In the field of metal casting, solute composition inhomogeneities at the macroscale are called macrosegregation, and the transition from the elongated grains in the outer portions of a casting to the more rounded grains in the center is termed Columnar to Equiaxed Transition (CET). Simultaneous prediction of macrosegregation and CET is still an important challenge in the field. One of the open questions is the role of melt convection on the CET and the effect of the CET on macrosegregation. A three-phase macroscale model for macrosegregation and CET was developed. The model accounts for numerous phenomena such as columnar dendrite tip undercooling, undercooling behind the columnar tips, and nucleation of equiaxed grains. This three-phase model was used to develop a less complex model that consists of two phases only and disregards undercooling behind the columnar tips and nucleation of equiaxed grains. An in-house parallel computing code on the OpenFOAM platform was developed to solve the equations of these models. The models were used to perform columnar solidification simulations of a numerical benchmark problem. It was found that the predictions of these models are nearly identical. It was also found that the dendrite tip selection parameter, which appears in the constitutive relation for the dendrite tip velocity, plays a key role in these models. With a realistic value for this parameter these models account for columnar dendrite tip undercooling, but as its value is increased in the simulations, predictions of these models converge to predictions of a model that neglects undercooling. Next, the three-phase model was used to perform CET simulations in the numerical solidification benchmark problem in the presence of melt convection. It was found that accounting for stationary equiaxed grains does not change the overall macrosegregation pattern nor the form of channel segregates. Finally, for the first time in the field of solidification, we developed accurate constitutive relations for macroscale solidification models that are based on a formal mesoscale analysis on the scale of a representative elementary volume that is used in developing volume-averaged macroscale models. This upscaling enabled us to present relations that incorporate changes in the shape of grains and solute diffusion conditions around them during growth. The models and constitutive relations we developed can now be used to predict critical phenomena such as macrosegregation, channel segregates, and CET in castings.

L'objectif de ce travail de thèse, est de développer un matériau d'apport et un mode opératoire de soudage associé permettant d'assembler des tôles d'acier K44X pour la fabrication de collecteurs d'échappement automobiles. Les propriétés de l'acier K44X ayant été optimisées pour répondre au mieux aux contraintes de l'application, les conditions de soudage recherchées devront, dans la mesure du possible, éviter de dégrader les caractéristiques de l'acier, en particulier en termes de tenue mécanique à haute température et de résistance à l'oxydation et à la fatigue thermique. Ce mémoire est divisé en quatre chapitres. Le premier chapitre est consacré à une présentation synthétique des évolutions dans le domaine de la fabrication des collecteurs d'échappement automobiles, et des connaissances actuelles dans les domaines des aciers inoxydables ferritiques et sa soudabilité, des procédés de soudage à l'arc. Le second chapitre présente les caractéristiques de l'acier K44X et la problématique de l'étude, puis décrit le travail d'élaboration des matériaux d'apport de différentes compositions. Ainsi que les résultats d'une caractérisation préliminaire des soudures obtenues avec les différents matériaux. Le chapitre 3 traite de la caractérisation de la tenue en service de l'assemblage retenu à l'issu du chapitre précédent. Les tests d'oxydation, de traction à chaud sur zone fondue des soudures ou sur assemblages complets, et de fatigue thermique, utilisés pour réaliser cette caractérisation sont décrits, et les résultats sont discutés. La fin de ce chapitre est consacrée à la caractérisation des précipités formés dans les zones fondues. Enfin, le dernier chapitre est consacré à la modélisation thermique du soudage et à la modélisation de la solidification, dans le but de tenter de prédire le type de microstructure de zone fondue formée lors d'une opération de soudage, en fonction des paramètres procédés. Cette modélisation, qui s'appuie sur les résultats d'un essai de soudage instrumenté, doit notamment permettre de prédire si les conditions de soudage, pour une composition d'acier donnée, permettent ou non de former une structure de grains équiaxe en zone fondue des soudures. / The objective of this work is to develop a filler metal and an associated