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School of Engineering, Computing and Mathematics
Faculty of Technology, Design and Environment
The current trend in solidification research is to develop a generic, energy-efficient, economical,sustainable, and pollution-free technology that can be applied to different alloy systems.Ultrasonic-cavitation melt treatment (UST) is a rather universal technology that can be appliedto conventional and advanced metallic materials, regardless of their composition, while beingenvironmentally friendly, cost effective, and ready to be implemented in conventional castingtechnologies such as direct-chill, continuous, or shape casting, as well as in emerging technologiesof additive manufacturing and nanocomposite materials.
The beneficial effects of UST—such as in assisted nucleation, activation of substrates (wetting),deagglomeration and fragmentation of solid phases, degassing of the melt, and grain refinement ofthe as-cast product—stem from the growth, collapse, and implosion of cavitation bubbles as a result ofalternate fluctuations in ultrasonic pressure. Although successfully demonstrated on the laboratoryand pilot scale, UST has not yet found widespread industrial implementation. This is mostly dueto the lack of in-depth understanding of the fundamental mechanisms behind the improved metalquality and structure refinement.
Thus, fundamental research is needed to answer the following practical questions: What is theoptimum melt flow rate that maximizes treatment efficiency whilst minimizing input power, cost, andplant complexity? What is the optimum operating frequency and acoustic power that accelerates thetreatment effects? What is the optimum location of an ultrasonic power source in the melt transfersystem in relation to the melt pool geometry? Answering these questions will pave the way forwidespread industrial use of ultrasonic melt processing with the benefit of improving the propertiesof lightweight structural alloys, simultaneously alleviating the present use of polluting (Cl, F) fordegassing or expensive (Zr, Ti, B, Ar) grain refinement additives.
The application of ultrasound and acoustic cavitation in liquid exfoliation of bulk layered materials is a widely used method. However, despite extensive research, the fundamental mechanisms remain far from being fully understood. A number of theories have been proposed to interpret the interactions between cavitation and bulk layered materials and hence to explain the mechanisms of ultrasound assisted exfoliation. Unfortunately, most of the research reported to date is ambiguous or inconclusive due to lack of direct real-time experimental evidence. In this paper, we report systematic work characterising cavitation emissions and observing the exfoliation of graphite in situ, in deionised water under the dynamic interaction with laser and ultrasound induced cavitation bubbles. Using ultra-high-speed optical imaging, we were able to determine the dynamic sequence of graphite exfoliation events on a time scale never reported before. Real-time observations also revealed that shock waves with a pressure magnitude up to 5 MPa and liquid-jets in the range of 80 ms−1, from transient cavitation bubble implosions, were essential for the initiation and propagation of the exfoliation process. On the other hand, bubble oscillations associated with stable cavitation were beneficial for promoting a gentler delamination of graphite layers.
Scaling up ultrasonic cavitation melt treatment (UST) requires effective flow management with minimised energy requirements. To this end, container dimensions leading to the resonance play a crucial role in amplifying pressure amplitude for cavitation. To quantify the importance of resonance length during the treatment of liquid aluminium, we used calibrated high-temperature cavitometers (in the range of 8–400 kHz), to measure and record the acoustic pressure profiles inside the cavitation-induced environment of liquid Al and deionized water (used as an analogue to Al) excited at 19.5 kHz. To achieve a comprehensive map of the acoustic pressure field, measurements were conducted at three different cavitometer positions relative to the vibrating sonotrode probe and for a number of resonant and non-resonant container lengths based on the speed of sound in the treated medium. The results showed that the resonance length affected the pressure magnitude in liquid Al in all cavitometer positions, while water showed no sensitivity to resonance length. An important practical application of UST in aluminium processing concerns grain refinement. For this reason, grain size analysis of UST-treated Al-Cu-Zr-Ti alloy was used as an indicator of the melt treatment efficiency. The result showed that the treatment in a resonance tank of (the wavelength of sound in Al) gave the best structure refinement as compared to other tested lengths. The data given here contribute to the optimisation of the ultrasonic process in continuous casting, by providing an optimum value for the critical compartment (e.g. in a launder of direct-chill casting) dimension.
