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

Zhiguo Zhang,Chuangnan Wang,Billy Koe,Christian M Schlepütz,Sarah Irvine,Jiawei Mi

doi:10.1016/j.actamat.2021.116796

Abstract

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.

Highlights

Ultrasound melt processing (USMP) is a physical field-based method for degassing in liquid metals and refining solidification microstructures [1,2]
Compared to the conventional methods routinely used in metal industry, for example, degassing of Al alloy melts using inert gases, and grain refinement using external grain refiners, the unique advantages of USMP are: (1) it is generic and applicable to virtually all alloy systems regardless of the alloy chemistry; (2) it is environmentally friendly and operationally efficient without the need of adding external chemicals into the processed melt [2,3]
Since the late 1990s, a number of the 3rd generation synchrotron X-ray facilities around the world have developed highspeed radiography and tomography capabilities that allow materials scientists and engineers to study in real time and in situ the dynamics of microstructure evolution in metal solidification processes

Summary

Introduction

Ultrasound melt processing (USMP) is a physical field-based method for degassing in liquid metals and refining solidification microstructures [1,2]. There has been a worldwide, renewed interest in the materials manufacturing and metallurgy sector to further develop and scale up ultrasound-based liquid metal processing technologies [3]. This motivates the uses of in situ and real time characterization techniques to study the dynamics in liquid melt under USMP. Since the late 1990s, a number of the 3rd generation synchrotron X-ray facilities around the world have developed highspeed radiography and tomography capabilities that allow materials scientists and engineers to study in real time and in situ the dynamics of microstructure evolution in metal solidification processes. The image acquisition speeds used in the above studies were in the range of 1 to 63 frames per second (fps), unable to reveal the true dynamic behaviour of ultrasonic bubbles and acoustic streaming flow in liquid metals

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Discussion

Conclusion