welding procedure allowing to join sheets of steels K44X for the manufacturing of exhaust manifolds for automotive. The properties of the steel K44X having been optimized to answer at best the constraints of the application, the welding conditions will have to, as possible, avoid degrading the characteristics of the steel, in particular in terms of mechanical strength with high temperature, oxidation resistance and in thermal fatigue.This report is divided into four chapters.The first chapter is dedicated to a synthetic presentation of the evolutions in the field of the manufacturing of the automotive exhaust manifolds, and current knowledge in the domains of ferritic stainless steels and its weldability, and in arc welding processes.The second chapter presents the characteristics of the steel K44X and the problem of the study, then described the work of elaboration of the filler metals with various compositions. As well as the results of a preliminary characterization of the welds obtained with the various materials.The chapter 3 is about the characterization of the in-service behavior of the assembly stemming from the previous chapter. The tests of oxidation, hot traction on molten zone of the welds or on complete assemblies, and of thermal fatigue, used to realize this characterization are described, and the associated results are discussed. The end of this chapter is dedicated to the characterization of precipitates formed in the molten zones.Finally, the last chapter is dedicated to the thermal modelling of the welding and to the modelling of the solidification, with the aim of trying to predict the type of microstructure of molten zone formed during a welding operation, according to the process parameters. This modelling, based on the results of a instrumented experimental test of welding, has to allow in particular to predict if the welding conditions, for a given composition of steel, allow or not to form a structure of grains equiaxed in molten zone of the welds.

Magnésio e suas ligas têm adquirido importância cada vez mais significativa como material estrutural de peso leve despertando um singular interesse pela indústria uma vez que oferece a melhor relação peso/resistência entre os metais. Os campos mais conhecidos de sua aplicação consistem na construção de veículos, na aeronáutica, manipulação industrial (robôs, automatização) e tecnologia de comunicação. Em particular, a indústria automobilística tem crescentemente ampliado a utilização de ligas de magnésio na produção de peças que vão desde caixas de câmbio até aros de rodas. As principais razões para este desenvolvimento são: mudanças na legislação ambiental, as exigências de cliente, e objetivos corporativos que requerem veículos mais leves diminuindo o consumo de combustível. O uso do magnésio para aplicações estruturais em altas temperaturas é limitado devido a sua baixa resistência à fluência. Isso se deve ao enfraquecimento do contorno de grão a partir da precipitação descontínua da fase b-Mg17Al12 de baixo ponto de fusão. Dentre as ligas de magnésio desenvolvidas para resistência à fluência, as ligas do sistema Mg-Al-RE-Ca oferecem ótimo desempenho com resultados similares à liga de alumínio ADC12. Muitos trabalhos sobre o sistema de ligas Mg-Al-RE-Ca foram realizados visando compreender a relação entre microestruturas e propriedades mecânicas. Entretanto, poucos estudos relacionaram a influência das variáveis de solidificação na formação das microestruturas. O presente trabalho tem como objetivo realizar um estudo em uma liga Mg-4%Al-3%La-1%Ca analisando a influência das variáveis térmicas tais como taxas de resfriamento, velocidade da isoterma liquidus e gradientes de temperatura, na formação de estruturas, na transição colunarequiaxial e espaçamento dendrítico durante o processo de solidificação. A previsão das distintas estruturas, tais como zona colunar e equiaxial é de grande interesse para avaliação e projeção das propriedades mecânicas dos fundidos. Dessa forma, a liga estudada foi submetida à solidificação unidirecional vertical ascendente e análise térmica. Foram realizadas análises metalográficas nos lingotes solidificados. Os resultados colaboram para uma melhor compreensão do fenômeno de solidificação da liga e serve como ferramenta no desenvolvimento de modelos de previsões de formação de micro e macroestruturas que influenciam diretamente nas propriedades mecânicas. / Due to their superior weight/resistance relation, magnesium and its alloys have been acquiring a great deal of importance in the modern industry, specially as lightweight structural materials in the fields of vehicle construction, aeronautics, industrial robotics, automation, and communication technologies. In particular, the automotive industry has been increasingly expanding the