208 research outputs found
Thermoelectric current and magnetic field interaction Influence on the structure of binary metallic alloys
No full textLorsqu'un champ magnétique est appliqué au cours de la solidification directionnelle, une convection dans la phase liquide peut être induite par l'effet thermoélectrique. En effet la présence d'un gradient de température le long du front de solidification peut provoquer la circulation du courant thermoélectrique, qui interagit avec le champ magnétique appliqué pour créer un écoulement (convection thermo électromagnétique-TEMC). Les conditions de transport de soluté et de l'énergie sont affectées par cette convection, donc il y a influence sur l'espacement des dendrites et la macro-ségrégation des composants de l'alliage. Dans ce travail, l'influence du champ magnétique sur la solidification directionnelle d'alliages métalliques est étudiée. Des travaux expérimentaux de la solidification directionnelle de Sn-Pb et Sn-Bi alliages sont réalisés. La solidification directionnelle dans la configuration Bridgman est effectuée avec ou sans champ magnétique appliqué. L'influence, sur la solidification, du champ magnétique et d'un courant électrique (AC et DC) appliqués est étudiée. Les mouvements du liquide provoquent de fortes macro-ségrégations ainsi qu'un modification des espacements interdendritiques. Les résultats expérimentaux sont interprétés à la lumière d'une modélisation heuristique. Le cas d'un champ magnétique tournant a été aussi étudié. Ainsi, la valeur de la rotation du champ est choisie pour ralentir assez brassage électromagnétique sans pour autant supprimer les effets de TEMC. À faible vitesse de tirage et faible vitesse de rotation faible champ une macro-ségrégation en forme de spirale a pu être obtenue.If magnetic field is applied during directional solidification, liquid phase convection can be induced by means of thermoelectromagnetic effect. Temperature gradient at the solidification front can cause thermoelectric current circulation, which then interacts with field and creates convection (Thermoelectromagnetic convection-TEMC). Solute and energy transport conditions are affected by this convection, thus it influences dendrite spacing and macrosegregation of the alloys. In this work magnetic field influence on the directional solidification of metallic alloys is studied. Experimental work of directional solidification of Sn-Pb and Sn-Bi alloys is done. Alloys are directionally solidified in Bridgman setup without or with applied magnetic field. Influence on the structure by magnetic field and applied electric current (AC and DC) is studied in this work. Analytical and experimental results are compared and interpreted. Bridgman solidification under rotating transverse magnetic field is studied as well, field rotation value is chosen to be slow enough that electromagnetic stirring does not fully suppress effects of TEMC. At low pulling velocity and low field rotation velocity spiral shaped component macrosegregation can be achieved
Fabrication of textured alumina by colloidal processing in a strong magnetic field
No full textTextured alumina ceramics were fabricated from different particle sized starting powders by slip casting in a strong magnetic field of 9.4 Tesla, followed by pressureless sintering at 1650°C for 2 hrs in air. The individual dispersed crystalline particles could be aligned inside the magnet when the energy of the crystal anisotropy is larger than the thermal motion energy. According to theory, the energy of the crystal anisotropy increases with increasing particle size, which was confirmed by the experimental results revealing that the orientation in the green deposit increased with increasing starting powder size. After sintering however, excessive grain growth accompanied with excellent texture formation was observed for the ceramic processed from the finest starting powder grade, whereas the modest orientation factor of the largest starting powder based ceramic hardly changed after sintering.sponsorship: COST Office
Staatsministerium für Wissenschaft und Kunst Freistaat Sachsen
SFB 609status: Publishe
Coarsening of the mushy zone in an AlCu30 alloy accelerated by a rotating magnetic field
