Contactless ultrasonic treatment in direct chill casting

Uniformity of composition and grain refinement are desirable traits in the direct chill (DC) casting of non-ferrous alloy ingots. Ultrasonic treatment (UST) is a proven method for achieving grain refinement, with uniformity of composition achieved with additional melt stirring. The immersed sonotrode technique has been employed for this purpose to treat alloys both within the launder prior to DC casting, and directly in the sump. In both cases mixing is weak, relying on buoyancy driven flow or in the latter case on acoustic streaming. In this work we consider an alternative electromagnetic (EM) technique used directly in the caster, inducing ultrasonic vibrations coupled to strong melt stirring. This ‘contactless sonotrode’ technique relies on a kilohertz frequency induction coil lowered towards the melt with the frequency tuned to reach acoustic resonance within the melt pool. The technique developed with a combination of numerical models and physical experiments has been successfully used in batch to refine the microstructure and degas aluminum in a crucible. In this work we extend the numerical model, coupling electromagnetics, fluid flow, gas cavitation, heat transfer and solidification to examine the feasibility of use in the DC process. Simulations show that a consistent resonant mode is obtainable within a vigorously mixed melt pool, with high pressure regions at the Blake threshold required for cavitation localized to the liquidus temperature. It is assumed extreme conditions in the mushy zone due to cavitation would promote dendrite fragmentation and that, coupled with strong stirring, would lead to fine equiaxed grains

Comparison of frequency domain and time domain methods for the numerical simulation of contactless ultrasonic cavitation

The use of a top-mounted electromagnetic induction coil has been demonstrated as a contactless alternative to traditional ultrasonic treatment (UST) techniques that use an immersed mechanical sonotrode for the treatment of metals in the liquid state. This method offers similar benefits to existing UST approaches, including degassing, grain refinement, and dispersion of nanoparticles, while also preventing contact contamination due to erosion of the sonotrode. Contactless treatment potentially extends UST to high temperature or reactive melts. Generally, the method relies on acoustic resonance to reach pressure levels suitable for inertial cavitation and as a result the active cavitation volume tends to lie deep in the melt rather than in the small volume surrounding the immersed sonotrode probe. Consequently, (i) with suitable tuning of the coil supply frequency for resonance, the treatment volume can be made arbitrarily large, (ii) the problem of shielding and pressure wave attenuation suffered by the immersed sonotrode is avoided. However, relying on acoustic resonance presents problems: (i) the emergence of bubbles alters the speed of sound, resonance is momentarily lost, and cavitation becomes intermittent, (ii) as sound waves travel through and reflect on all the materials surrounding the melt, the sound characteristics of the crucible and supporting structures need to be carefully considered. The physics of cavitation coupled with this intermittent behaviour poses a challenge to sonotrode modelling orthodoxy, a problem we are trying to address in this publication. Two alternative approaches will be discussed, one of which is in the time domain and one in the frequency domain, which couple the solution of a bubble dynamics solver with that of an acoustics solver, to give an accurate prediction of the acoustic pressure generated by the induction coil. The time domain solver uses a novel algorithm to improve simulation time, by detecting an imminent bubble collapse and prescribing its subsequent behaviour, rather than directly solving a region that would normally require extremely small time steps. This way, it is shown to predict intermittent cavitation. The frequency domain solver for the first time couples the nonlinear Helmholtz model used for studying cavitation, with a background source term for the contribution of Lorentz forces. It predicts comparable RMS pressures to the time domain solver, but not the intermittent behaviour due to the underlying harmonic assumption. As further validation, the frequency domain method is also used to compare the generated acoustic pressure with that of traditional UST using a mechanical sonotrode

Fabrication of hollow polymer microstructures using dielectric and capillary forces

Electric Field Assisted Capillarity is a novel one-step process suitable for the fabrication of hollow polymer microstructures. The process, demonstrated to work experimentally on a microscale using Polydimethylsiloxane (PDMS), makes use of both the electrohydrodynamics of polymers subject to an applied voltage and the capillary force on the polymers caused by a low contact angle on a heavily wetted surface. Results of two-dimensional numerical simulations of the process are discussed in this paper for the special case of production of microfluidic channels. The paper investigates the effects of altering key parameters including the contact angle with the top mask, the polymer thickness and air gap, the permittivity of the polymer, the applied voltage and geometrical variations on the final morphology of the microstructure. The results from these simulations demonstrate that the capillary force caused by the contact angle has the greatest effect on the final shape of the polymer microstructures

Enhancement of mechanical properties of pure aluminium through contactless melt sonicating treatment