use of magnesium alloys in the production of auto-parts, ranging from gearbox housings to steering wheels. The main reasons for this developments are changes in environmental legislations, new customer requirements, and corporate policies regarding fuel consumption and weight/power relations. The use of magnesium alloys for structural applications at high temperatures is limited due to the precipitation of the discontinuous phase b-Mg17Al12, which in fact, weakens the grain boundary during service resulting in a low creep resistance. Among the magnesium alloys developed for creep resistance, the alloys of the system Al-Mg-RE-Ca offer optimum performance with results similar to the ADC12 aluminum alloy. Many studies on the Al-Mg-RE-Ca system alloys were aimed to understand the relationship between microstructure and mechanical properties. However, few studies undertake the influence of the solidification variables in the microstructure formation. This work aims to study the influence of some thermal variables such as temperature gradients, solidification and growth tip rate on the formation of microstructures, the columnar/equiaxial transition and dendrite arm spacing, during the solidification process of a Mg-4%Al-3%La- 1%Ca alloy. The prediction of the different structures, such as the columnar and the equiaxial regions is of great interest for the assessment and projection of the mechanical properties of the casts. Therefore, the alloy studied in this work were submitted to thermal analysis during an unidirectional vertical ascending solidification, as well as optical and scanning electron microscopy characterization. The results contribute to a better understanding of the solidification phenomena of the magnesium alloys, as well as a tool in the development of numerical models for the prediction of structures which directly influence the mechanical properties of the parts.

Magnésio e suas ligas têm adquirido importância cada vez mais significativa como material estrutural de peso leve despertando um singular interesse pela indústria uma vez que oferece a melhor relação peso/resistência entre os metais. Os campos mais conhecidos de sua aplicação consistem na construção de veículos, na aeronáutica, manipulação industrial (robôs, automatização) e tecnologia de comunicação. Em particular, a indústria automobilística tem crescentemente ampliado a utilização de ligas de magnésio na produção de peças que vão desde caixas de câmbio até aros de rodas. As principais razões para este desenvolvimento são: mudanças na legislação ambiental, as exigências de cliente, e objetivos corporativos que requerem veículos mais leves diminuindo o consumo de combustível. O uso do magnésio para aplicações estruturais em altas temperaturas é limitado devido a sua baixa resistência à fluência. Isso se deve ao enfraquecimento do contorno de grão a partir da precipitação descontínua da fase b-Mg17Al12 de baixo ponto de fusão. Dentre as ligas de magnésio desenvolvidas para resistência à fluência, as ligas do sistema Mg-Al-RE-Ca oferecem ótimo desempenho com resultados similares à liga de alumínio ADC12. Muitos trabalhos sobre o sistema de ligas Mg-Al-RE-Ca foram realizados visando compreender a relação entre microestruturas e propriedades mecânicas. Entretanto, poucos estudos relacionaram a influência das variáveis de solidificação na formação das microestruturas. O presente trabalho tem como objetivo realizar um estudo em uma liga Mg-4%Al-3%La-1%Ca analisando a influência das variáveis térmicas tais como taxas de resfriamento, velocidade da isoterma liquidus e gradientes de temperatura, na formação de estruturas, na transição colunarequiaxial e espaçamento dendrítico durante o processo de solidificação. A previsão das distintas estruturas, tais como zona colunar e equiaxial é de grande interesse para avaliação e projeção das propriedades mecânicas dos fundidos. Dessa forma, a liga estudada foi submetida à solidificação unidirecional vertical ascendente e análise térmica. Foram realizadas análises metalográficas nos lingotes solidificados. Os resultados colaboram para uma melhor compreensão do fenômeno de solidificação da liga e serve como ferramenta no desenvolvimento de modelos de previsões de formação de micro e macroestruturas que influenciam diretamente nas propriedades mecânicas. / Due to their superior weight/resistance relation, magnesium and its alloys have been acquiring a great deal of importance in the modern industry, specially as lightweight structural materials in the fields of vehicle construction, aeronautics, industrial robotics, automation, and communication technologies. In particular, the automotive industry has been increasingly expanding the use of magnesium alloys in the production of auto-parts, ranging from gearbox housings to steering