Effect of Ultrasonic Melt Treatment on Solidification Microstructure of Al–5Ti–1B Alloy Containing Numerous Inoculant Particles

The effect of ultrasonic melt treatment (UST) on the solidification microstructure of an Al–5Ti–1B alloy containing high-volume fractions of Al₃Ti and TiB₂ particles is investigated for various UST times with different melt holding times. The as-cast Al–5Ti–1B alloy is composed of TiB₂ and polygonal Al₃Ti particles (present prior to UST), plate-like Al₃Ti particles, and Al grains (formed during UST and/or solidification). The UST causes a size reduction and homogeneous distribution of the TiB₂-agglomerated region containing many submicron-sized TiB₂ particles pushed to the grain boundaries. The UST slightly decreases the size and improves the distribution of polygonal Al₃Ti particles enriched in the TiB₂-agglomerated region. Unlike the TiB₂ and polygonal Al₃Ti particles, which exhibit a minor refining effect, the plate-like Al₃Ti particles show a significant refinement with UST application. The UST has a significant effect on the size distribution of Al grains by inducing the formation of medium-sized grains at the expense of small and large grains; however, it only has a slight effect on grain refinement. The degree of microstructure modification increases with increasing UST time but decreases with melt holding time after UST. The mechanisms for the refinement and dispersion of the TiB₂ and Al₃Ti particles and Al grains are discussed considering fragmentation, nucleation, and growth behaviors induced by the UST and subsequent solidification. Graphic abstract: [Figure not available: see fulltext.].