Instability of a plane conducting free surface submitted to an alternating magnetic field

This paper considers the stability of a horizontal liquid-metal free surface in the presence of a horizontal alternating magnetic field. A weak formulation is used to derive a generalized Mathieu–Hill equation for the evolution of surface perturbations. Previous studies which rely on time-averaging the electromagnetic force over one field cycle have predicted a generally weak instability, but we find much larger growth rates near the resonances, where the surface wave frequency is an integral multiple of the field frequency. The method can be extended to include viscous and ohmic damping; the former has little effect, while the latter damps all waves except those whose frequency is close to the field frequency. Growth rates can be closely approximated by simple algebraic formulae, as can the critical magnetic field strength for the onset of instability

The starfish experiment: a Lagrangian approach

The present paper analyses the free surface deformation of a liquid metal drop under the influence of an alternating magnetic field. The analysis is restricted to the first axi- symmetric mode oscillation. In the low frequency case, the electromagnetic forces are of gradient type and purely oscillatory. Without any viscous damping, it is then possible to build a Lagrangian function, which involves the kinetic energy, the gravitational energy, the surface energy and the electromagnetic energy. The time evolution of the pool height is easily obtained from the Lagrange equation. It is shown that the pool height behaves like a non-linear forced oscillator

Refinement and growth enhancement of Al2Cu phase during magnetic field assisting directional solidification of hypereutectic Al-Cu alloy.

International audienceUnderstanding how the magnetic fields affect the formation of reinforced phase during solidification is crucial to tailor the structure and therefor the performance of metal matrix in situ composites. In this study, a hypereutectic Al-40 wt.% Cu alloy has been directionally solidified under various axial magnetic fields and the morphology of Al2Cu phase was quantified in 3D by means of high resolution synchrotron X-ray tomography. With rising magnetic fields, both increase of Al2Cu phase's total volume and decrease of each column's transverse section area were found. These results respectively indicate the growth enhancement and refinement of the primary Al2Cu phase in the magnetic field assisting directional solidification. The thermoelectric magnetic forces (TEMF) causing torque and dislocation multiplication in the faceted primary phases were thought dedicate to respectively the refinement and growth enhancement. To verify this, a real structure based 3D simulation of TEMF in Al2Cu column was carried out, and the dislocations in the Al2Cu phase obtained without and with a 10T high magnetic field were analysed by the transmission electron microscope

Electromagnetic processing – from AC to DC Field - A way for process improvement and innovation

International audienceTechnology of Electromagnetic processing of Materials (EPM) is relatively well known and mature. This knowledge gives the possibility to improve such processes and to integrate them in specific application (aeronautics, automotive sector, metallurgy...) at industrial scale. Thanks to a transverse and multiphysics approach which combined fluid dynamics, heat transfer, process metallurgy, solidification…, it is possible to know how to design new innovative processes with the integration of new design of electromagnetic system. An association of more complex EM configurations can be achieved: combination of AC and DC field, or two AC field… Future development of EPM technologies are in agreement with energy savings and CO 2 reduction demand. In metallurgy industry, the integration of EPM technologies is more basically associated with productivity improvement, maintenance reduction and also safety consideration. EM processing can be classified by type of magnetic field involved: from AC to DC field. The choice depends on the desired action on the electro conductive materials. These processes are suitable for heating, melting, flow and shape control, solidification control (stirring, pumping)…but for each application a specific configuration needs to be defined, selected and optimized. This paper gives some examples of EPM technologies applications for the development of new industrial process and main challenge to succeed in order to be relevant: (i) cold crucible technology or levitation, (ii) electromagnetic pump and (iii) DC electromagnetic brake. A combined approach by using numerical multiphysics modeling and experimental validation is used in order to give some guidelines for process improvement, or new electromagnetic design: feasibility and potentiality for integration at industrial scale

