Synergistic effect of ultrasonic melt treatment and fast cooling on the refinement of primary Si in a hypereutectic Al–Si alloy

A mechanism describing the synergistic effect of ultrasonic melt treatment (UST) and subsequent fast cooling on the refinement of primary Si particles in a hypereutectic Al−Si alloy was investigated by examining inoculant particles via high-resolution transmission electron microscopy, serial sectioning, and melt filtration. The application of UST activated non-wetting MgAl₂O₄ particles with diameters of ∼0.5 μm to nucleate the primary Si phase. The cavitation-enhanced wetting of MgAl₂O₄ particles caused the formation of the AlP phase at the MgAl₂O₄ interface, further improving the nucleation potential. The cavitation-enhanced wetting and dispersion of inclusions (such as MgAl₂O₄) also resulted in the refinement and de-agglomeration of AlP particles. The UST-induced changes to the inoculant particles ultimately increased their number density, and the observed effects became more pronounced after increasing the degree of undercooling up to 20 K, leading to enhanced refinement of primary Si particles at higher cooling rates (up to 10² K s⁻¹).