Impact of melt convection induced by ultrasonic wave on dendrite growth in Sn–Bi alloys

• Direct observation of microstructural response to ultrasonic vibration was performed.
• Simultaneously with the circulating convection, the dendrite oscillation occurred.
• The dendrite fragmentation was significantly enhanced by the secondary effects.
• The secondary effects have an important role in modifying the microstructure.

The effects of an ultrasonic wave (20 kHz) on the dendritic microstructure of Sn–13 at%Bi alloys were studied using time-resolved in situ X-ray imaging. Sequential images clearly showed the secondary effects such as the ultrasonic streaming and the oscillation with much lower frequency comparing to the ultrasonic vibration frequency. The ultrasonic wave caused a circulating convection, whose domain size was almost the same as the specimen size, and simultaneously periodic convection (40 Hz), which was identified from the longitudinal oscillation of the dendrites. The fragmentation of the primary and secondary dendrite arms in the columnar zone was significantly enhanced under the ultrasonic wave. The secondary effects of the ultrasonic wave play a significant role for modifying the solidification structure.