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Aiming at improving the efficiency of cavitation treatment, this work investigates characteristics of acoustic streaming and cavitation generated in water by dumbbell-shaped sonotrodes with plane, truncated and conical tips. The main emphasis was placed on elucidating the effects of tip shape and vibration amplitude ranged from 40 to 60  μ m . The PIV technique and Weissler reaction were used to measure flow pattern and velocity of acoustic streaming, and cavitation efficiency, respectively. To provide a theoretical explanation to the experimental results, a self-developed mathematical model was used to simulate the acoustic streaming and predict the size of cavitation zone numerically. Both the experimental and numerical results revealed that the sonotrode tip shape affects the acoustic streaming significantly, altering the flow magnitude and direction from fast and downward under the plane and truncated tips to relatively slow and upward near the conical tip. Besides, the conical tip provides a more efficient cavitation treatment in comparison with the plane and truncated tips. The simulation results showed that widening of cavitation zone and altering of acoustic streaming velocity and direction near the sonotrode tip are responsible for the enhancement of cavitation treatment efficiency. [ABSTRACT FROM AUTHOR]

It is well known that solidification of molten metals occurs under the influence of transient unsteady phenomena, which are difficult to control in actual casting operation. Examples include buoyancy flows, natural heat convection, repeated remelting and solidification in mushy zone and solid shell at the mold interface. This paper presents the results suggesting that ultrasound waves, irradiated into the hot-top mold offers an attractive way to control the transient phenomena in DC casting of aluminum alloy billets. A novel mathematical model was developed to simulate the DC casting with considering transient melt flow, heat transfer, ultrasound propagation, acoustic streaming and solidification. It was found that ultrasound irradiation alters the solidification behavior of melt and sump evolution especially in the earlier stage of casting, however as the billet length increases, the sump profile becomes almost the same for both the ultrasonic and conventional castings. In this condition, the casting speed becomes the key parameter influencing the sump evolution and mushy zone volume. The results reveal that ultrasound-driven acoustic streaming and turbulent oscillations result in a suppression of buoyancy flow in the sump and lead to an increase of frequency of the melt temperature oscillations at the solidification interface. One of the useful results of these phenomena can be a more frequent repetition of remelting/solidification cycle and improvement of the billet quality, particularly the billet surface morphology. The effect of ultrasound irradiation on the billet morphology was verified experimentally using a pilot DC caster. [ABSTRACT FROM AUTHOR]

• Cavitation bubbles implode causing fine droplets of bismuth to form emulsion. • Acoustic streaming transfers bismuth droplets throughout the melt. • After solidification, 90% of bismuth particles have size under 10 m. • Most bismuth particles remain embedded inside aluminum grains. The aim of this study is to develop an ultrasound-assisted process for fabrication of frozen emulsion alloys, which are a type of miscibility gap alloys (MGAs). Al-Bi system was used to produce 2-kg ingots of Al-Bi frozen emulsion alloys in the following two steps: (1) emulsification treatment of Al-Bi melt by introducing high-intense ultrasound vibrations into it in a temperature range of Al-Bi binodal decomposition, and (2) casting of the melt into a book mold. The ingot samples were analyzed with SEM/EDX for the alloy microstructure and bismuth particle distribution. Besides, cold model experiments, thermodynamic and CFD numerical simulations were performed to elucidate the underlying mechanisms of emulsification including droplet fragmentation and acoustic streaming. The results reveal that the alloys with Bi concentration of 4%~9.4% can be fabricated by the proposed technique. Within this range, 90% of Bi particles have a diameter under 10 µm, and the majority of them are incorporated into aluminum grains. Especially, a uniform microstructure is obtained when concentration of Bi is less than 6%. Emulsification was found to occur at the interface between two liquids when cavitation bubbles collapse in the interface vicinity followed by high-velocity micro-jetting which is responsible for the emulsion droplet formation. Another positive effect of ultrasound is acoustic streaming which is responsible for the effective agitation of molten bath preventing the sedimentation of Bi droplets during the ultrasound treatment and casting operation. [ABSTRACT FROM AUTHOR]