No full textAnnealing experiments were performed on a near-eutectic Al-30wt%30 alloy held in temperature gradients from 0.1 to 3 K/mm for annealing times of 10 to 500 minutes, with and without the application of rotating magnetic fields. After annealing, metallographic analysis was carried out on sections of the samples having a constant fraction solid of 10%. X-ray tomography was also used to create three-dimensional reconstructions of the microstructures with a spatial resolution of approximately 1µm. The specific surface area was measured in 2D and 3D, as well as the local curvature distribution on the 3D reconstructed images. The microstructures clearly show that fluid flow accelerates coarsening of the mush, and that the temperature gradient has a pronounced and unexpected influence on the coarsening kinetics, which is explained qualitatively with a simple model of secondary dendrite arm growth
The effect of fluid flow on the dendritic microstructure in two AlSiCu alloys
No full textDirectional solidification experiments were performed with AlSi6Cu4 and AlSi9Cu4 alloys. The solidification velocity was varied by a factor of ten while the temperature gradient was kept constant at 3 K/mm over the processing length. Artificial fluid flow was induced in the samples by a rotating magnetic field device with flow velocities up to 10 mm/s being able to interact with the mushy zone. After processing, the microstructures were analyzed with respect to the primary dendrite stem spacing, the secondary dendrite arm spacing (SDAS), the specific surface area of the primary aluminium phase, and the local distribution of Si and Cu to quantify the flow-induced segregation of solute species. We generally observe that fluid flow leads to a reduction of the primary stem spacing, while the functional dependence on velocity does not change from the well-known models described in the literature. The SDAS, however, reacts differently. The usually observed cube root dependence of the spacing on solidification time is replaced by a square root dependence in agreement with newer theories on accelerated coarsening by fluid flow. The rotating magnetic field induces so-called secondary flows, which promote segregation of Si and Cu towards the sample center. The EDX analysis shows a pronounced segregation of Si and Cu, in agreement with numerical modeling
Magnetic interaction between two non-magnetic particles migrating in a conductive fluid under a strong magnetic field
No full textA theoretical analysis of the interaction between two non-magnetic particles (Al2O3) migrating in a conductive fluid due to an imposed strong magnetic field is presented based on our recent work [1]. Two kinds of magnetic forces are induced: an attractive force due to the magnetisation and a repulsive force due to the conductive fluid flow around the Al2O3 particles. The forces are calculated with the consideration of the magnetic field effect on the particle migration and become significant with the increase of the magnetic flux density. A counteracting behavior with a critical particle size of the interparticle magnetic forces exists. In addition, the critical particle size as a function of the magnetic field gradient value is calculated. It renders a better understanding of some recent experimental results and provides a theoretical basis to control the particles assembly using strong magnetic fields for materials processing.sponsorship: COST Office
Staatsministerium für Wissenschaft und Kunst Freistaat Sachsen
SFB 609status: Publishe
Modelling of the draining of a molten glass heated by induction
No full textLe présent travail de thèse s'inscrit dans le cadre de la mise au point d'un nouveau procédé de fusion d'oxydes dans un four chauffé par induction. Le procédé étudié met en jeu des fortes interactions entre des phénomènes électromagnétique, thermique et hydrodynamique dans un milieu fluide aux propriétés physiques fortement dépendantes de la température. L'objectif de la thèse est de modéliser le procédé en couplant étroitement le chauffage par effet Joule, le brassage mécanique et la vidange du four. La modélisation de l'évolution temporelle de l'interface entre le verre et l'air lors de vidange du creuset froid a été réalisée. S'agissant de la méthodologie, nous avons choisi de coupler deux logiciels : Flux® pour le calcul électromagnétique et Fluent® pour la thermo-hydraulique. L'évolution de la surface libre a été traitée par la méthode multiphasique « Volume-Of-Fluid – VOF », et l'agitation mécanique par les modèles « Moving Reference Frame » et « Sliding Mesh ». Nous avons d'abord considéré la vidange sans agitation mécanique d'une cuve remplie d'une huile silicone de haute viscosité. Ce modèle initial prend en compte des études de similitude hydraulique entre l'huile silicone et le verre. Puis nous avons superposé l'écoulement forcé crée par un agitateur mécanique et les phénomènes électromagnétique et thermique afin modéliser l'écoulement du bain de verre fondu. Le modèle final permet de fournir de diverses grandeurs, notamment des