wheels. The main reasons for this developments are changes in environmental legislations, new customer requirements, and corporate policies regarding fuel consumption and weight/power relations. The use of magnesium alloys for structural applications at high temperatures is limited due to the precipitation of the discontinuous phase b-Mg17Al12, which in fact, weakens the grain boundary during service resulting in a low creep resistance. Among the magnesium alloys developed for creep resistance, the alloys of the system Al-Mg-RE-Ca offer optimum performance with results similar to the ADC12 aluminum alloy. Many studies on the Al-Mg-RE-Ca system alloys were aimed to understand the relationship between microstructure and mechanical properties. However, few studies undertake the influence of the solidification variables in the microstructure formation. This work aims to study the influence of some thermal variables such as temperature gradients, solidification and growth tip rate on the formation of microstructures, the columnar/equiaxial transition and dendrite arm spacing, during the solidification process of a Mg-4%Al-3%La- 1%Ca alloy. The prediction of the different structures, such as the columnar and the equiaxial regions is of great interest for the assessment and projection of the mechanical properties of the casts. Therefore, the alloy studied in this work were submitted to thermal analysis during an unidirectional vertical ascending solidification, as well as optical and scanning electron microscopy characterization. The results contribute to a better understanding of the solidification phenomena of the magnesium alloys, as well as a tool in the development of numerical models for the prediction of structures which directly influence the mechanical properties of the parts.

Magnésio e suas ligas têm adquirido importância cada vez mais significativa como material estrutural de peso leve despertando um singular interesse pela indústria uma vez que oferece a melhor relação peso/resistência entre os metais. Os campos mais conhecidos de sua aplicação consistem na construção de veículos, na aeronáutica, manipulação industrial (robôs, automatização) e tecnologia de comunicação. Em particular, a indústria automobilística tem crescentemente ampliado a utilização de ligas de magnésio na produção de peças que vão desde caixas de câmbio até aros de rodas. As principais razões para este desenvolvimento são: mudanças na legislação ambiental, as exigências de cliente, e objetivos corporativos que requerem veículos mais leves diminuindo o consumo de combustível. O uso do magnésio para aplicações estruturais em altas temperaturas é limitado devido a sua baixa resistência à fluência. Isso se deve ao enfraquecimento do contorno de grão a partir da precipitação descontínua da fase b-Mg17Al12 de baixo ponto de fusão. Dentre as ligas de magnésio desenvolvidas para resistência à fluência, as ligas do sistema Mg-Al-RE-Ca oferecem ótimo desempenho com resultados similares à liga de alumínio ADC12. Muitos trabalhos sobre o sistema de ligas Mg-Al-RE-Ca foram realizados visando compreender a relação entre microestruturas e propriedades mecânicas. Entretanto, poucos estudos relacionaram a influência das variáveis de solidificação na formação das microestruturas. O presente trabalho tem como objetivo realizar um estudo em uma liga Mg-4%Al-3%La-1%Ca analisando a influência das variáveis térmicas tais como taxas de resfriamento, velocidade da isoterma liquidus e gradientes de temperatura, na formação de estruturas, na transição colunarequiaxial e espaçamento dendrítico durante o processo de solidificação. A previsão das distintas estruturas, tais como zona colunar e equiaxial é de grande interesse para avaliação e projeção das propriedades mecânicas dos fundidos. Dessa forma, a liga estudada foi submetida à solidificação unidirecional vertical ascendente e análise térmica. Foram realizadas análises metalográficas nos lingotes solidificados. Os resultados colaboram para uma melhor compreensão do fenômeno de solidificação da liga e serve como ferramenta no desenvolvimento de modelos de previsões de formação de micro e macroestruturas que influenciam diretamente nas propriedades mecânicas. / Due to their superior weight/resistance relation, magnesium and its alloys have been acquiring a great deal of importance in the modern industry, specially as lightweight structural materials in the fields of vehicle construction, aeronautics, industrial robotics, automation, and communication technologies. In particular, the automotive industry has been increasingly expanding the use of magnesium alloys in the production of auto-parts, ranging from gearbox housings to steering wheels. The main reasons for this developments are changes in environmental legislations, new