Experimental and numerical study of anomalous thermocapillary convection in liquid gallium

Thermocapillary Marangoni convection of liquid gallium was studied experimentally and numerically. A specially designed experimental setup ensured an oxide-free surface of the liquid gallium for a very long time. The convective flow at the free surface was found to be directed opposite to both buoyancy-driven and ordinary thermocapillary convection. The anomalous direction of the thermocapillary flow was explained by the presence of a small amount of a surface-active contaminant—lead adsorbed at the free surface. Two different approaches were used to describe the observed phenomenon. First, the flow was treated as a pure thermocapillary convection with a modified dependence of the surface t…

SEGREGATION CONTROL AT DIRECTIONAL SOLIDIFICATION USING MAGNETIC FIELD AND ELECTRIC CURRENT

International audience

Velocity measurements in the liquid metal flow driven by a two-phase inductor

We present the results of velocity measurements obtained by ultrasonic Doppler velocimetry and local potential probes in the flow of GaInSn eutectic melt driven by a two-phase inductor in a cylindrical container. This type of flow is expected in a recent modification to the floating zone technique for the growth of small-diameter single intermetallic compound crystals. We show that the flow structure can be changed from the typical two toroidal vortices to a single vortex by increasing the phase shift between the currents in the two coils from 0 to 90 degrees. The latter configuration is thought to be favourable for the growth of single crystals. The flow is also computed numerically and a …

Arrays of Rotating Permanent Magnet Dipoles for Stirring and Pumping of Liquid Metals

AbstractMultiple configurations of synchronously rotating permanent magnet cylinders magnetized across the axes are proposed for liquid metal stirring for homogenization as well as for pumping. Universal analytical model is used for an initial parameter analysis. Then experimental setups were built to perform physical modelling of the industrial applications, e.g. large-scale metallurgical furnaces. Velocity distribution in the liquid metal was measured using different methods: the Ultrasound Doppler anemometry and the potential difference probes. The study shows that the cylindrical permanent magnet setups can achieve up to 10 times higher energy efficiency compared to AC inductors and hav…

HOW TO MAKE LIQUID METAL TRANSPARENT? A NEUTRON RADIOGRAPHY METHOD FOR SCALED-DOWN METALLURGICAL EQUIPMENT

Permanent magnet dipole stirrer for aluminium furnaces

Use of permanent magnet systems for metallurgical applications have been proposed before. We have continued development of such systems by investigation use of single magnetic dipole which is realized as permanent magnet cylinder. Magnet is magnetized orthogonal to its axis and has been mounted to the side of the liquid metal reservoir. System first was modelled using small scale experiment with galinstan melt. Velocity distributions and flow patterns in reservoir using Ultrasound Doppler Anemometer were acquired and will be presented here. Additionally, numerical model using Comsol and Cenos modelling software were developed alongside analytical, non-dimensional estimation calculations. Ex…

Influence on the macrosegregation of binary metallic alloys by thermoelectromagnetic convection and electromagnetic stirring combination

International audience; Influence of a slowly rotating 0.5 T transverse magnetic field on the directionally solidified metallic alloy has been experimentally studied in this work. Main idea is to study an influence on the melt flow and material structure caused by the simultaneous electromagnetic stirring and thermoelectromagnetic convection with comparable magnitudes. Electromagnetic stirring and thermoelectromagnetic convection intensities have been estimated analytically to find optimal experimental parameters. It is experimentally demonstrated that with such an interaction it is possible to modify component macrosegregation of Sn-10 wt% Pb alloy. Helical macrosegregation within cylindri…

Design of a System to Compose 50 Hz Alternating and Static Magnetic Field From Induction Coil and Permanent Magnets

We demonstrate a technical solution to achieve an intense 50 Hz alternating magnetic field and a static magnetic field made by a permanent magnet at the same place. A compact coil built using the Bitter coil concept is customized to support alternating current, thus creating an alternating magnetic field. A complementary permanent magnet assembly is built around it to achieve the superposition of both fields along the axis of symmetry. Results from the experimental model are compared to both numerical and analytical models.

Casting technology for ODS steels – dispersion of nanoparticles in liquid metals

Dispersion of particles to produce metal matrix nanocomposites (MMNC) can be achieved by means of ultrasonic vibration of the melt using ultrasound transducers. However, a direct transfer of this method to produce steel composites is not feasible because of the much higher working temperature. Therefore, an inductive technology for contactless treatment by acoustic cavitation was developed. This report describes the samples produced to assess the feasibility of the proposed method for nano-particle separation in steel. Stainless steel samples with inclusions of TiB2, TiO2, Y2O3, CeO2, Al2O3 and TiN have been created and analyzed. Additional experiments have been performed using light metals…

Application of AC and DC magnetic field for surface wave excitation to enhance mass transfer

Abstract Refining the metallic melts and removal of contaminants from the free surface may be sped up by increasing the surface area of mass exchange interface. It is relevant to both the case of volatile impurity evaporation in a vacuum as well as the case of chemical reactions when the continuous crust of reaction products should be interrupted. Previously proposed low-frequency AC magnetic field excitation of intense surface waves is limited by skin-effect. The induced sign-alternating forcing at high dimensionless frequency values becomes dominantly pulsing at considerably higher than unity values, delivering time-averaged stirring without intense surface waves. Superimposing DC magneti…

Contactless magnetic excitation of acoustic cavitation in liquid metals

A steady axial magnetic field is applied to a liquid metal zone heated by induction currents. The resulting alternating Lorentz force causes pressure oscillations that being strong enough lead to cavitation in the molten metal. Amplitude of the pressure oscillations is proportional to the product of the induced currents and the steady axial magnetic field induction. We follow an approach where the acoustic pressure is maximized by the induction currents. The onset of cavitation is identified by the occurrence of sub-harmonics of the drive frequency in sound recorded at the surface of the experimental cell. It is demonstrated that cavitation in a liquid metal may be excited by a superimposed…

ABSOLUTE THERMOELECTRIC POWER OF Pb–Sn ALLOYS

International audience; In this work, absolute thermoelectric power (ATP) of Pb, Sn, Pb-20 wt.% Sn, Pb-40 wt.% Sn, Pb-60 wt.% Sn, Pb-80 wt.% Sn are measured. Measurements are performed in a temperature gradient furnace from 20 degrees C to 500 degrees C, for both solid and liquid states. Temperatures are measured with T-type copper-constantan thermocouples, while voltage signal between copper electrodes of those thermocouples is recorded in order to calculate ATP of the sample metal.

