Containerless Solidification Of Bi-Ga And Bi-In Alloys Under Acoustic Levitation Conditions

Bi-Ga hypermonotectic and Bi-In eutectic alloys have been successfully processed by a single-axis acoustic levitator. The solidification process under acoustic levitation conditions has been investigated systematically in comparison with static conditions. The main results are obtained as follows.Under static conditions, the second phase (Ga) in the Bi-Ga hypermonotectic alloy has prominent upward floating tendency, leading to serious macrosegeration at low cooling rate. Calculation indicates that Stokes motion is the main influence factor. But under acoustic conditions, the second phase (Ga) has no upward floating tendency and the average size is larger. The second phase (Ga) distributes homogeneously in the matrix (Bi). Analyses indicate that surface vibration and shape oscillation of the levitated droplets may have the effect of preventing Stokes motion, reducing macrosegeration, and other unstable factors promote collision and consolidation, increasing the average size of the second phase. Moreover, the content of Ga, heating temperature and cooling rate can affect significantly the final microstructures. With the increase of undercooling, the average size of the second phase decreases obviously.Based on the experimental observation on the solidification microstructures of Bi-60wt%In hypoeutectic alloy, it can be found that under static conditions, the primary phase BiIn2 is characterized by dendrite structures, but under acoustic levitation conditions, the primary phase is characterized by tip-splitting structures. Analyses indicate that the instability of solid-liquid interface aroused by ultrasound may cause a morphological transition of primary phase BiIn2 from dendrite growth to tip-splitting growth. It has also been found that dendrite breaking has close relationship with tip-splitting structures, which may be the substantial reason resulting in spontaneous grain refinement. Microstructures of Bi-72wt%In hypereutectic alloy indicate that the vibration aroused by ultrasound can break the dendrites of s primary phase and make them coarser. Moreover, the ultrasound tends to increase the nucleation temperature.