This study concerns the numerical simulation of two competing ultrasonic treatment (UST) strategies for microstructure refinement in the direct-chill (DC) casting of aluminium alloys. In the first, more conventional, case, the sonotrode vibrating at 17.3 kHz is immersed in the hop-top to treat the sump melt pool, in the second case, the sonotrode is inserted between baffles in the launder. It is known that microstructure refinement depends on the intensity of acoustic cavitation and the residence time of the treated fluid in the cavitation zone. The geometry, acoustic field intensity, induced flow velocities, and local temperature are factors which affect this treatment. The mathematical model developed in this work couples flow velocity, acoustics modified by cavitation, heat transfer, and solidification at the macroscale, with Lagrangian refiner particles, used to determine: (a) their residence time in the active zones, and (b) their eventual distribution in the sump as a function of the velocity field. This is the first attempt at using particle models as an efficient, though indirect, alternative to microstructure simulation, and the results indicate that UST in the launder, assisted with baffle separators, yields a more uniform distribution of refining particles, avoiding the strong acoustic streaming jet that, otherwise, accompanies hot-top treatment, and may lead to the strong segregation of refining particles. Experiments conducted in parallel to the numerical studies in this work appeared to support the results obtained in the simulation.
In this work, we study how ultrasonic cavitation melt treatment (UST) affects the temperature distribution, sump profile, and resulting microstructure in the direct-chill (DC) casting of an AA6008 aluminum alloy. Two 152 mm diameter billets were cast; one was treated with UST (UST-DC casting) in the hot top while the other was not (conventional DC casting). To investigate the temperature distribution, temperature was measured at multiple points in both billets. The sump profile was visualized by pouring Zn into the sump during casting. The microstructure was analyzed by measuring the grain size of as-cast billets. A numerical model of DC casting and UST-DC casting has been validated with the temperature measurements across the billets, and the experimental results agrees well with the numerical model. It is found that the sump profile quantification with thermocouple measurements is more accurate and less prone to interpretation than with Zn tracing. Numerical simulation results show that UST application in the hot top with sonotrode position at 20 mm above the graphite ring level depresses the liquidus isotherm but does not affect the solidus isotherm, resulting in a thinner transition region compared with conventional DC casting. Grain structure analysis verifies that structure refinement with UST has been achieved at the given sonotrode position. The strongest grain refinement was at the center of the billet with the average grain size 50% smaller than that without UST. The results are discussed in terms of the known mechanisms of UST, i.e. dendrite fragmentation and deagglomeration of nucleating substrates.
The application of cavitation-induced shock waves generated at low driving frequencies, known as power ultrasound, is essential for a wide range of fields, such as sonochemistry, lithotripsy, nanomaterials, emulsions and casting, to name but a few. In this paper, we present measurements of the shock wave pressures emitted by cavitating bubbles in water, under ultrasonic excitation produced by an immersed probe oscillating at 24 kHz. A broad-spectrum fibre-optic hydrophone calibrated in the range of 1–30 MHz was used for this purpose. Spectral analysis of the data reveals a consistent resonance peak at a very narrow range of frequencies (3.27–3.43 MHz). Results were confirmed using real-time analysis of high-speed recordings. By eliminating other possible sources, we propose that this new peak might be associated with shock wave emissions from collapsing bubbles. Spatial maps obtained by collating individual shock wave pressures highlight the effect of pressure shielding with increasing input power, attributed to a cloud of bubbles surrounding the probe. This work contributes towards the elucidation of the key properties of cavitation-driven shock waves and the underlying mechanisms, essential in controlling the effectiveness of the external processing conditions on various physical, chemical and biological systems.
The performance augmentation of pairs of vertical axis wind turbines (VAWTs) is known to be dependent on incident wind direction, turbine spacing and direction of rotation. Yet, there is a lack of robust numerical models investigating the impact of these parameters. In this study two-dimensional CFD simulations of an isolated VAWT and of co- and counter-rotating pairs of VAWTs were performed with the aim to determine turbine layouts that can increase the power output of VAWT farms. More than 11,500 h of simulations were conducted at a turbine diameter Reynolds number of 1.35 · 107. A mesh convergence study was conducted, investigating the influence of mesh size, domain size, azimuth increment, number of iterations per time step, and domain cell density. Results showed that mesh size, domain size, and azimuth increment proved to have the biggest impact on the converged results. For the configurations analysed, pairs of VAWTs exhibited a 15% increase in power output compared to operating in isolation, when the second rotor was spaced three turbine diameters downstream and at an angle of 60° to the wind direction. Furthermore, when three turbines were positioned in series, the power output was greater than a pair by an additional 3%.