customer requirements, and corporate policies regarding fuel consumption and weight/power relations. The use of magnesium alloys for structural applications at high temperatures is limited due to the precipitation of the discontinuous phase b-Mg17Al12, which in fact, weakens the grain boundary during service resulting in a low creep resistance. Among the magnesium alloys developed for creep resistance, the alloys of the system Al-Mg-RE-Ca offer optimum performance with results similar to the ADC12 aluminum alloy. Many studies on the Al-Mg-RE-Ca system alloys were aimed to understand the relationship between microstructure and mechanical properties. However, few studies undertake the influence of the solidification variables in the microstructure formation. This work aims to study the influence of some thermal variables such as temperature gradients, solidification and growth tip rate on the formation of microstructures, the columnar/equiaxial transition and dendrite arm spacing, during the solidification process of a Mg-4%Al-3%La- 1%Ca alloy. The prediction of the different structures, such as the columnar and the equiaxial regions is of great interest for the assessment and projection of the mechanical properties of the casts. Therefore, the alloy studied in this work were submitted to thermal analysis during an unidirectional vertical ascending solidification, as well as optical and scanning electron microscopy characterization. The results contribute to a better understanding of the solidification phenomena of the magnesium alloys, as well as a tool in the development of numerical models for the prediction of structures which directly influence the mechanical properties of the parts.

Melt convection and solid phase movement play an important role in solidification processes, which significantly influence the formation of grain structures and solute segregations. In general, the melt convection and grain movement are a result of buoyancy forces. The densities within melt are different due to the variation of temperature and concentration, leading to thermally and solutally driven melt convection. Similarly, the density differences between the grains and the bulk melt cause the grain movement, leading to solid sedimentation or grain floating, as the case may be. Free, unattached solid grains are produced by partial remelting and fragmentation of dendrites, by mechanical disturbances such as stirring or vibration and by heterogeneous nucleation of grains in solidification of grain-refined alloys. In this way, movement of solid crystals during solidification can be ascertained in the following two cases. In the first case, during columnar solidification of non-grain-refined alloys, solid movement is possible in the form of dendrite fragments detached from the columnar stalks by the process of remelting and fragmentation. Movement of grains during columnar solidification gives rise to altogether different microstructure from columnar to equiaxed. In the second case, during equiaxed solidification of grain-refined alloys, the movement of solid crystals is possible in the form of equiaxed dendrite crystals nucleated due to presence of grain refiners. The rate and manner by which the free solids settle (or float) will influence macrosegregation in metal castings. Control of the solidification process is possible through an understanding of the solid movement and its effect on macrosegregation and microstructure. With this viewpoint, the overall objective of the present thesis is to study, experimentally and numerically, the phenomenon of solid phase movement during solidification. Through this study, deeper insights of the role of solid phase movement in solidification are developed which can be used for possible control of quality in castings. Both columnar and equiaxed solidification are considered. Models for transport phenomena associated with columnar solidification with solid phase movement are rarely found in the literature, because of inherent difficulty associated with consideration of microscopic features such as remelting and fragmentation. To tackle this problem, solidification modules for remelting and fragmentation are developed first, followed by integration of these molecules in a macroscopic solidification model. A Rayleigh number based fragmentation criterion is developed for detachment of dendrite fragments from the developing mushy zone, which determines the conditions favorable for fragmentation of dendrites. The criterion developed is a function of net concentration difference, liquid fraction, permeability, growth rate of mushy layer, and thermophysical properties of the material. The effect of various solidification parameters on fragmentation is highlighted. The integrated continuum model developed is applied to