Flow behaviors driven by a rotating spiral permanent magnetic field

International audienceA rotating spiral magnetic field, constructed by a series of permanent magnets, is presented in this study, which is used to drive liquid metal flow. The rotation of such magnetic stirrer can then exert spatial- and tempo- electromagnetic force and drive three dimensional turbulent flow within the liquid metal bulk. Its velocity field was measured via the ultrasonic Doppler velocimetry. Two typical toroidal vortices flow patterns: the secondary flow and the globally axial flow in the meridian plane have been validated, which depends on the several structural and operating parameters, i.e., radius of the liquid metal bulk. The critical transition conditions between these two flow patterns have been discussed according to the experimental results

Influence of strong magnetic field on distribution of solid particles in BiZn immiscible alloys with a metastable miscibility gap

International audienceCompositions located in the metastable miscibility gap of BiZn immiscible alloy was investigated under a high static magnetic field (HSMF). BiZn immiscible alloys with uniformly distribution of solid particles in the matrix were obtained under HSMF with 29 T. The results show that the solid Bi particles were uniformly distributed in the matrix because of complete suppression of Stokes sedimentation under the HSMF with 29 T. Segregation in the alloys solidified under 0 T, 1 T and 6 T was mainly owning to Stokes sedimentation, but that solidified under 17.4T and 29 T was dominated by nucleation, growth and Marangoni migration processes of liquid Bi droplets. The segregation mechanism under the effects of HSMF was discussed

Influence of a transverse magnetic field on solidification structure during directional solidification

International audienceThree alloys were directionally solidified at low growth speeds under a transverse magnetic field. The results show that the application of the transverse magnetic field significantly modified the solidification structure. Indeed, we found that along with the refinement of cells/dendrites, the magnetic field caused the deformation of liquid-solid interfaces, extensive segregations (i.e., freckles and channels) in the mushy zone. Moreover, we observed that dendrite fragments and equiaxed grains were moved approximately along the direction perpendicular to the magnetic field. Modification of the solidification structure under a weak magnetic field is attributed to a TEMC-driven heat transfer and interdendritic solute transport and a TEMF-driven motion of dendrite fragments

Magnetic field, temperature and velocity distribution measurements in an electromagnetic induction pump using a small ferrite core coil system

International audienceIn this paper technique of local AC magnetic field, temperature and velocity measurements in the gap between electromagnetic induction pump (EMIP) channel and inductor using a single ferrite core coil is discussed. Described method has several important advantages: high signal/noise ratio due to ferrite core, small size of a sensor, three parameter measurement, non-intrusiveness, low cost etc. However, some significant limitations should be also considered: saturation of ferrite core with relatively low fields and temperature dependence on magnetic permeability characteristic of ferrite material. Finally, this perspective measurement method was tested in University of Latvia (UL) and implemented in measurement system of TESLA-EMP loop in Institute of Physics of University of Latvia (IPUL). Some relevant results of these experiments are presented

Parametric instability of an elongated pool of liquid metal in a low frequency magnetic field

Le travail expérimental préliminaire démontre l'existence d'instabilités paramétriques sur le bord d'une goutte ovale de métal liquide soumis à un champ magnétique vericale à frequence basse. Ces instabilités causent la formation des modes variqueux (V) et les modes sinueux (S) sur le bord de la goutte. Nous présentons une investigation théorique des instabilités paramétriques dans une band de métal liquide soumise à un champ magnétique vertical de fréquence basse. A partir de notre modèle nous déterminons la valeur du seuil du champ magnétique necessaire pour exciter de modes divers

Modeling of the effect of a thermoelectric magnetic force onto conducting particles immersed in the liquid metal

International audienceSimulation of a thermo-electromagnetic force which acts on a conducting particle immersed into liquid metal is performed using multi-gird multi-physics software AEQUATIO. To verify numerical solutions a model thermoelectric problem is solved using two methods. In the first one a phase function is used to indicate the phase transition whereas in the second the solid particle is described with a real frontier of a simplified shape. Numerical and analytical solutions for a model problem qualitatively agree but strong oscillations are observed in a numerical solution with a phase function. Further AEQUQTIO is applied for calculation of the velocity of a dendrite fragment observed in-situ in experiment of solidification of AlCu alloy. Numerical solution gives a good agreement with the experimental observation