Ultrasonic emulsification (USE) assisted by cavitation is an effective method to produce emulsion droplets. However, the role of gas bubbles in the USE process still remains unclear. Hence, in the present paper, high-speed camera observations of bubble evolution and emulsion droplets formation in oil and water were used to capture in real-time the emulsification process, while experiments with different gas concentrations were carried out to investigate the effect of gas bubbles on droplet size. The results show that at the interface of oil and water, gas bubbles with a radius larger than the resonance radius collapse and sink into the water phase, inducing (oil–water) blended liquid jets across bubbles to generate oil-in-water-in-oil (O/W/O) and water-in-oil (W/O) droplets in the oil phase and oil-in-water (O/W) droplets in the water phase, respectively. Gas bubbles with a radius smaller than the resonance radius at the interface always move towards the oil phase, accompanied with the generation of water droplets in the oil phase. In the oil phase, gas bubbles, which can attract bubbles nearby the interface, migrate to the interface of oil and water due to acoustic streaming, and generate numerous droplets. As for the gas bubbles in the water phase, those can break neighboring droplets into numerous finer ones during bubble oscillation. With the increase in gas content, more bubbles undergo chaotic oscillation, leading to smaller and more stable emulsion droplets, which explains the beneficial role of gas bubbles in USE. Violently oscillating microbubbles are, therefore, found to be the governing cavitation regime for emulsification process. These results provide new insights to the mechanisms of gas bubbles in oil–water emulsions, which may be useful towards the optimization of USE process in industry.
This work focuses on ultrasonic melt treatment (UST) in a launder upon pilot-scale direct chill (DC) casting of 152-mm-diameter billets from an AA6XXX alloy with Zr addition. Two casting temperatures (650°C and 665°C) were used to assess their effect on the resulting microstructure (grain size, particle size, and number density). Structure refinement results show the feasibility of UST in the DC casting launder. This is quantified through the corresponding reduction of grain size by around 50% in the billet center, or more towards the billet surface, reduction of the average Al3Zr particle size, and increase in the particle number density. A higher Al3Zr particle density was obtained when the alloy was cast at 665°C. Numerical simulation results and suggestions on how to improve the treatment quality of UST in DC casting launder are also provided.
One of the main applications of ultrasonic melt treatment is the grain refinement of aluminium alloys. Among several suggested mechanisms, the fragmentation of primary intermetallics by acoustic cavitation is regarded as very efficient. However, the physical process causing this fragmentation has received little attention and is not yet well understood. In this study, we evaluate the mechanical properties of primary Al3Zr intermetallics by nano-indentation experiments and correlate those with in-situ high-speed imaging (of up to 1 Mfps) of their fragmentation process by laser-induced cavitation (single bubble) and by acoustic cavitation (cloud of bubbles) in water. Intermetallic crystals were chemically extracted from an Al-3 wt% Zr alloy matrix. Mechanical properties such as hardness, elastic modulus and fracture toughness of the extracted intermetallics were determined using a geometrically fixed Berkovich nano-diamond and cube corner indenter, under ambient temperature conditions. The studied crystals were then exposed to the two cavitation conditions mentioned. Results demonstrated for the first time that the governing fragmentation mechanism of the studied intermetallics was due to the emitted shock waves from the collapsing bubbles. The fragmentation caused by a single bubble collapse was found to be almost instantaneous. On the other hand, sono-fragmentation studies revealed that the intermetallic crystal initially underwent low cycle fatigue loading, followed by catastrophic brittle failure due to propagating shock waves. The observed fragmentation mechanism was supported by fracture mechanics and pressure measurements using a calibrated fibre optic hydrophone. Results showed that the acoustic pressures produced from shock wave emissions in the case of a single bubble collapse, and responsible for instantaneous fragmentation of the intermetallics, were in the range of 20–40 MPa. Whereas, the shock pressure generated from the acoustic cavitation cloud collapses surged up to 1.6 MPa inducing fatigue stresses within the crystal leading to eventual fragmentation.