A new contactless ultrasonic sonotrode method was previously designed to provide cavitation conditions inside liquid metal. The oscillation of entrapped gas bubbles followed by their final collapse causes extreme pressure changes leading to de-agglomeration and the dispersion of oxide films. The forced wetting of particle surfaces and degassing are other mechanisms that are considered to be involved. Previous publications showed a significant decrease in grain size using this technique. In this paper, the authors extend this research to strength measurements and demonstrate an improvement in cast quality. Degassing effects are also interpreted to illustrate the main mechanisms involved in alloy strengthening. The mean values and Weibull analysis are presented where appropriate to complete the data. The test results on cast Al demonstrated a maximum of 48% grain refinement, a 28% increase in elongation compared to 16% for untreated material and up to 17% increase in ultimate tensile strength (UTS). Under conditions promoting degassing, the hydrogen content was reduced by 0.1 cm3/100 g

High-speed imaging of the ultrasonic deagglomeration of carbon nanotubes in water

Ultrasonic treatment is effective in deagglomerating and dispersing nanoparticles in various liquids. However, the exact deagglomeration mechanisms vary for different nanoparticle clusters, owing to different particle geometries and inter-particle adhesion forces. Here, the deagglomeration mechanisms and the influence of sonotrode amplitude during ultrasonication of multiwall carbon nanotubes in de-ionized water were studied by a combination of high-speed imaging and numerical modeling. Particle image velocimetry was applied to images with a higher field of view to calculate the average streaming speeds distribution. These data allowed direct comparison with modeling results. For images captured at higher frame rates and magnification, different patterns of deagglomeration were identified and categorized based on different stages of cavitation zone development and for regions inside or outside the cavitation zone. The results obtained and discussed in this paper can also be relevant to a wide range of carbonaceous and other high aspect ratio nanomaterials

Coupling acoustic cavitation and solidification in the modeling of light alloy melt ultrasonic treatment

The space industry requires strong lightweight alloys to decrease launching costs and to increase the reliability of components. One promising technique is the application of ultrasound to a solidifying melt, which has been demonstrated to enhance the thermo-physical qualities of the treated sample through grain refinement. The underlying mechanism is through acoustic cavitation; however, it is not well understood how cavitating bubbles disrupt the microstructure. Further understanding of the fundamentals of ultrasonic melt processing is required to optimize treatment parameters, thus enabling the efficient production of lighter, stronger alloys at an industrial scale. To achieve this goal and investigate the effect of cavitating bubbles on the solidification front, we present a high-order micro-scale acoustic cavitation model. This model is applied to the interaction between cavitating bubbles and a needle dendrite of succinonitrile 1 wt. % camphor organic transparent alloy for which high-speed digital imaging is available in the literature

Alloy grain refinement by means of electromagnetic vibrations

A tuned electromagnetic induction coil generates ultrasonic pressure waves is an alloy melt. Under acoustic resonance conditions, cavitation of dissolved gases is observed leading to microstructure refinement and dispersion of added particles. The method as an alternative to the immersed sonotrode technique offers several advantages. Being contactless, it can be applied equally to high temperature or reactive melts, avoiding contamination due to probe erosion; consequently, it is maintenance free. The consequent electromagnetic induction stirring means that larger volumes of melt can be treated (a major limitation of the traditional method), as the liquid is forced to pass repeatedly through zones of cavitation activity. The coil configuration used depends on application. Here, a top conical coil immersed in aluminium melt (contactless due to EM repulsion) was used. Simulations of sound, flow and EM fields are given, compared with experiments and indicating strong stirring, evidence of cavitation and grain refinement

Digital modelling in the performance evaluation of 19th Century clipper ships: the Thermopylae and the Cutty Sark

The Cutty Sark, built in 1869 in Dumbarton, is the last intact extreme clipper ship of composite construction. She famously took part in a race with another composite tea clipper the Thermopylae which was launched in 1868. As only one of these ships exists today, and she no longer sails, some computational modelling has been undertaken. Using 3D geometries built from the lines plan of each ship CFD models were built looking at the viscous pressure and frictional resistances. Results suggest that the Thermopylae had the lower hull resistance when sailing windward, however the Cutty Sark appears to have a lower resistance at high speeds with the wind behind her

Controlling solute channel formation using magnetic fields

Solute channel formation introduces compositional and microstructural variations in a range of processes, from metallic alloy solidification, to salt fingers in ocean and water reservoir flows. Applying an external magnetic field interacts with thermoelectric currents at solid/liquid interfaces generating additional flow fields. This thermoelectric (TE) magnetohydrodynamic (TEMHD) effect can impact on solute channel formation, via a mechanism recently drawing increasing attention. To investigate this phenomenon, we combined in situ synchrotron Xray imaging and Parallel-Cellular-Automata-Lattice-Boltzmann based numerical simulations to study the characteristics of flow and solute transport under TEMHD. Observations suggest the macroscopic TEMHD flow appearing ahead of the solidification front, coupled with the microscopic TEMHD flow arising within the mushy zone are the primary mechanisms controlling plume migration and channel bias. Two TE regimes were revealed, each with distinctive mechanisms that dominate the flow. Further, we show that grain orientation modifies solute flow through anisotropic permeability. These insights led to a proposed strategy for producing solute channelfree solidification using a time-modulated magnetic field