stimulate the solidification of aqua-ammonia system in a side-cooled rectangular cavity. The numerical results are in good qualitative agreement with those of experiments reported in literature. A gentle ramp of the mushy zone due to settling of solid crystals, as also noticed in experimental literature, is observed towards the bottom of the cavity. The influence of various modeling parameters on solid phase movement and resulting macrosegregation is investigated through a parametric study. Movement of grains during columnar solidification gives rise to altogether different microstructure and sometimes may initiate a morphological transition of the microstructure from columnar to equiaxed if the number and size of equiaxed grains ahead of the columnar front become sufficient to arrest the columnar growth. The generalised model developed, considering solid phase movement during columnar solidification is used to predict columnar-to-equiaxed transition (CET) based on a prescribed cooling rate criterion. It is found that presence of convection significantly affects the solidification behaviour. Moreover, the movement of dendrite fragments and their accumulation at the columnar front further trigger the occurrence of CET. Cooling configuration, too significantly affects the nature of CET. In unidirectional solidification cases, the locations of CET are found to be in a plane parallel to the chill face. However, for the case of the non-unidirectional solidification (as in side-cooled cavity), the locations of CET need not be in a plane parallel to the chill face. In contrast to fixed columnar solidification, equiaxed solidification is poorly understood; in particular, the phenomena associated with solid crystal movement. Movement of unattached solid crystals, formed due to heterogeneous nucleation on grain-refiners, is induced by the convective currents as well as by buoyancy effects, causing the solid to sediment or to float, depending on density of solid compared to that of the bulk melt. While moving in the bulk melt these crystals can also remelt or grow. A series of casting experiments with AI-based alloys are performed to investigate the role and influence of movement of solid crystals on macrosegregation and microstructure evolution during equiaxed solidification. Controlled experiments are designed for studying, separately, settling and floatation of equiaxed crystals for different cooling conditions and configurations. Further, these experiments are carried out in convective and non-convective cases to understand the effect of convection on solid phase movement. Temperature measurements are performed at various locations in the mould during the experiments. After the cavity is solidified, microstructural and chemical analyses of the experimental samples are carried out, several notable features are observed in temperature histories, macrosegregation pattern, and microstructures due to settling/flotation phenomenon of solid crystals. It is found that the flow behavior of solid grains has a profound influence on the progress of solidification (in terms of grain size distribution and fraction eutectic) and macrosegregation distribution. In some cases, the induced flow due to solid phase movement can cause a flow reversal. The observations and quantitative data obtained from experiments, with the help of detailed solidification conditions provided, can be used for future validations of models for equiaxed solidification. Subsequently, numerical studies are carried out, using a modified version of the macroscopic model developed for columnar solidification with motion of solid crystals, to predict the transport phenomena during equiaxed solidification. The model is applied to simulate the solidification processes corresponding to each of the experimental cases performed in this study. For a better understanding of the phenomenon of movement of solid crystals, the following two special cases of solidification are also presented: 1) without movement of solid crystals and 2) movement of solid crystals without any relative velocity between solid and liquid phases. The numerical predictions showing nature of flow field and progress of solidification are substantiated by the experimental data for the thermal analysis, qualitative microstructural Images and quantitative microstructural analysis. It is concluded, with the help of various experiments and simulations, that movement of solid crystals influences the casting quality appreciably, in terms of macrosegregation and microstructures. It is expected that the improved understanding of the role and influence of solid phase movement during solidification processes (both columnar and equiaxed) obtained through this thesis will be useful for possible control of quality of as-cast products.