Liquid Phase Exfoliation (LPE) is an efficient method for graphene flake exfoliation and considered to be compatible with industrial production requirements. However, most of available LPE methods require the use of harmful and expensive solvents for chemical exfoliation prior to mechanical dispersion of the flakes, and therefore an additional step is needed to remove the contamination caused by the added chemicals, making the process complex, costly, unsafe and detrimental to the environment.
By studying the effects of key ultrasonic LPE parameters, our study demonstrates the possibility to control the production and quality of few-layer graphene flakes in pure water in a relatively short period of time. The driving frequency of an ultrasonic source, a higher acoustic cavitation intensity and uniform distribution of the cavitation events in the sonicated volume are the key parameters for controlling the thickness, surface area and production yield of few-layer graphene flakes. The results are discussed in the context of mechanical exfoliation. This opens a direction for developing LPE into a cost effective, clean, environmentally friendly, and scalable manufacturing process for the next generation of two-dimensional nanomaterials for industrial-scale applications.
Quantitative understanding of the interactions of ultrasonic waves with liquid and solidifying metals is essential for developing optimal processing strategies for ultrasound processing of metal alloys in the solidification processes. In this research, we used the synchrotron X-ray high-speed imaging facility at Beamline I12 of the Diamond Light Source, UK to study the dynamics of ultrasonic bubbles in a liquid Sn-30wt%Cu alloy. A new method based on the X-ray attenuation for a white X-ray beam was developed to extract quantitative information about the bubble clouds in the chaotic and quasi-static cavitation regions. Statistical analyses were made on the bubble size distribution, and velocity distribution. Such rich statistical data provide more quantitative information about the characteristics of ultrasonic bubble clouds and cavitation in opaque, high-temperature liquid metals.
The prediction of the acoustic pressure field and associated streaming is of paramount importance to ultrasonic melt processing. Hence, the last decade has witnessed the emergence of various numerical models for predicting acoustic pressures and velocity fields in liquid metals subject to ultrasonic excitation at large amplitudes. This paper summarizes recent research, arguably the state of the art, and suggests best practice guidelines in acoustic cavitation modelling as applied to aluminium melts. We also present the remaining challenges that are to be addressed to pave the way for a reliable and complete working numerical package that can assist in scaling up this promising technology.
Acoustic streaming and its attendant effects in the sump of a direct-chill (DC) casting process are successfully predicted under ultrasonic treatment for the first time. The proposed numerical model couples acoustic cavitation, fluid flow, heat and species transfer, and solidification to predict the flow pattern, acoustic pressure, and temperature fields in the sump. The model is numerically stable with time steps of the order of 0.01 s and therefore computationally attractive for optimization studies necessitating simulation times of the order of a minute. The sump profile is altered by acoustic streaming, with the slurry region depressed along the centreline of the billet by a strong central jet. The temperature gradient in the transition zone is increased, potentially interfering with grain refinement. The cooling rate in the sump is also altered, thereby modifying the dendrite arm spacing of the as-cast billet. The relative position of the sonotrode affects the sump profile, with the sump depth decreased by around 5 mm when the sonotrode is moved above the graphite ring level by 100 mm. The acoustic streaming jet penetrates into the slurry zone and, as a result, the growth direction of dendritic grains in the off-centre position is altered.
The acoustic streaming behaviour below an ultrasonic sonotrode in water was predicted by numerical simulation and validated by experimental studies. The flow was calculated by solving the transient Reynolds-Averaged Navier-Stokes equations with a source term representing ultrasonic excitation implemented from the predictions of a nonlinear acoustic model. Comparisons with the measured flow field from Particle Image Velocimetry (PIV) water experiments revealed good agreement in both velocity magnitude and direction at two power settings, supporting the validity of the model for acoustic streaming in the presence of cavitating bubbles. Turbulent features measured by PIV were also recovered by the model. The model was then applied to the technologically important area of ultrasonic treatment of liquid aluminium, to achieve the prediction of acoustic streaming for the very first time that accounts for nonlinear pressure propagation in the presence of acoustic cavitation in the melt. Simulations show a strong dependence of the acoustic streaming flow direction on the cavitating bubble volume fraction, reflecting PIV observations. This has implications for the technological use of ultrasound in liquid metal processing.
