Andris Bojarevics

Thermoelectric magnetic force acting on the solid during directional solidification under a static magnetic field

Thermoelectric magnetic force (TEMF), which is induced by the interaction between the thermoelectric current and the applied magnetic field, acting on the solid during directional solidification under a static magnetic field was derived. Equipping the derived equation, an analytical calculation of the velocity of a solid spherical particle submitted to the TEMF was carried out. The experiment with corresponding phenomenon was performed and recorded by the in situ synchrotron X-ray imaging, which permitted a direct measurement of the velocity of the TEMF-driven motion of detached fragments. The measurement of the velocities showed a reasonable agreement with the calculation results.

Modification of liquid/solid interface shape in directionally solidifying Al–Cu alloys by a transverse magnetic field

Al-0.85wt%Cu and Al-2.5wt%Cu alloys were directionally solidified under different transverse magnetic field (TMF) intensities to investigate the influence of TMF on the liquid/solid interface shape with respect to the various length scales appearing (planar, cellular, and dendritic interfaces). Results show that planar and cellular interfaces tilt to one side and then level off with increasing TMF although the dendritic interface appears not to behave in this manner. In situ synchrotron X-ray imaging was applied during directional solidification of the Al-4wt%Cu alloy under a 0.08T TMF, revealing leveling of the initially sloped interface. Solute redistribution, caused by thermoelectric magnetic convection (TEMC), responds to the changes in the interface shape. Because different typical length scales should be used in estimating the velocity of TEMC for planar, cellular, and dendritic interfaces, the maximum velocity of the convection ahead of the interface is obtained under different TMF intensities; correspondingly, leveling of the interface’s degree of slop varies with TMF.

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 tension on temperature so that to reproduce the measured velocity distribution. Second, a novel physical model was devised for the flow driven by the gradient of the surface tension induced by the temperature dependence of the concentration of the adsorbed layer of contaminant. In contrast to the ordinary thermocapillary...

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 axial magnetic field of a moderate 0.5 T induction.

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

Abstract Multiple 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 have potential of wide-range industrial application, e.g. can be used as stirrers for secondary aluminium furnaces with up to 50 cm thick walls.

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 with an increased value of the steady magnetic field using a superconducting magnet with a field strength of up to 5 T.

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°. The latter configuration is thought to be favourable for the growth of single crystals. The flow is also computed numerically, and a reasonable agreement with the experimental results is found. The obtained results may be useful for the design of combined two-phase electromagnetic stirrers and induction heaters for metal or semiconductor melts.

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 tests using GaInSn alloy has been done and are presented in the article. Model experiments are used to elaborate dimensionless parameters to verify the feasibility of this technology for liquid aluminium.

Magnetic Fields, Convection and Solidification

In solidification processes the fluid flow occurs almost at every scale from the bulk, near the interfaces and deeply in the mushy zone. Numerical modeling is a valuable tool for understanding and master the solidification processes, however, macro-scale models are not always able to predict in detail the random behavior of the solidification process whereas models for micro scales are not capable to take into account a complex structure of flows which enter into the mushy zone. In the present paper the variety of the flows and imprints they left on solidification structure are discussed and illustrated with experimental data which naturally comprise every flow occurring in the process.

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, TiB2 as well as others, have been created. It has been demonstrated that the cavitation onset in the liquid steel varies extensively and depends on the cavitation nuclei rather than the strength of acoustic pressure. The microstructure of the produced samples has been analyzed using SEM and EDS.