Longitudinal ultrasound modulated interface microstructure evolution for directionally solidifying Al-Cu-Si ternary alloy

Longitudinal ultrasound with 20 kHz frequency and 0.5-3.5 μm amplitude was introduced into the directional solidification process of ternary Al-4.5%Cu-4.5%Si alloy under the processing conditions of 200 K·cm -1 temperature gradient and 0.1-20 μm·s -1 growth velocity. The intensive ultrasound was found to elevate solid/liquid interface temperature, reduce liquid temperature gradient and concentrate cavitation zone to the 20 mm range near solidification front. These dynamic effects stimulated the instability of planar interface and promoted the cell-to-dendrite (CDT) transition, resulting in a special ultrasound-driven instability regime within microstructure selection map. This ultrasonic modulation proved more potent for dendritic growth than cellular growth and facilitated the columnar-to-equiaxed (CET) transition. Moreover, the mushy zone length, intercellular spacing and dendritic arm spacings always increased with the rise of ultrasonic amplitude.