Hydraulic components are coated by thermal spraying to protect them against cavitation erosion. These coatings are built up by successive deposition of single splats. The behavior of a single splat under mechanical loading is still very vaguely understood. Yttria-stabilized zirconia (YSZ) and stainless-steel splats were obtained by plasma spraying onto stainless steel substrates. The velocity and temperature of particles upon impact were measured and the samples were subsequently exposed to cavitation erosion tests. An acoustic cavitation simulation estimated the water jet velocity and hammer stresses exerted by bubble collapse on the surface of the specimen. Although the results suggested that high stress levels resulted from cavitation loading, it was clear that weak adhesion interfaces played a crucial role in the accelerated cavitation-induced degradation.
Ultrasonic (cavitation) melt processing attracts considerable interest from both academic and industrial communities as a promising route to provide clean, environment friendly and energy efficient solutions for some of the core issues of the metal casting industry, such as improving melt quality and providing structure refinement. In the last 5 years, the authors undertook an extensive research programme into fundamental mechanisms of cavitation melt processing using state-of-the-art and unique facilities and methodologies. This overview summarises the recent results on the evaluation of acoustic pressure and melt flows in the treated melt, direct observations and quantitative analysis of cavitation in liquid aluminium alloys, in-situ and ex-situ studies of the nucleation, growth and fragmentation of intermetallics, and de-agglomeration of particles. These results provide valuable new insights and knowledge that are essential for upscaling ultrasonic melt processing to industrial level.
The application of ultrasound to the processing of liquids and slurries has a long history. This chapter considers the main mechanisms of ultrasonic processing of metallic alloys as well as principal applications of this technology to processing of liquid metals, casting of alloys and manufacturing of new materials. Some theoretical background is given as well, The text is illustrated with historical and new results including those obtained with most advanced techniques such as high-temperature cavitometry, high-speed in-situ observations and X-ray synchrotron imaging.
This work focuses on the effects of ultrasonic melt treatment (UST) during direct-chill (DC) casting on the temperature distribution across the billet, sump profile, and the resulting microstructure. Two AA6008 billets were cast; one was treated with UST in the hot top while the other was not. To determine the temperature distribution along the billet, multi-point temperature measurements were made across the radii of both billets. The sump profile was also analyzed through macrostructure analysis, after Zn was poured into the sump, while structure refinement was quantified through grain-size measurements. A numerical model of ultrasound-assisted DC casting is validated using the temperature measurements. As an outcome, this study provides information on the extent to which UST affects the sump profile and the corresponding changes in the microstructure. The knowledge gained from this study paves the way towards optimization of UST parameters in DC casting.
Ultrasonic melt treatment (UST) using a single sonotrode source in a launder is an efficient way to treat a large-volume melt. One key parameter is the melt processing temperature. Melt processing temperature affects the acoustic pressure generated by the sonotrode, which ultimately defines the cavitation development as well as the resulting acoustic streaming. Experimental results also show that processing temperature affects intermetallic number density and the final grain size. This work presents a numerical model covering acoustic cavitation, flow (including acoustic streaming), and heat transfer in direct-chill (DC) casting, to better understand this process. The UST effectiveness is quantified through the size of the high-pressure acoustic region and the melt residence time, a result reflected in experimental grain size data. The output of this work is useful for optimizing the selection of process parameters for UST DC casting.
