Numerical and Experimental Investigation of Ultrasonic-Assisted Semi-continuous Casting for Microstructure Homogenization in Large-Scale Al–Cu Alloy Cylindrical Hollow Billets
摘要
During the semi-continuous casting process of large-diameter Al–Cu alloy cylindrical hollow billets, issues such as coarse grains, eutectic phase segregation, and solute macro-segregation are highly prone to occur due to the massive melt volume and complex double-sided cooling conditions, which restricts their application in high-end aerospace components. In this paper, a dual-source ultrasonic-assisted casting strategy optimized by numerical simulation is proposed to improve the solidification microstructure uniformity of large-diameter cylindrical hollow billets with an outside diameter of 800 mm and an inside diameter of 400 mm. First, a 3D thermal-fluid numerical model coupling the cavitation model and the acoustic streaming source term was established to systematically reveal the influence of ultrasonic power, insertion depth, and angle of the sonotrode on the acoustic pressure field, flow field, and sump profile of the melt. The simulation results indicate that adopting a power of 350 W combined with a 45° inclined insertion method can induce the optimal acoustic streaming stirring effect, effectively weakening natural convection and significantly reducing the sump depth. Based on the process parameters optimized by numerical simulation, industrial-scale casting experiments were conducted. The results verify that under the optimized ultrasonic treatment process, the average grain size of the billet is reduced from 238 μm in conventional casting to 111 μm, achieving a refinement rate of 53.4%. The coarse network-like Al2Cu eutectic phase is broken into fine dispersed particles, with its area fraction decreased from 11.5 to 5.6%. Furthermore, the segregation of the Cu element is effectively suppressed, and the maximum circumferential positive segregation rate is controlled within 4.4%. The synergistic effect of the strong shear acoustic streaming and cavitation effect induced by ultrasound not only effectively fragments the primary dendrites and the network-like Al2Cu eutectic phase but also promotes the convection of solutes at the solidification front, thereby achieving a high degree of homogenization in both the microstructure and composition across the full cross-section of the large-diameter hollow billet.