Abstract
Acoustic streaming and its attendant effects in the sump of a direct-chill (DC) casting process are successfully predicted under ultrasonic treatment for the first time. The proposed numerical model couples acoustic cavitation, fluid flow, heat and species transfer, and solidification to predict the flow pattern, acoustic pressure, and temperature fields in the sump. The model is numerically stable with time steps of the order of 0.01 s and therefore computationally attractive for optimization studies necessitating simulation times of the order of a minute. The sump profile is altered by acoustic streaming, with the slurry region depressed along the centreline of the billet by a strong central jet. The temperature gradient in the transition zone is increased, potentially interfering with grain refinement. The cooling rate in the sump is also altered, thereby modifying the dendrite arm spacing of the as-cast billet. The relative position of the sonotrode affects the sump profile, with the sump depth decreased by around 5 mm when the sonotrode is moved above the graphite ring level by 100 mm. The acoustic streaming jet penetrates into the slurry zone and, as a result, the growth direction of dendritic grains in the off-centre position is altered.
Highlights
Ultrasonic melt treatment is applied to direct-chill (DC) casting for degassing, reducing the macrosegregation level, and refining the grain structure
The model is first run without the sonotrode, but with the same operating conditions, to obtain the initial conditions for the ultrasonic treatment in DC casting (USDC) simulations
The results from this conventional DC casting simulation are compared with the USDC results
Summary
Ultrasonic melt treatment is applied to direct-chill (DC) casting for degassing, reducing the macrosegregation level, and refining the grain structure. Lebon et al [13] used such a model to compute the acoustic pressures in water and aluminium and obtained a good agreement with measured values This set of non-linear equations is computationally expensive to solve and require the solution of ordinary differential equations that describe bubble dynamics in each computational cell. We apply Louisnard’s acoustic streaming model [19] coupled with Trujillo’s non-linear Helmholtz equation [18] to the ultrasonic treatment in DC casting (USDC) of an AA6XXX series aluminium alloy to predict the acoustic streaming pattern in the sump This model extends our previously validated model of acoustic streaming in water [20] by including the heat transfer and species conservation equations and considering the effect of acoustic radiation in non-linear pressure propagation. Results show that the flow effect is weaker for the higher position, resulting in less melt penetration into the slurry
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