Ultrasonic cavitation melt treatment (UST) of aluminium alloys has received considerable attention in the metal industry due to its simple and effective processing response. The refined primary intermetallic phases formed in the treated alloys during controlled solidification, govern alloy structural and mechanical properties for applications in the automotive and aerospace industries. Since the UST is performed close to the liquidus temperatures of the alloys, understanding the refinement mechanism of the primary intermetallic phases has been beset by difficulties in imaging and handling of liquid metals. In this paper, the sonofragmentation behaviour of primary intermetallic Al3Zr crystals extracted from the matrix of an Al-3 wt% Zr alloy and fixed on a solid substrate was investigated. The intermetallics were exposed to cavitation action in deionized water at 24 kHz of ultrasound frequency. The fragmentation mechanism from the nearby collapsing cavitation bubbles was studied with in-situ high speed imaging. Results revealed that the main fragmentation mechanism is associated with the propagation of shock wave emissions from the collapsing bubble clouds in the vicinity of the crystal. The mechanical properties of the Al3Zr phase determined previously were used for the fracture analysis. It was found that an Al3Zr intermetallic undergoes low cycle fatigue fracture due to the continuous interaction with the shock wave pressure. The magnitude of the resulting shear stress that leads to intermetallic fragmentation was found to be in the range of 0.6 – 1 MPa.
Mechanical properties of primary Al3Zr crystals and their in situ fragmentation behaviour under the influence of a single laser induced cavitation bubble have been investigated using nanoindentation and high-speed imaging techniques, respectively. Linear loading of 10 mN was applied to the intermetallics embedded in the Al matrix using a geometrically well-defined diamond nano-indenter to obtain the mechanical properties at room temperature conditions. Primary Al3Zr crystals were also extracted by dissolving the aluminium matrix of an Al-3wt% Zr alloy. The extracted primary crystals were also subjected to cavitation action in deionized water to image the fracture sequence in real time. Fragmentation of the studied intermetallics was recorded at 500,000 frames per second. Results showed that the intermetallic crystals fail by brittle fracture mode most likely due to the repeatedly-generated shock waves from the collapsing bubbles. The results were interpreted in terms of fracture mechanics using the nanoindentation results.
The current challenge for upscaling the ultrasonic melt processing (USP) technology to industrial scale is in improving the treatment efficiency using a single-sonotrode setup. To achieve this, we suggest two innovative approaches: increasing the melt residence time and exploiting acoustic resonance. This can be achieved through flow management in a launder by partitions where the resonance length within the partitions is equal or at integer steps to the wavelength of the incident sound wave. This study focuses on acoustic pressure measurements at different partition configurations and flow conditions combined with numerical modelling of the process. The measurements are done both in liquid aluminum and in water as its transparent analogue. The acoustic pressure measurements are then used to assess melt treatment improvement through cavitation activity and pressure distribution in the launder as well as to verify and further develop the numerical model.
Abstract. Ultrasonic processing (USP) during direct-chill (DC) casting of light metal alloys istypically applied in the sump of a billet. This approach, though successful for structurerefinement and modification, has two main drawbacks: (a) mixture of mechanisms that relyheavily on dendrite fragmentation and (b) a limited volume that can be processed by a singleultrasonic source. We suggest moving the location of USP from the sump to the launder andapplying it to the melt flow for continuous treatment. The apparent benefits include: (a)degassing of the melt volume, (b) grain refinement through activation of non-metallic inclusions,fragmentation of primary crystals, and deagglomeration of grain refining substrates, and (c) apossibility to use a single ultrasonic source for processing large melt volumes. To optimize thisprocess with regard to the acoustic intensity and melt residence time in the active cavitation zone,flow modification with baffles as well as informed location of the ultrasonic source are required.In this paper, we demonstrate the results of experimental trials where the degassing degree andgrain refinement have been the indicators of the USP efficiency for two aluminium alloys, i.e.LM25 and AA7050. The results are supported by acoustic measurements and computersimulations.
The quantification of acoustic pressures in liquid metals is of paramount interest for the optimization of ultrasonic melt treatment (UST) of large volumes. Until recently, the measurements of acoustic pressure and cavitation intensity in a melt were cumbersome and unreliable due to the high temperatures and the lack of suitable instruments. These difficulties imposed strict limitations on the experimental and numerical investigation of cavitation and bubble dynamics within liquid metals. In recent years, our group used a unique calibrated high temperature cavitometer to measure cavitation activity and acoustic pressures in liquid aluminum. Phenomena such as acoustic attenuation, shielding, and cavitation intensity have been studied. These measurements were also used to validate a non-linear acoustic numerical model applicable to flow in bubbly liquids subject to acoustic cavitation. Both experimental and numerical characterization of the acoustic and flow fields provides a powerful tool to optimize cavitation processing in liquid metals.