estimations pour le temps de vidange, le flux thermique et l'évolution temporelle du débit massique et de la température dans le four.This thesis is part of the development of a new technology of oxides melting in a furnace heated by induction. The technology studied involves strong interactions between electromagnetic, thermal and hydrodynamic phenomena in a flow with physical properties strongly dependents of the temperature. The aim of the thesis is the modelling of the process by coupling closely the Joule heating, the mechanical stirring and the draining of the furnace. The modeling of the time evolution of the interface between glass and air during the emptying of the cold crucible was performed. Regarding the methodology, we chose to combine two scientific codes: Flux® for the electromagnetic calculation and Fluent® for thermal-hydraulics. The evolution of the free surface was treated by the multiphasic method "Volume -Of- Fluid - VOF" and the mechanical stirring by the “Moving Reference Frame” and the “Sliding Mesh”. First of all, we considered the draining of a tank filled with a silicon oil of high-viscosity without mechanical stirring. This initial model took into account studies of hydraulic similarity between the silicon oil and the glass. Then we superimposed the forced flow creates by the mechanical stirrer, the thermal and the electromagnetic phenomena in order to model the flow of the molten glass. The final model can provide various parameters, including the time needed to drain the furnace, the heat transfer flux and the time evolution of the mass flow rate and of the temperature inside de furnace
Study on the thermoelectric magnetic effect in directional solidification of Al-Cu alloy
No full textNous étudions l'effet thermo-électrique et les phénomènes qui en résultent, forces et les courants thermoélectriques (TEC) sous l'action d'un champ magnétique externe imposé lors de la solidification d'alliages métalliques. Nous avons utilisé des simulations numériques, des observations directes et des examens de laboratoire. L'interaction entre les courants thermo-électriques et le champ magnétique externe lors de la solidification se produit des forces électromagnétiques et donc un écoulement du métal liquide. Le résultat est nommé effet magnétique thermoélectrique (TEME). Les formulations de TEC, les forces et les équations gouvernant les écoulements TEM sont donnés. Afin de mieux prouver l'existence de la TEME, des expériences par méthode d'imagerie à rayons X menées au synchrtron ont été utilisées pour observer in-situ et en temps réel l'action directe des forces et les mouvements TEM pendant la solidification directionnelle des alliages Al-Cu. Nous avons montré la cohérence raisonnable entre les calculs analytiques et des simulations numériques qui ont exécuté avec les mêmes conditions de traitement. En outre, la capacité des écoulements thermo-électriques à influer sur la microstructure lors de la solidification directionnelle sont expérimentalement évaluées dans les autres cas en réalité. La solidification directionnelle d'une seule phase de formation des alliages Al-Cu sous divers champs magnétiques montre que les écoulements TEM sont capables de modifier la forme de l'interface liquide-solide conduisant à des morphologies différentes. L'effet le plus intense se produit dans différents champs magnétiques pour différentes morphologies, en effet, le champ magnétique élevé est nécessaire pour la morphologie a une plus petite longueur typique. Ceci est en accord avec le comportement des vitesses de TEM qui varient avec les champs magnétiques imposés ainsi que les différentes échelles de longueur typique. Cette variation est confirmée par des simulations numériques 3D. Nous montrons que les dendrites primaires et à l'avant de la phase eutectique, peuvent être modifiés par les mouvements TEM et les forces de TEM dans le solide pour améliorer la croissance de la phase de Al2Cu facettes primaire pendant la solidification des Al-40wt%Cu hypereutectiques. Le mécanisme de renforcement de la croissance de la phase facettes Al2Cu est confirmé par la transmission électronique observation au microscope, et la raison de la formation de la structure de croissance de couple de Al-26wt% Cu alliages est vérifiée par le test de l'analyse thermique différentielle. Ainsi, nous pouvons affirmer que le champ magnétique élevé facilite la formation de la structure de la croissance de couple pour hypoeutectiques alliages Al-Cu, et favorise la croissance de la phase Al2Cu primaire pour hypereutectiques Al-Cu alliages.We have investigated the thermoelectric magnetic (TEM) forces and flows resulting