Influence d'un champ magnétique glissant sur la solidification dirigée des alliages métalliques binaires

In the field of the metallic alloys development, principal industrial goals are the control of the metallurgical structure and defects. Hydrodynamic movements in the liquid phase have a significant influence on properties of the solidified product. In order to study the influence of a forced convection on the directional solidification of binary metallic alloys, a new device made up of a Bridgman type furnace and a Bitter coil to produce a travelling magnetic field was designed and developed. We focused our attention on two major effects influenced by the forced convection : the macrosegregation and the grain structure. The velocity and the direction of the forced flow can be controlled thanks to the application of the travelling magnetic field. We have shown that this configuration can control macrosegregations and that moreover, the dendritic primary spacing is modified according to the applied field. With regard to the grain structure, using an analytical model, we have shown that the extension of the solutale layer in front of the solidification front varies according to the direction of the flow near the mushy zone. This mechanism enhances the nucleation and the growth of equiaxed grains. In the case of non refined alloys, a mode of elongated free grains could be probably obtained by fragmentation under a range of magnetic field.Dans le domaine de l'élaboration des alliages métalliques, les principaux enjeux industriels résident dans la possibilité de maîtriser la structure métallurgique ainsi que les défauts qui surviennent lors de la phase de solidification. Lors de la solidification, les mouvements hydrodynamiques dans la phase liquide ont une influence importante sur les propriétés du produit solidifié. Dans cette étude, la conception d'un nouveau dispositif BATMAF (Bridgman Apparatus with a Travelling MAgnetic Field) constitué d'un four de solidification dirigée et d'une bobine de Bitter permettant de produire un champ magnétique glissant a permis d'étudier l'influence d'une convection forcée sur la solidification dirigée des alliages métalliques binaires. Notre attention s'est plus particulièrement portée sur deux effets majeurs influencés par la présence de convection forcée ou non : la macroségrégation et la structure de grains pour un alliage d'Al-3,5%pdsNi en présence ou non de particules affinantes. La vitesse ainsi que le sens de l'écoulement forcé peuvent être contrôlés par l'application du champ magnétique glissant. Nous avons montré que dans le cas de notre alliage, la macroségrégation peut être contrôlée et que de plus, l'espacement primaire dendritique est modifié en fonction du champ appliqué. En ce qui concerne les structures de grains, à l'aide d'un modèle analytique, nous montrons que l'extension de la couche solutale en avant du front de solidification varie en fonction du sens de l'écoulement au voisinage de la zone pâteuse. Ceci conditionne la possibilité pour les grains équiaxes de germer puis de croître et, par conséquent, l'obtention du régime équiaxe ou non. Enfin, dans le cas des alliages non affinés, dans un domaine d'intensité de brassage, un rgime de grains libres allongés a pu être obtenu probablement par fragmentation. Cette étude démontre s'il en était besoin, l'importance cruciale de la maîtrise de la convection sur les macroségrégations et la structure des grains et ouvre des perspectives quant à l'utilisation du champ magnétique glissant pour leur contrôle

Experimental investigation of the natural and forced convection on solidification of Sn-3wt. %Pb alloy using a benchmark experiment

International audienceWe deal with the development of a solidification benchmark experiment in order to investigate the structure formation as well as solute macro-mesosegregation, by means of a well-controlled solidification experiment. The experiment consists in solidifying a rectangular ingot of Sn-3wt.%Pb alloy, by using two lateral heat exchangers which allow extraction of the heat flux from one or two vertical sides of the sample. The domain is a quasi two dimensional parallepipedic ingot (100x60x10) mm. The temperature difference ΔT between the two lateral sides is 40 K and the cooling rate CR= 0.03 K/s. The instrumentation consists in recording the instantaneous temperature maps by means of an array of 50 thermocouples in order to provide the time evolution of the isotherms. After each experiment the patterns of the segregations have been obtained by X-ray radiograph and confirmed by eutectic fraction measurements. The local solute distribution determined by means of induction coupled plasma analysis is provided. The originality of the present study is to examine the effect of the forced convection driven by a travelling magnetic field (TMF) induced by a linear inductor located on the bottom part of the sample. A periodically reversed stirring with a modulation frequency equal to 0.5 Hz stirring have been investigated. This study allows us to evaluate the evolution due to the forced convection induced by a TMF field, as well as its influence on the initial conditions, the solidification macrostructure and the segregation behavior. Measurements of the velocity field by ultrasonic Doppler velocimetry (UDV) method in a Ga-In-Sn pool were performed and transposed to the tin-lead alloy case before solidification. Post-mortem patterns of the macro-mesosegregations have been obtained by X-ray radiography. The results show the transport effects of the flow on both the macrosegregations and the channel formation. The reversal of the TMF produces a decrease of the level of mesosegregations, namely channel formation