• Acoustic streaming velocity in aluminum melt and water was measured through Karman vortex frequency measurement. • The velocity of acoustic streaming is independent of amplitude of horn tip oscillation both in water and aluminum melt. • The velocity of acoustic streaming in aluminum melt is approximately the same as that in water. It is well known that ultrasonic cavitation causes a steady flow termed acoustic streaming. In the present study, the velocity of acoustic streaming in water and molten aluminum is measured. The method is based on the measurement of oscillation frequency of Karman vortices around a cylinder immersed into liquid. For the case of acoustic streaming in molten metal, such measurements were performed for the first time. Four types of experiments were conducted in the present study: (1) Particle Image Velocimetry (PIV) measurement in a water bath to measure the acoustic streaming velocity visually, (2) frequency measurement of Karman vortices generated around a cylinder in water, and (3) in aluminum melt, and (4) cavitation intensity measurements in molten aluminum. Based on the measurement results (1) and (2), the Strouhal number for acoustic streaming was determined. Then, using the same Strouhal number and measuring oscillation frequency of Karman vortices in aluminum melt, the acoustic streaming velocity was measured. The velocity of acoustic streaming was found to be independent of amplitude of sonotrode tip oscillation both in water and aluminum melt. This can be explained by the effect of acoustic shielding and liquid density. [ABSTRACT FROM AUTHOR]

The present work investigated melt flow pattern and temperature distribution in the sump of aluminum billets produced in a hot-top equipped direct chilling (DC) caster conventionally and with ultrasonic irradiation. The main emphasis was placed on clarifying the effects of acoustic streaming and hot-top unit type. Acoustic streaming characteristics were investigated first by using the earlier developed numerical model and water model experiments. Then, the acoustic streaming model was applied to develop a numerical code capable of simulating unsteady flow phenomena in the sump during the DC casting process. The results revealed that the introduction of ultrasonic vibrations into the melt in the hot-top unit had little or no effect on the temperature distribution and sump profile, but had a considerable effect on the melt flow pattern in the sump. Our results showed that ultrasound irradiation makes the flow velocity faster and produces a lot of relatively small eddies in the sump bulk and near the mushy zone. The latter causes frequently repeated thinning of the mushy zone layer. The numerical predictions were verified against measurements performed on a pilot DC caster producing 203 mm billets of Al-17%Si alloy. The verification revealed approximately the same sump depth and shape as those in the numerical simulations, and confirms the frequent and large fluctuations of the melt temperature during ultrasound irradiation. However, the measured temperature distribution in the sump significantly differed from that predicted numerically. This suggests that the present mathematical model should be further improved, particularly in terms of more accurate descriptions of boundary conditions and mushy zone characteristics. [ABSTRACT FROM AUTHOR]

• The phenomena occurring near or on the melt free surface during ultrasonic treatment for molten aluminum are examined. • Ultrasonic treatment can greatly promote the formation of alumina particles on the melt surface around sonotrode. • A part of formed alumina particles can be entrained from the free surface into the melt bulk. • The entrained alumina particles can serve as nucleation sites for primary Al3Zr compounds. Although the ultrasonic treatment of molten aluminum has been studied for long period, there is still much to be revealed for this process. Many studies have focused on the investigation of acoustic cavitation and streaming under the horn tip and their effects on the treatment efficiency. However, to the best of our knowledge, no attempt has been done to explain phenomena occurring near or on the melt free surface. Thus, the goal of this study is to investigate phenomena occurring at the free surface during ultrasound irradiation and clarify their possible influence on the ultrasound treatment performance. The results of high temperature and water model experiments reveal that ultrasound irradiation significantly promotes the formation of alumina particles on the melt free surface around sonotrode, and part of these particles can be entrained into aluminum melts. Furthermore, TEM observation results suggested that the entrained alumina inclusions can serve as nucleation sites for the primary Al 3 Zr compounds. Most importantly, the oxidation and entrainment of particles from free surface are likely to be controllable by the immersion depth of sonotrode into molten aluminum. [ABSTRACT FROM AUTHOR]