from the interaction between the internal thermoelectric currents (TEC) and the imposed external magnetic field during solidification. Numerical simulations, direct observations and experimental examinations were undertaken. As the natural phenomenon, TEC was discovered almost 200 years ago, therefore, our introduction begins from then on. It is shown that the interaction between TEC and external magnetic field during solidification in the cont put forth new interesting phenomena in the context of a rising field named Electromagnetic Processing of Materials. After that, it is discussed how the TEC appear and the TEM effect (TEME, referring to both TEM forces and flows) behaves at the liquid-solid interface in directional solidification under external magnetic field. Meanwhile, formulations of TEC, TEM forces and flows are given, and numerical simulations of TEME are performed to visually display the TEM forces and flows. In order to further prove the existence of TEME, in situ synchrotron X-ray imaging method was used to observe the direct resultant of TEM forces and flows during directionally solidifying the Al-Cu alloys. The observations show reasonable consistency with the analytical calculations and numerical simulations performed with the same process conditions. Except confirmation the existence of TEME, its abilities to affect the microstructure during directional solidification are experimentally investigated in the more realistic cases. The single phase forming Al-Cu alloys are directionally solidified under various magnetic fields, which shows that TEM flows are capable to modify the shape of liquid-solid interface, and the most intensive affect occurs under different magnetic fields for different interface morphologies. Indeed, the smaller the typical length of the morphology is the higher the magnetic field is needed. This agrees with the estimating regulation of the velocity of TEM flows changing with magnetic fields for different typical length scales, and is confirmed by 3D numerical simulations. Directional solidification of multiphase forming Al-Cu alloys under various magnetic fields shows that the mushy zone length (distance between the front of primary dendrites and eutectic phases) varies with the magnetic fields, which can be attributed to the redistribution of rejected solutes by TEM flows. In addition, apparent enhanced growth of the primary faceted Al2Cu phase is founded when Al-40wt%Cu alloys are solidified under sufficient high magnetic fields, this should be ascribed to the TEM forces acting on the solid because strains are able to lead the formation of defects and thus benefit to the growth of faceted phase. This is confirmed by comparison of the dislocations in samples solidified without and with a 10T magnetic field via transmission electron microscopy observation. In another aspect, an almost entire couple growth structure is achieved when Al-26wt%Cu alloys are directionally solidified under a 4T magnetic field, which can be explained by the effect of high magnetic field on changing the nucleation temperature and growth velocity of each phase. Moreover, the differential thermal analysis test on the nucleation temperature of both α-Al and eutectic phases verified this explanation. Therefore, we conclude that high magnetic field facilitates the formation of couple growth structure for hypoeutectic Al-Cu alloys, reversely, enhances the growth of primary dendrite for hypereutectic Al-Cu alloys
Dynamics of Liquid Metal Drops Influenced by Electromagnetic Fields
No full textZusammenfassung
Diese Arbeit ist den Effekten gewidmet, die an der Oberfläche von Flüssigmetall im Magnetfeld auftreten
können. Im Prinzip erlauben Magnetfelder, Lorentzkräfte auf flüssiges Metall auszuüben und in seinem Innern
Induktionswärme zu generieren. Es ist aber auch bekannt, dass Flüssigmetall-Oberflächen durch Magnetfelder
dramatische Formänderungen oder Schwingungen erfahren können. Ein Verständnis dieser Phänomene ist wichtig
für sämtliche metallurgische Anwendungen, bei denen freie Oberflächen vorkommen. Als repräsentatives Problem
untersuchen wir einen Tropfen aus Flüssigmetall, der eine freie Oberfläche mit einem endlichen Volumen verbindet.
Wir schliessen Temperatureffekte aus und konzentrieren uns auf die Wirkung der Lorentzkraft. Wir erarbeiten
ein Schema zur Klassifikation von Tropfen-Magnetfeld-Problemen basierend auf der Frequenz des Magnetfeldes
und dem Shielding-Parameter des Tropfens in diesem Feld. Anhand dieses Schemas wählen wir fünf Fallstudien
aus und studieren das Tropfenverhalten im i) transienten, ii) hochfrequenten und iii) mittelfrequenten Magnetfeld.
