UST performed in direct-chill casting launder was close to 100 ms meaning that every fragment should essentially recirculate almost 50 times before moving downstream towards a hot top mould. Therefore, the synergy of strong collapses inside the cavitation zone and adjacent to the sonotrode-tip, in conjunction with the rapid collision of fragments with the bubble cloud may lead to crystal breakage after few acoustic cycles.…”

“…The majority of the fragments of sizes<100 μm produced after 9 sec of treatment, were more likely to be the result of delamination and breakage of the fractured crystal surface, as observed in Fig. 6 f. The results indicate that the effective treatment time of intermetallics within the active fragmentation zone (defined in section 3.1) in water medium should be in the range of 10–15 sec which coincides well with the time required for efficient intermetallic fragmentation upon casting of Al alloys [3] , [33] . It is important to note here that in a real casting processes, the choice of effective treatment time will also depend on the process scale dimensions and various other important factors such as casting speed, horn dimension etc.…”

“…The final step in this study was to identify the statistical distribution of the fragments based on the sonication time, which is important to control the treatment duration within the cavitation zone and optimise flow management techniques for continuous casting as recently discussed in [3] , [42] .…”

“…Ultrasonic melt treatment (UST) has been found very beneficial in this respect, by improving the properties of the as-cast alloys through grain refinement, melt degassing, and improved structure uniformity. UST combines with conventional solidification processes such as direct-chill (DC) casting to offer an environmentally friendly and cost effective treatment [1] , [2] , [3] . Although UST is a well-established technique in processing melts [1] , [4] , the exact mechanisms of grain refinement are still under scrutiny.…”

Comparison of Frequency Domain and Time Domain Methods for the Numerical Simulation of Contactless Ultrasonic Cavitation

“…At the point of collapse, extreme temperatures [3] and pressures [4] occur. Traditionally, UST experiments induce sound waves using an immersed mechanical sonotrode operating at low ultrasonic frequencies (17)(18)(19)(20). This technique induces strong pressure waves, which are nevertheless attenuated rapidly away from the sonotrode tip due to intense inertial cavitation and gas shielding.…”