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