Le système Ti-Al est prometteur pour la substitution aux superalliages base-Ni aéronautiques. Dans la gamme de compositions d’intérêt pour ces applications, la solidification de ces alliages débute par la phase ß (structure cubique centrée) suivie d’une transition péritectique ß + L → α (structure hexagonale). En fonction de la teneur en Al, les proportions des phases ß et α au cours de la solidification varient, et le procédé de coulée centrifuge pour fabriquer des aubes de turbine pour l’aéronautique introduit d’autres variations structurales. Le présent travail explore l’influence de la teneur en Al et de la centrifugation sur la solidification de ces alliages. Des expériences de refusion en creuset froid sous induction, de solidification dirigée en centrifugeuse de grand diamètre et de coulée centrifuge sont présentées et caractérisées. Des mécanismes potentiels pour les différences structurales observées sont proposés. Une fragmentation assistée par une teneur en Al plus élevée est observée dans les essais en creuset froid, et une réaction péritectique démarrant plus tôt est associée à une fragmentation plus efficace pour provoquer la transition colonnaire-équiaxe (TCE). Une compétition entre la convection et la sédimentation est observée lors des essais de solidification dirigée sous centrifugation, et la refusion des bras secondaires est responsable de la TCE. En coulée centrifuge, des structures hétérogènes en « ailes de mouette » à faibles teneurs en Al dépendent de la cinétique de refroidissement et de la convection. Un comportement différent sous les mêmes conditions est constaté à plus fortes teneurs en Al, transition qui semble coïncider avec le péritectique / The Ti-Al system is a promising substitute for Ni-based aeronautical superalloys. In the composition range of interest for these applications, the solidification of these alloys begins with a ß phase (body-centered cubic structure) followed by a peritectic transition ß + L → α (hexagonal close-packed structure). As a function of Al content, the proportions of ß and α phases over the course of solidification change, and the centrifuge casting process for aeronautical turbine blade fabrication introduces other structural variations. The present work explores the influence of Al content and centrifugation on the solidification process in these alloys. Remelting experiments in a cold-crucible induction furnace, directional solidification experiments in a large-diameter centrifuge and centrifuge casting experiments are presented and characterized. Potential mechanisms for the observed structural differences are proposed. Fragmentation assisted by a higher Al content is observed in cold crucible casting, and an early onset of the peritectic reaction is associated with fragmentation that more effectively provokes a columnar-to-equiaxed transition (CET). Competition between convection and sedimentation is observed in directional solidification under centrifugation, and secondary arm remelting is responsible for CET. In centrifuge casting, heterogeneous “seagull wing” structures for low Al contents depend on cooling rates and convection. A different behavior under the same conditions is noted for higher Al contents, and the transition seems to coincide with the peritectic

Les coulées industrielles permettent de distinguer deux types de structures : colonnaires et équiaxes. La mise en place de ces structures a des conséquences importantes sur les autres hétérogénéités, particulièrement les macroségrégations chimiques. Le code SOLID, développé à l’Institut Jean Lamour, permet de décrire de manière couplée la convection naturelle du liquide ainsi que la germination, la croissance et le transport des grains équiaxes. Le travail présenté a pour but de proposer une modélisation de l’apparition et de la croissance des structures colonnaires couplées à celles des grains équiaxes, permettant ainsi de prédire la Transition Colonnaire-Equiaxe (TCE) et Equiaxe-Colonnaire (ECT). La particularité du modèle est de considérer la croissance couplée des structures uniquement au niveau des pointes primaires colonnaires car c’est à cet endroit que les gradients de soluté sont les plus importants. Après validation, le modèle est appliqué à des cas de coulées industrielles de lingots d’acier et comparé à des mesures expérimentales. Il en ressort en premier lieu que sans la modélisation du mouvement des grains équiaxes, les morphologies et les ségrégations de carbone prédites ne correspondent pas à l’expérience. Par la suite, on montre que les résultats obtenus dépendent fortement du scénario d’apparition des grains équiaxes. Une germination hétérogène volumique des grains équiaxes ne permet pas de prédire la TCE expérimentale. En revanche, la fragmentation des grains, associée à un critère pour le début de la fragmentation, prédit une TCE et des ségrégations en carbone en accord avec l’expérience. On montre alors que la masselotte des lingots peut ainsi être une source importante de grains / It is possible to distinguish two main types of structures in castings: columnar and equiaxed. The dynamic set up of these structures has a strong impact on other heterogeneities, especially the chemical macrosegregations. Developed at the Institut Jean Lamour, SOLID is a numerical code that accounts for natural convection as well as the germination, growth and transport of equiaxed grains. The purpose of this work is to model the appearance and the growth of the columnar structures coupled with the description of the equiaxed grains. The model can therefore predicts the Columnar-to-Equiaxed (CET) and Equiaxed-to Columnar (ECT) Transitions. The main characteristic of the model is to consider the coupled growth of both structures only