Analytic solutions to determine critical magnetic fields for thermoelectric magnetohydrodynamics in alloy solidification

During alloy solidification, it has been observed that the morphology of microstructures can be altered by applying an external DC magnetic field. This structural change can be attributed to solutal convective transport introduced by thermoelectric magnetohydrodynamics (TEMHD) which drives fluid motion within the inter-dendritic region. Complex numerical models with grid resolutions on the microscopic scale have been constructed to solve the equations governing TEMHD. To complement these computationally intensive numerical models, analytic solutions were sought. Specifically, the analytic solutions presented herein are asymptotic solutions derived for TEMHD under low and high magnetic field intensities. Combination of these asymptotic solutions leads to simple formulae for estimating critical magnetic fields which can be readily evaluated in terms of characteristic lengths of materials that have been identified in experiments as key parameters of critical fields. Indeed, the critical magnetic fields predicted with the asymptotic solutions exhibit magnitudes consistent with those applied in current ongoing experiments where significant changes in microstructure have been observed. The capability to predict accurate results indicates that the analytic solutions described herein are valuable precursors not only for detailed numerical simulations but also for experimental design to study critical magnetic fields in alloy solidification

Investigation of liquid phase motion generated by the thermoelectric current and magnetic field interaction.

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Effect of slowly rotating transverse magnetic field on a directionally solidified binary metallic alloy

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Absolute thermoelectric power of PbSn alloys.

International audienceIn 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

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

International audienceInfluence 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 cylindrical sample is obtained as a result of simultaneous influence of thermoelectromagnetic convection (TEMC) and electromagnetic stirring of the liquid melt. Obtained experimental results allow to determine TEMC velocity by comparing the intensities of TEMC and electromagnetic stirring. (C) 2014 Elsevier B.V. All rights reserved

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

International audienceIn 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 distribution along the diameter of the sample is quantitatively measured by scanning electron microscopy. Results show that significant influence on the macrosegregation and dendrite spacing of a metallic alloy is achieved if sample is solidified with applied transverse magnetic field at low solidification velocity. (C) 2013 Elsevier B.V. All rights reserved

CURRENT AND MAGNETIC FIELD INTERACTION INFLUENCE ON LIQUID PHASE CONVECTION.

International audienceThis paper analyzes the influence of the electric current and magnetic field interaction on the liquid metal motion. At the crystallization front of a metallic alloy, temperature variations can exist in length scales comparable with dendrite spacing order of magnitude. If an external magnetic field is applied, a Lorentz force appears and a flow of the liquid phase is induced (thermoelectromagnetic convection). A similar effect can be achieved by the interaction of electric current and magnetic field of the same direction. Results of mathematical modelling of the liquid phase motion and analytical estimation of expected velocity order of magnitude are presented here

Sensitivity of contactless ultrasound processing to variations of the free surface of the melt with induction heating

Contactless ultrasound treatment can be useful for high-temperature or reactive alloys in the liquid state, where contact with an immersed vibrating probe is undesirable. The alternating component of the Lorentz force can generate sound pressure levels leading to cavitation of gas bubbles in the melt, through acoustic resonance. Resonance is a function of the speed of sound in the liquid, the shape of the volume containing it and surrounding boundary conditions. Induction forces applied to a crucible lead to bulk stirring and in general deform the free surface, whose precise shape may influence the resonance conditions. This effect is investigated here by multi-physics computer modelling. Calculated results for aluminium melts are compared with experimental data and conclusions are drawn as to the sensitivity to process parameters and the reliability of this type of ultrasound metal processing

Thermoelectric magnetic flows in melt during directional solidification.

International audienceThermoelectric magnetic (TEM) flows in melts, which are generated by TEM forces in liquids, were uncovered by the shape evolution of the planar solid/liquid interface during directional solidification. The solid/liquid interface developing from an initially tilted shape to a nearly flat one has been in situ and real-time observed by means of synchrotron X-ray radiography. The corresponding numerical 3D simulations and velocity measurements of flows in the melt confirm that TEM flows exist and respond to this interface shape change. This observation provides visible evidence for TEM flows in melt and their influence on the solid/liquid interface dynamics when directional solidification is conducted in a magnetic field. (C) 2014 AIP Publishing LL