This paper presents an investigation of the evolution of flow structures and cavitation intensity in water as an analogue for a liquid metal under ultrasonic excitation. Results are presented for 20 kHz high-power ultrasound. The input power ranged from 50% (8.5 μm p-p) to 100% (17 μm p-p). To identify the streaming structures and understand the recirculation flows for different vibrational amplitudes of the sonotrode, particle image velocimetry (PIV) measured the velocity field. Simultaneously, a calibrated cavitometer probe measured acoustic intensity in the fluid. The cavitation intensity away from the acoustic source decreased with increasing input acoustic power, but was relatively constant inside the cavitation zone (irrespective of the input power). PIV measurements showed that the direction of the flow pattern was strongly related to the vibrational amplitude of the sonotrode. These results are compared with the predictions of an acoustic cavitation model. The outcome of the present work will help to determine the efficient optimization of ultrasonic processing of liquid metals that is of increasing technological importance.
Ultrasonic (cavitation) melt processing is attracting considerable interest from both academia and industry as a promising technological route to provide clean, environment-friendly, and energy efficient solutions for some of the core issues of the metal casting industry, such as improved melt quality and structure refinement. Upscaling of this technology to industrial level is still hindered by insufficient understanding of the underlying mechanisms. In the last 5 years, we undertook an extensive programme of fundamental research into the mechanisms of cavitation melt processing using state-of-the-art techniques and facilities, including calibrated high-temperature cavitometry, high-speed filming, particle-image velocimetry, synchrotron X-rays imaging, and advanced electron microscopy. This paper gives an overview of recent results on measuring the acoustic pressure and melt flows in the treated melt, real-time observations of cavitation in liquid aluminium alloys, and in-situ and ex-situ studies of the nucleation and fragmentation of intermetallics.
Ultrasonic melt processing is a promising technique for microstructural refinement in castings. Several mechanisms have been proposed for the observed effects, including cavitation-induced nucleation, activation of substrates and fragmentation. Until now, however, real-time experimental observations which could clarify any of the above mechanisms are very limited. For the first time we directly observed the fragmentation of primary crystals formed in alloys by ultrasonic cavitation. The primary crystals were extracted from real Al alloys and subjected to ultrasonic processing in water with in situ high-speed filming. The recordings of fragmentation of the primary crystals allowed us to observe the different mechanisms of fragmentation, depending on the mechanical properties and morphology of the primary crystals. The collapse of cavitation bubbles in water is less violent than that in liquid aluminum due to the lower cavitation threshold, viscosity and surface tension. Therefore the fragmentation mechanisms for the primary crystals observed in water should also be present for the same primary crystals in the more violent cavitation situation in liquid aluminum.
The refining effect of ultrasonic melt processing on primary Al3Ti particles was investigated by applying ultrasound to an Al0.4 wt% Ti alloy over two temperature ranges: 810 to 770 oC (above liquidus) and 730 to 690 oC (below liquidus). In both cases, significant refinement of primary Al3Ti particles was observed. Based on the examination of the quenched samples taken before and after ultrasonication, it was proposed that cavitation-enhanced nucleation on indigenous particles plays the dominant role when the ultrasonication is applied above the liquidus, while cavitation-induced fragmentation prevails when the ultrasonication is carried out below the liquidus. Using scanning electron microscopy, energy dispersive X-ray spectroscopy, and transmission electron microscopy, the nucleating sites for the primary Al3Ti were identified as α-Al2O3particles. The refinement observed after ultrasonication above the liquidus is hence attributed to cavitation-enhanced nucleation on α-Al2O3 particles. In supplementary room temperature experiments, the cavitation-induced fragmentation of primary Al3Ti intermetallics was studied in-situ by high-speed imaging of the deeply-etched alloy samples immersed in distilled water. The dynamic interaction between the cavitation bubbles and primary Al3Ti particles was imaged in real time. A fragmentation process caused by oscillating bubble continuously interacting with a primary Al3Ti particle was clearly observed.