in the zone near the tips of the primary columnar dendrites. It is indeed there that the strongest solute gradients are located. The model is verified by comparing it to experiments and other models of the literature. The model is then applied to the case of industrial steel ingots and compared to experimental measurements. The first result is that without taking into account the movement of the equiaxed grain the results for equiaxed grain morphology and for macrosegregation do not agree with the measurements. Next, we find that the phenomenon considered for equiaxed grain formation is decisive for the CET prediction. When heterogeneous volumic nucleation is considered, we were not able to predict the CET correctly. However, when fragmentation at the columnar front is considered – along with a criterion for the onset of fragmentation – the results agree quite well with the experiments. It is also shown that the hot-top of ingots is consequently an important source of equiaxed grains

This study aims to improve the Additive Manufacturing (AM) design space for the popular multi-component Ni alloy Inconel 625 (IN625) thorough investigating the microstructural evolution, namely the solidification microstructure and the solid-state phase transformations during the Laser Powder Bed Fusion (LPBF) process. Highly non-equilibrium solidification and the complex reheating conditions during the LPBF process result in the formation of various types of solidification microstructures and grain morphologies which consequently lead to a wide range of mechanical properties. Understanding the melt’s thermal conditions, alloy chemistry, and thermodynamics during the rapid solidification and solid-state phase transformation in AM process will help to control material properties and even produce a material with specific microstructural features suited to a given application. This research helps to better understand the process-microstructure-property relationships of LPBF IN625. First, a set of simple but effective analytical solidification models were employed to evaluate their ability to predict the solidification microstructure in AM applications. As a case study, Solidification Microstructure Selection (SMS) maps were created to predict the solidification growth mode and grain morphology of a ternary Al-10Si-0.5Mg alloy manufactured by the LPBF process. The resulting SMS maps were validated against the experimentally obtained LPBF microstructure available in the literature for this alloy. The challenges, limitations, and potential of the SMS map method to predict the microstructural features in AM were comprehensively discussed. Second, The SMS map method was implemented to predict the solidification microstructure and grain morphology in an LPBF-built multi-component IN625 alloy. A single-track LPBF experiment was performed utilizing the EOSINT M280 machine to evaluate the SMS map predictions. The resulting microstructure was characterized both qualitatively and quantitatively in terms of the solidification microstructure, grain morphology, and Primary Dendrite Arm Spacing (PDAS). Comparing the experimentally obtained solidification microstructure to the SMS map prediction, it was found that the solidification mode and grain morphology were correctly predicted by the SMS maps. Although the formation of precipitates was predicted using the CALculation of PHAse Diagrams (CALPHAD) approach, it was not anticipated from the analytical solution results. Third, to further investigate the microsegregation and precipitation in IN625, Scanning Transmission Electron Microscopy (STEM) using Energy-Dispersive X-ray Spectroscopy (EDS), High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), Scheil-Gulliver (with solute trapping) model, and DIffusion-Controlled TRAnsformations (DICTRA) method were employed. It was found that the microstructural morphology mainly consists of the Nickel-Chromium (gamma-FCC) dendrites and a small volume fraction of precipitates embedded into the interdendritic regions. The precipitates predicted with the computational method were compared with the precipitates identified via HAADF-STEM analysis inside the interdendritic region. The level of elemental microsegregation was overestimated in DICTRA simulations compared to the STEM-EDS results; however, a good agreement was observed between the Scheil and STEM-EDS microsegregation estimations. Finally, the spatial variations in mechanical properties and the underlying microstructural heterogeneity of a multi-layer as-built LPBF part were investigated to complete the process-structure-properties relationships loop of LPBF IN625. Towards this end, numerical thermal simulation, electron microscopy, nano hardness test, and a CALPHAD approach were utilized to investigate the mechanical and microstructural heterogeneity in terms of grain size and morphology, PDAS, microsegregation pattern, precipitation, and hardness along the build direction. It was found that the as-built microstructure contained mostly columnar (Nickel–Chromium) dendrites were growing epitaxially from the substrate along the build direction. The hardness was found to be minimum in the middle and maximum in the bottom layers of the build’s height. Smaller melt pools, grains, and PDAS and higher thermal gradients and cooling rates were observed in the bottom layers compared to the top layers. Microsegregation patterns in multiple layers were also simulated using DICTRA, and the results were compared with the STEM-EDS results. The mechanism of the formation of precipitates in different regions along the build direction and the precipitates’ corresponding effects on the mechanical properties were also discussed. / Thesis / Doctor of Philosophy (PhD)