Analytical solution for the diffusion of a capacitor discharge generated magnetic field pulse in a conductor

Powerful forces arise when a pulse of a magnetic field in the order of a few tesla diffuses into a conductor. Such pulses are used in electromagnetic forming, impact welding of dissimilar materials and grain refinement of solidifying alloys. Strong magnetic field pulses are generated by the discharge current of a capacitor bank. We consider analytically the penetration of such pulse into a conducting half-space. Besides the exact solution we obtain two simple self-similar approximate solutions for two sequential stages of the initial transient. Furthermore, a general solution is provided for the external field given as a power series of time. Each term of this solution represents a self-sim…

Direct chill casting of aluminium alloys under electromagnetic interaction by permanent magnet assembly

Direct chill casting is one of the methods used in industry to obtain good microstructure and properties of aluminium alloys. Nevertheless, for some alloys grain structure is not optimal. In this study, we offer the use of electromagnetic interaction to modify melt convection near the solidification interface. Solidification under various electromagnetic interactions has been widely studied, but usually at low solidification velocity and high thermal gradient. This type of interaction may succeed fragmentation of dendrite arms and transport of solidification nuclei thus leading to improved material structure and properties. Realization of experimental small-scale crystallizer and electromag…

Neutron Radiography Visualization of Solid Particles in Stirring Liquid Metal

Abstract This paper presents the analysis of the first dynamic neutron radiography experiment that visualized motion of solid particles in liquid metal, which was stirred by a system of four counter-rotating magnets. The paper also contains the quantitative results derived from neutron images: the distribution of particle concentration, number of admixed particles and velocities as functions of the magnet rotation speed.

Nanoparticle dispersion in liquid metals by electromagnetically induced acoustic cavitation

Abstract Aim of this study is to investigate experimentally the effect of magnetically induced cavitation applied for the purpose of nanoparticle dispersion in liquid metals. The oscillating magnetic force due to the azimuthal induction currents and the axial magnetic field excites power ultrasound in the sample. If the fields are sufficiently high then it is possible to achieve the acoustic cavitation threshold in liquid metals. Cavitation bubble collapses are known to create microscale jets with a potential to break nanoparticle agglomerates and disperse them. The samples are solidified under the contactless ultrasonic treatment and later analyzed by electron microscopy and energy-dispers…

Permanent magnet bottom-stirred swirling flow in coaxial shallow cylindrical containers

Here, an original rotating permanent magnet (RPM) system placed coaxially with the liquid metal container is studied as an effective means of generating flow in shallow cylinders for potential application in aluminum metallurgy (e.g., for ladle stirring and metal dosing). The studied RPM system generates volume force with strong axial variation and force maximum near the radial midpoint. The numerical and experimental data show that, in the shallow cylinder case, the azimuthal velocity follows the force radial distribution. The resulting velocity maximum occurs near the radial midpoint, unlike in the traditional rotating magnetic field (RMF) stirrer systems, where the velocity maximum occur…

Thermoelectric current and magnetic field interaction influence on the structure of directionally solidified Sn-10 wt.%Pb alloy

International audience; In this experimental work Sn-10 wt.% Pb alloy is directionally solidified in Bridgman setup at various growth velocities (from 0.5 mu m/s to 20 mu m/s) under transverse 0.4 T magnetic field. Temperature gradient of 8 K/mm is maintained perpendicular to the solidification direction during experiments. Liquid phase convection and its influence on the structure and segregation of an alloy, caused by magnetic field and thermoelectric current interaction (thermoelectromagnetic convection or TEMC) is studied experimentally and estimated theoretically in this work. Detailed velocity order of magnitude estimation is carried out. Besides optical microscopy, component distribu…

Permanent magnet centrifugal pump for liquid aluminium stirring

In liquid aluminium processing, it is important to ensure continuous mixing of the molten metal to ensure temperature and composition homogeneity, and to improve degassing of the metal. Aluminium and its alloys are often processed in special degassing chambers after preparation in order to avoid segregation and to improve material structure, and to avoid gas pores. This extra step increases material costs and may cause material contamination. In this article, we present the idea to use centrifugal permanent magnet stirrer to induce liquid aluminium motion through thick furnace wall by creating high-speed local liquid metal jet. Analytical estimation, numerical modelling and experimental tes…

Magnetically Induced Cavitation for the Dispersion of Particles in Liquid Metals

A contactless excitation of cavitation is possible by superposition of induction heating with a static axial magnetic field. This creates an alternating electromagnetic body force in a liquid metal which in turn produces pressure oscillations. Using this method, the onset of cavitation has been clearly observed in various liquid metals (tin, zinc, aluminum, steel SAE 304) at pressure oscillations in the range of 28…50 kPa. The present study aims to extend the previous work by producing steel metal matrix composites (MMC) and assessing the feasibility of the proposed method for particle dispersion in steel. Stainless steel (SAE 316L) samples with different ceramic inclusions, e.g. TiN, Al2O3…