Synchrotron X-ray imaging and ultrafast tomography in situ study of the fragmentation and growth dynamics of dendritic microstructures in solidification under ultrasound

High speed synchrotron X-ray imaging and ultrafast tomography were used to study in situ real time the fragmentation growth dynamics of dendritic microstructures an Al-15%Cu alloy solidification under ultrasound. An ultrasound 30 kHz with vibration amplitude 29 µm was applied into melt produced a strong swirling acoustic flow ~0.3 m/s. Efficient dendrite occurred due dominant mechanism is thermal perturbation remelting plus mechanical fracture separation effect. Acoustic fatigue impact phase collision effects found play minor role causing fragmentation. Just 10 s application at early stage ~100% more fragments compared case without ultrasound, resulting 20~25% reduction average grain size solidified samples. Furthermore, morphology tip velocity mainly affected by initial fragment number density their distribution. The systematic real-time datasets obtained near operando conditions provided valuable 4D information for validation numerical models assistance developing optimisation strategy processing industry.