Direct Simulation of a Solidification Benchmark Experiment

International audienceA solidification benchmark experiment is simulated using a three-dimensional cellular automaton-finite element solidification model. The experiment consists of a rectangular cavity containing a Sn-3 wt pct Pb alloy. The alloy is first melted and then solidified in the cavity. A dense array of thermocouples permits monitoring of temperatures in the cavity and in the heat exchangers surrounding the cavity. After solidification, the grain structure is revealed by metallography. X-ray radiography and inductively coupled plasma spectrometry are also conducted to access a distribution map of Pb, or macrosegregation map. The solidification model consists of solutions for heat, solute mass, and momentum conservations using the finite element method. It is coupled with a description of the development of grain structure using the cellular automaton method. A careful and direct comparison with experimental results is possible thanks to boundary conditions deduced from the temperature measurements, as well as a careful choice of the values of the material properties for simulation. Results show that the temperature maps and the macrosegregation map can only be approached with a three-dimensional simulation that includes the description of the grain structure

Influence d'un champ magnétique glissant sur la solidification dirigée des alliages métalliques binaires

M. J.F. Pinton, Président M. G. Faivre, Rapporteur M. B. Billia, Rapporteur M. D. Camel, Examinateur M. C.A Gandin Examinateur M. R. Moreau, Directeur de thèse Mme N. Mangelinck-Noel, Co-encadrantIn the field of the metallic alloys development, principal industrial goals are the control of the metallurgical structure and defects. Hydrodynamic movements in the liquid phase have a significant influence on properties of the solidified product. In order to study the influence of a forced convection on the directional solidification of binary metallic alloys, a new device made up of a Bridgman type furnace and a Bitter coil to produce a travelling magnetic field was designed and developed. We focused our attention on two major effects influenced by the forced convection: the macrosegregation and the grain structure. The velocity and the direction of the forced flow can be controlled thanks to the application of the travelling magnetic field. We have shown that this configuration can control macrosegregations and that moreover, the dendritic primary spacing is modified according to the applied field. With regard to the grain structure, using an analytical model, we have shown that the extension of the solutale layer in front of the solidification front varies according to the direction of the flow near the mushy zone. This mechanism enhances the nucleation and the growth of equiaxed grains. In the case of non refined alloys, a mode of elongated free grains could be probably obtained by fragmentation under a range of magnetic field.Dans le domaine de l'élaboration des alliages métalliques, les principaux enjeux industriels résident dans la possibilité de maîtriser la structure métallurgique ainsi que les défauts qui surviennent lors de la phase de solidification. Lors de la solidification, les mouvements hydrodynamiques dans la phase liquide ont une influence importante sur les propriétés du produit solidifié. Dans cette étude, la conception d'un nouveau dispositif BATMAF (Bridgman Apparatus with a Travelling MAgnetic Field) constitué d'un four de solidification dirigée et d'une bobine de Bitter permettant de produire un champ magnétique glissant a permis d'étudier l'influence d'une convection forcée sur la solidification dirigée des alliages métalliques binaires. Notre attention s'est plus particulièrement portée sur deux effets majeurs influencés par la présence de convection forcée ou non : la macroségrégation et la structure de grains pour un alliage d'Al-3.5%pdsNi en présence ou non de particules affinantes. La vitesse ainsi que le sens de l'écoulement forcé peuvent être contrôlés par l'application du champ magnétique glissant. Nous avons montré que dans le cas de notre alliage, la macroségrégation peut-être contrôlée et que de plus, l'espacement primaire dendritique est modifié en fonction du champ appliqué. En ce qui concerne les structures de grains, à l'aide d'un modèle analytique, nous montrons que l'extension de la couche solutale en avant du front de solidification varie en fonction du sens de l'écoulement au voisinage de la zone pâteuse. Ceci conditionne la possibilité pour les grains équiaxes de germer puis de croître et, par conséquent, l'obtention du régime équiaxe ou non. Enfin, dans le cas des alliages non affinés, dans un domaine d'intensité de brassage, un régime de grains libres allongés a pu être obtenu probablement par fragmentation. Cette étude démontre s'il en était besoin, l'importance cruciale de la maîtrise de la convection sur les macroségrégations et la structure des grains et ouvre des perspectives quant à l'utilisation du champ magnétique glissant pour leur contrôl

Multiscale statistical analysis of the tin-lead alloy solidification process

International audienceThis paper presents a quasi-two-dimensional solidification benchmark experiment with controlled thermal boundary conditions. The experiment consists in solidifying a rectangular ingot of Sn-3wt.%Pb using two lateral heat exchangers to extract the heat flux from one or both vertical sides of the sample. The aim is to assess the reproducibility of the solidification experiments performed under rigorously identical conditions. The temperature difference (Delta T) between the two vertical sides is 40 K and the cooling rate (CR) is 0.03 K/s. This slow-cooling condition favours segregation formation. A lattice of fifty thermocouples placed on the front of the sample between the two vertical sides is used to determine the instantaneous temperature distribution. During solidification, the time evolution of the temperature field is recorded, and analyzed. This allows us to estimate the time evolution of the temperatures due to natural convection and assess the velocity field, the solidification macrostructure and the segregation behaviour. Post-mortem segregation patterns are obtained both by X-ray radiography and induction coupled plasma analysis. The recorded data is analyzed statistically. An ensemble average procedure is applied to both instantaneous temperature and post-mortem composition measurements. This makes it possible to determine the mean values of the solidification parameters as well as their standard deviations. Temperature fluctuation standard deviations are deterministic and characterize the solidification dynamics. Among other results, it is shown that mesoscale segregation locations are not fully predictable. (C) 2016 Elsevier Masson SAS. All rights reserved