In Situ Synchrotron X-ray Study of Ultrasound Cavitation and Its Effect on Solidification Microstructures

Abstract

Considerable progress has been made in studying the mechanism and effectiveness of using ultrasound waves to manipulate the solidification microstructures of metallic alloys. However, uncertainties remain in both the underlying physics of how microstructures evolve under ultrasonic waves, and the best technological approach to control the final microstructures and properties. We used the ultrafast synchrotron X-ray phase contrast imaging facility housed at the Advanced Photon Source, Argonne National Laboratory, US to study in situ the highly transient and dynamic interactions between the liquid metal and ultrasonic waves/bubbles. The dynamics of ultrasonic bubbles in liquid metal and their interactions with the solidifying phases in a transparent alloy were captured in situ. The experiments were complemented by the simulations of the acoustic pressure field, the pulsing of the bubbles, and the associated forces acting onto the solidifying dendrites. The study provides more quantitative understanding on how ultrasonic waves/bubbles influence the growth of dendritic grains and promote the grain multiplication effect for grain refinement. have been widely used in industry, e.g., in ultrasonic cleaning, sonochemistry, and medical treatment. In the past few decades, extensive laboratory results have demonstrated that applying ultrasound waves into solidifying liquid alloys can lead to the refinement of alloy microstructures. [1][2][3] Although ultrasound-induced grain refinement has been shown effective in many metallic alloy systems, almost all previous research have interpreted the mechanism of grain refinement based on post-mortem microstructural characterization of the solidified alloys and empirical correlation, if any, between the measured grain size and the input ultrasonic power. [4,5] A very recent high-speed imaging study of ultrasonic treatment of a solidifying organic transparent alloy revealed that the shock wave emitted from imploding bubbles can fracture the growing dendrites, [6] increasing the grain multiplication effect that leads to the enhancement of grain refinement. However, in situ and real time studies of the fundamentals of how the highly dynamic ultrasonic waves and the ultrasonic bubbles interact with the liquid metal, the semisolid and solid phases nucleated during solidification have not been reported mainly due to the difficulties in studying the bubble dynamics in the opaque liquid metal. In this paper, we report a number of in situ imaging studies of the dynamic behavior of ultrasonic bubbles in a Sn-13 wt pct Bi and a Bi-8 wt pct Zn alloy using the ultrafast X-ray phase contrast imaging (PCI) facility housed at the Advanced Photon Source (APS), Argonne National Laboratory, US. The real-time imaging studies are complemented by a numerical simulation of the bubble dynamics using the classical Gilmore model. [7] The experiments were carried out at the beamline 32-ID-B of APS, and the detailed description of the experiment can be found in References 8, 9. The undulator gap was set to 14 ...

Evolution of the As‐Cast Grain Microstructure of an Ultrasonically Treated Al–2Cu Alloy

“…One is the inoculation of nucleants via master alloys and through modifying the alloying chemistry . The other is the application of external fields by physical means . One of these is ultrasonic vibration or what will henceforth be called Ultrasonic Treatment (UST).…”

Dynamics of two interacting hydrogen bubbles in liquid aluminum under the influence of a strong acoustic field

“…Treating liquid metals near their liquidus temperature with ultrasound is known to signicantly improve the functional quality and properties of the processed metallic materials [14]: benecial eects of the ultrasonic treatment include degassing of dissolved gases, improved wetting and/or the activation of inclusions by cleaning the solid-liquid interface, enhancing nucleation, and rening the grain structure of the solidied sample [1,5]. These improvements are attributed to acoustic cavitation [6].…”

“…Mutual interaction aects coalescence, formation of bubble streamers and clouds, and shielding of the sonotrode source. Therefore, a more complete understanding of ultrasonic treatment in liquid metal processing and of the dierent bubble structures observed by X-ray tomogra- * G.S.B.Lebon@greenwich.ac.uk phy [12] and radiography [13,14] in aluminum alloys requires the modeling of bubble interaction.…”

“…Large values of pressures, of the order of MPa, are registered a few millimeters away from the cavitating bubbles. These shock waves are a possible cause for the disruption of the growth of the solidication front (including the rupture of secondary dendrite arms [14]) and the random occurrence of cavitation implosions contributing to the grain rening process of the treated melt, in addition to the dominant eect of the cavitation-induced multiplication of nucleation sites [1]. Each pressure signal attributed to unstable cavitating bubbles contains a strong component of the forcing frequency (17.7 kHz) and only minute traces of harmonics and broadband noise.…”

Effects of ultrasonic vibration on the microstructure and mechanical properties of high alloying TiAl

“…In the cavitation-enhanced nucleation theory, the ultrasonic treatment could enlarge the melt supercooling and activate the inclusion, and the grain refinement is contributed by the enhanced heterogeneous nucleation6. In the cavitation-induced dendrite fragmentation mechanism, the ultrasonic treatment could generate powerful shock wave and jet stream, which will break the dendrite leading to the grain multiplication, and the fine grains are resulted from the dendrite fragmentation7.…”