Influence of ultrasonic cavitation treatment on fragmentation and de-agglomeration of primary intermetallics and oxides formed in Al alloys

The utilisation of ultrasound in material processing, whether it is machining, welding, cleaning, mixing, disintegration, degassing, atomisation, emulsification, crystallisation and/or for accelerating diffusion in solid and liquid phases, has always mesmerised the scientific community with the remarkable effects that it produces during its application in fields ranging from automotive, aerospace, military, construction to even medical (therapeutics) and food sector. One such prevalent and industrially accepted processing technique is ultrasonic melt treatment (UST) which aids in improving the mechanical properties of metals and alloys by influencing their final microstructure. UST effectively promotes grain refinement by utilising the phenomena of acoustic cavitation that leads to fragmentation/de-agglomeration of solid phases, such as primary intemetallics and oxides, formed during solidification of as-cast alloys. Knowledge of the governing mechanisms associated with fragmentation and de-agglomeration of solid phases is prerequisite and instrumental for upscaling of UST in order to realise its substantial economical and sustainability benefits. At present, there is a dearth of fundamental understanding of the process mechanism by which these solid phases found typically within the metallic melts fragment/de-agglomerate and gets refined during UST. Moreover, the output and yield of UST is primarily dependent on the optimisation of operating parameters such as sonication frequency, treatment time, amplitude and processing conditions i.e. temperature, residence time within plus the extent of the cavitation zone. Huge efforts have been made to systematically comprehend and characterise in real-time the in-situ cavitation dynamics occurring during ultrasonic processing of liquid metals through the use of advanced synchrotron X-ray facilities. Nonetheless, the use of such approach is frequently afflicted by complications associated with the opacity and handling of liquid metals, spatial and temporal resolution limitations and narrow field of view available for capturing and recording the multi-phase interactions during a continuously growing solid phase during imaging. To overcome these non-negligible restraints, use of transparent liquid systems becomes necessary to decode the fragmentation and de-agglomeration process of intermetallics and oxides. Over the years, water has been recognised as a suitable analogue to mimic molten aluminium showing similar acoustic cavitation behaviour under ultrasonic excitation. This dissertation presents a novel approach that aims to investigate the interaction dynamics of cavitating bubbles with the fixed and floating Al3Zr primary intermetallics and oxides such as silica and magnesia in water medium at room temperature conditions. In this work, we have attempted to elucidate in detail the mechanisms of ultrasound-induced fragmentation and de-agglomeration of primary and secondary solid phases. Prior to conducting such studies, depth sensing indentation experiments were performed to measure and evaluate the mechanical properties of such intermetallic crystals that served as a tool for understanding the complexity of the overall process. Results disclosed that sono-fragmentation of intermetallics is primarily governed by the propagating shock waves emanating from inertial cavitation cloud collapses. Intermetallic fracture then occurs via low cycle fatigue loading, followed by catastrophic brittle failure. In contrast, de-agglomeration of oxide clusters is primarily governed by the sono-capillary effect and facilitated by the cavitation bubble activity and induced-acoustic streaming leading to its breakage and erosion. Having recognised the crucial role of shock waves in fragmentation of intermetallic crystals, we also explored the detailed characteristic of acoustic emissions produced during ultrasonic cavitation in water and liquid aluminium by using state-of-the art advanced cavitation sensors calibrated from kHz to MHz broadband frequency range. The results obtained from these fundamental work motivated us to carry out pilot-scale experiments to assess the acoustic cavitation response in a continuous melt flow system by implementing UST in a conventional direct-chill casting setup. The casting trials demonstrated the positive impact of cavitation melt treatment on the grain refinement characteristics of end cast billet/ingots. The promising results from both fundamental and real-scale experiments are currently being utilised to develop numerical simulation tools to improve the output and yield of UST.

Grantor dc:publisher

Oxford Brookes University

Year dc:date

2022

Author dc:creator

• Priyadarshi, Abhinav

Contributors dc:contributor

• Tzanakis, Iakovos

Statement dc:rights

• All rights reserved

Language dc:language

en

DOI dc:identifier

https://doi.org/10.24384/qn73-1721

OAI identifier oai:identifier

tle:8307a6fe-5e4b-4bc8-b7b0-2f14d85f67c1:d6bd9758-527a-46cd-bfe2-c433766e8fca:1