Die Untersuchungen sind vorwiegend analytischer Art, nur die Mittelfrequenz-Studie ist experimentell. Die beiden
wichtigsten Probleme, welche die vorliegende Arbeit zum Gegenstand hat, sind das symmetrische Zusammendrücken
oder Halten von Flüssigmetalltropfen einerseits und deren azimutale Verformungen andererseits. Für das transiente
Magnetfeld werden zwei Studien präsentiert, jede zu einem der beiden Hauptprobleme. Eine Verbindung zwischen
transientem und hochfrequentem Feld besteht darin, das mit beiden Feldtypen stationäre Kräfte im Metall erzeugt
werden können. Ein wichtiger Unterschied ist jedoch, dass transiente Felder das Metall durchdringen können,
während hochfrequente Felder vom Metall abgeschirmt werden, wodurch eine Kopplung zwischen Tropfenform und
Magnetfeld entsteht. Die Effekte im hochfrequenten Feld sind daher schwieriger zu modellieren. Wir präsentieren
eine Hochfrequenz-Studie, in der es um das Zusammendrücken und Halten von Tropfen in einem gegebenen Magnetfeld
geht. Eine zweite Hochfrequenz-Studie beschäftigt sich mit longitudinaler Levitation. Dort geben wir als einfache
Tropfenform einen Flüssigmetall-Zylinder vor und ermitteln das Magnetfeld, welches die vorausgesetzte Tropfenform
tatsächlich ermöglichen würde. Im mittelfrequenten Feld bieten sich für theoretische Betrachtungen die grössten
Schwierigkeiten, da das Magnetfeld den Tropfen nun partiell durchdringt und kaum noch vereinfacht werden kann.
Dieser Bereich wurde daher durch die fünfte Studie experimentell erkundet. Dabei wurde eine Flüssigmetall-Scheibe
verwendet, welche nur zweidimensionale Verformungen ausführen kann. Die Ergebnisse der Arbeit zeigen, dass
insbesondere transiente Magnetfelder gangbare Wege der analytischen Modellierung bieten. Ebenso wie hochfrequente
Magnetfelder eignen sie sich zum Formen und Stützen freier Flüssigmetall-Oberflächen. Für das Studium der
azimutalen Verformungen hat sich die Scheiben-Geometrie als günstig erwiesen, sowohl analytisch als auch
experimentell. Insgesamt zeigt sich, dass eine Fortführung der Arbeit auf dem Gebiet der Wechselwirkung zwischen
Magnetfeldern und Flüssigmetall-Oberflächen lohnenswert ist.This work is devoted to the free surface effects that occur when liquid metal is placed in a magnetic field.
Principally, magnetic fields allow to exert Lorentz forces on liquid metal and to generate induction heat inside
it. But it is also known that liquid metal surfaces in magnetic fields can undergo dramatic shape changes or
experience oscillations. An understanding of these phenomena is crucial to all metallurgical applications showing
free surfaces. As a representative problem we examine a liquid metal drop that combines a free surface with a
finite volume. We exclude heat effects and focus on the consequences of the Lorentz force. To this end, we
elaborate a classification scheme for liquid metal drop - magnetic field problems comprising the frequency of
the magnetic field and the Shielding parameter of the drop in this field. On that basis we select five case
studies involving i) transient, ii) middle-frequency and iii) high-frequency magnetic field to explore the
behavior of liquid metal drops in it. We mainly use analytical means - only the middle-frequency study is
experimental. The major problems we tackle concern the symmetric squeezing and supporting of drops and its
azimuthal deformations, respectively. Two studies are presented for the transient magnetic field, each accounting
for one of the two problems. A connection between transient and high frequency magnetic field is the possibility
to exert a steady force on the liquid metal. An important difference is that transient fields can penetrate the
metal while high-frequent fields are shielded by the metal resulting in a coupling between surface shape and
magnetic field distribution. Therefore, the effects of high frequency magnetic fields are more difficult to model.
We present one high frequency study where we presuppose the magnetic field and ask for the resulting drop shape
(forward problem) and another one where we presuppose a simple surface shape and ask for the best suited magnetic
field to obtain it (reverse problem). The most difficulties arise in middle-frequent magnetic fields. Here we
have partial shielding which makes it necessary to solve the magnetic diffusion equation and to account for the
coupling between magnetic field and drop surface at the same time. In this field, the fifth study reports
experimental results on the azimuthal deformations of a liquid metal disc in an inhomogeneous inductor field.
The results of the work show that especially the transient fields provide feasible ways for analytical modeling.
Like high frequency fields they are suited to shape and to support liquid metal surfaces. To study azimuthal
deformations, the disc geometry has proven useful - both analytically and experimentally. Overall, it still seems
worthwhile to further investigate the behavior liquid metal surfaces in magnetic fields
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