Mechanisms of ultrasonic de-agglomeration of oxides through in-situ high-speed observations and acoustic measurements

Abhinav Priyadarshi,Mohammad Khavari,Tungky Subroto,Paul Prentice,Koulis Pericleous,Dmitry Eskin,John Durodola,Iakovos Tzanakis

doi:10.1016/j.ultsonch.2021.105792

Abstract

Ultrasonic de-agglomeration and dispersion of oxides is important for a range of applications. In particular, in liquid metal, this is one of the ways to produce metal-matrix composites reinforced with micron and nano sized particles. The associated mechanism through which the de-agglomeration occurs has, however, only been conceptualized theoretically and not yet been validated with experimental observations. In this paper, the influence of ultrasonic cavitation on SiO2 and MgO agglomerates (commonly found in lightweight alloys as reinforcements) with individual particle sizes ranging between 0.5 and 10 μm was observed for the first time in-situ using high-speed imaging. Owing to the opacity of liquid metals, a de-agglomeration imaging experiment was carried out in de-ionised water with sequences captured at frame rates up to 50 kfps. In-situ observations were further accompanied by synchronised acoustic measurements using an advanced calibrated cavitometer, to reveal the effect of pressure amplitude arising from oscillating microbubbles on oxide de-agglomeration. Results showed that ultrasound-induced microbubble clusters pulsating chaotically, were predominantly responsible for the breakage and dispersion of oxide agglomerates. Such oscillating cavitation clusters were seen to capture the floating agglomerates resulting in their immediate disintegration. De-agglomeration of oxides occurred from both the surface and within the bulk of the aggregate. Microbubble clusters oscillating with associated emission frequencies at the subharmonic, 1st harmonic and low order ultra-harmonics of the driving frequency were deemed responsible for the breakage of the agglomerates.

Highlights

Ultrasonic treatment (UST) of metallic melts favourably impacts material quality through grain refinement, particle mixing and, disper sion, cluster fragmentation, and degassing [1,2]
Note that this is important for the metalmatrix composites and for “conventional” grain refinement that is based on the distribution of secondary phases or activated substrates, which is facilitated by de-agglomeration/dispersion
As the agglomerates were uplifted from the base, some of them moved towards the ultrasonic source and de-agglomerated instantly upon interacting with the strong cavitation cloud formed below the tip, generating a fine suspension in this region (Fig. 3e)

Summary

Introduction

Ultrasonic treatment (UST) of metallic melts favourably impacts material quality through grain refinement, particle mixing and, disper sion, cluster fragmentation, and degassing [1,2] This applies to liquid metal processing with added nanoparticles/powders or grain refining agents used to produce metal matrix composite materials [3,4]. Owing to large surface tension, poor wettability, oxidation and hydrogen adsorption, particles added externally, or indigenous oxides, often form agglomer ates with absorbed hydrogen on their surface [1] These agglom erated particles cannot be wetted by the surrounding melt and are difficult to disperse and distribute uniformly within the liquid metal matrix. De-agglomeration using ultrasonic effects can be important in other applications ranging from pharmaceuticals to food industry [15]

Methods

Results

Conclusion