A comparative study of ternary Al–Sn–Cu immiscible alloys prepared by conventional casting and casting under high-intensity ultrasonic irradiation

Highlights: ► Systematic investigation on the solidification microstructure of ternary Al–Sn–Cu immiscible system aided by computational thermodynamics calculations. ► Comparative study of conventional casting and casting under high-intensity ultrasonic irradiation. ► Demonstrated the high effectiveness of ultrasound exposure during solidification. ► Effect of cavitation on nucleation and the relative effects of cavitation and acoustic streaming on the dispersion of Sn-rich liquid phases have been explained from the experimental observation. Cavitation was found to promote fragmentation and dispersion of Sn-rich liquid leading to homogeneous dispersion of refined Sn phase. Microstructural modification was found to be contributed by cavitation and associated shockwave generation while bulk fluid flow under acoustic streaming was found to be less effective on the microstructure evolution. ► Globular and highly refined α-Al formed near the radiator through enhanced heterogeneous nucleation in contrast to dendritic α-Al observed in conventional solidification. - Abstract: A comparative study on the microstructure of four ternary Al–Sn–Cu immiscible alloys, guided by the recent thermodynamic assessment of the system, was carried out with specific focus on the soft Sn particulate distribution in hard Al-rich matrix in the presence and absence of ultrasonic irradiation during solidification. The results clearly demonstrate high effectiveness of ultrasonication in promoting significantly refined and homogeneously dispersed microstructure, probably aided by enhanced nucleation and droplet fragmentation under cavitation. While conventional solidification produced highly segregated Sn phase at the centre and bottom of Sn-rich alloy ingots, ultrasonic treatment produced effective dispersion irrespective of the alloy constitution in the region around the radiator. At distance >20 mm from the radiator, presumably outside the region of active cavitation, any bulk fluid flow from acoustic streaming appears to be less efficient in preventing segregation of liquids even though still more effective than quiescent solidification conditions.