Effect of Ultrasonication on the Solidification Microstructure in Al and Mg-Alloys

Solidification microstructures formed under high-intensity ultrasonication are systematically investigated in various Al and Mg-alloys. The range of microstructures examined include primary grain structure, intermetallic compounds (IMCs), regular and irregular eutectics. Extensive grain refinement is observed in all alloys, especially in the areas of cavitation, further facilitated by the presence of solute. Heat treatability of the ultrasonicated alloys is enhanced as the fine-grained microstructure reduced diffusion distance. Regular lamellar eutectic microstructure is observed to degenerate under cavitation. Irregular eutectic and IMCs are modified from coarse plates or complex morphology into compact polygonal shape. Result suggests that ultrasonication increases nucleating particles and potency of nucleants. The modification of eutectic and IMCs predominantly occurs through coarsening and spheroidisation in the area of intense cavitation. While grain refinement effect is noticed in the entire ingots assisted by acoustic streaming, eutectic modification effect dissipates rapidly outside the area of cavitation.