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

Manufacturing of light-weight materials is associated with several types of casting defects during solidification. Porosity defects are common, especially in aluminum and its alloys, which initiate crack propagation and thereby cause drastic deterioration in the mechanical properties. These defects, classified as micro and macro defects (based on their sizes), are mainly governed by release of hydrogen into the liquid at the solid-liquid interface, which triggers the nucleation and growth of hydrogen bubbles in the melt. Subsequently, these bubbles interact with solidifying interfaces such as dendritic arms and eutectic fronts, leading to the formation of pores. Macroscopic defects in the form of voids are created due to solidification shrinkage. The primary focus of the present work is to develop phenomenological models for the evolution of microporosity and microstructures during solidification. The issues outlined above typically occur in multi-phase environments comprising of solid, liquid and gaseous phases, and over a range of length and time scales. Any phenomenological prediction would, therefore, require a multi-phase-scale approach. Principles of volume averaging are applied to equations of conservation to obtain single-field formulations. These are then solved with appropriate interface tracking techniques such as Enthalpy, Level-set, Volume-of-fluid and Immersed-boundary methods. The framework is built up on a standard pressure based incompressible fluid flow solver (SIMPLER algorithm) and coupled modeling strategies are proposed to address the interfacial dynamics. A two-dimensional framework is considered with a fixed-grid Cartesian co-ordinate system. Scaling analyses are performed to bring out the relative effects of various competing parameters in order to obtain further insights into this complex phenomenon. The numerical results and scaling predictions are validated against experimental observations published in literature. In literature, numerical predictions of microporosity mainly include criteria based models based on empirical relations and deterministic/stochastic models based on diffusion driven growth assuming spherical bubbles. The dynamic evolution of non-spherical bubble-metal interface in a three-phase system is yet to be captured. Moreover, several in-situ experiments have shown elongated bubble shapes during the engulfment phase, therefore a criterion to define the dependence on cooling rates and the resulting bubble morphology can possibly deliver further practical insights. We propose a numerical model for hydrogen bubble growth, its movement and subsequent engulfment by a solidifying front, combining the features of level-set and enthalpy methods for tracking bubble-metal and solid-liquid interfaces, respectively. The influx of hydrogen into heterogeneously nucleated bubbles results in growth of bubbles to sizes up to a few hundreds of microns. In the first part of this numerical study, a methodology based on the level-set approach is developed to simultaneously capture hydrogen bubble growth and movement in liquid aluminum. The solidification is first assumed to occur outside the micro-domain providing a specified hydrogen influx to the bubble-in-liquid system. The level-set equation is formulated in such a way as to account for simultaneous growth and movement of the bubble. The growth of a bubble with continuous and fixed hydrogen levels in the melt is studied. The rates of growth of bubble-liquid and solidifying interfaces are compared using an order of magnitude analysis. This scaling analysis explains the thought experiment proposed in the literature, where difference in bubble shapes was attributed to the cooling rate. Moreover, it shows explicit dependence on bubble radius and cooling rate leading to a new criterion for bubble elongation proposed in this thesis. This also highlights the comparison between solidification and hydrogen diffusion time-scales which primarily govern the competitive growth behavior. The bubble-in-liquid model is coupled with microscopic enthalpy method to incorporate effects of solidification and study the interaction of solid-liquid and bubble-liquid interfaces. The phenomena of bubble engulfment and elongation are successfully captured by the proposed model. A parametric study is carried out to estimate the bubble elongation based on different initial bubble sizes and varying cooling rates encountered in typical sand, permanent mold and die casting processes. Although simulation of microstructures has been extensively studied in the literature, very few models address the phenomena of simultaneous growth and movement of equiaxed dendrites. The presence of different flow environments and multiple dendrites are known to alter the position and shape of the dendrites. The proposed model combines the features of the following methods, namely, the Enthalpy method for modeling growth; the Immersed Boundary Method (IBM) for handling the rigid solid-liquid interfaces; and the Volume of Fluid (VOF) method for tracking the advection of the dendrite. The algorithm also performs explicit-implicit coupling between the techniques used. Validation with available literature is performed and dendrite growth in presence of rotational and buoyancy driven flow fields is studied. The expected transformation into globular microstructure in presence of stirring induced flows is successfully simulated. A simple order estimate for time required for stirring is performed which agrees with numerical predictions. In buoyancy driven environment of a settling dendrite, the arm tip speeds show expected higher velocity of the upstream tip compared to its counterpart. The model is extended to study thermal and hydrodynamic interactions between multiple dendrites with appropriate considerations for different orientations and velocities of the dendritic solid entities. The present model can be used for the prediction of grain sizes and shapes and to simulate morphological transformations due to different melt flow scenarios. In the final part, the methodology presented for growth and engulfment of hydrogen bubbles is extended to study the phenomenon of diffusion driven bubble growth occurring in direct foaming of metals. The source of hydrogen is determined by the rate of decomposition of the blowing agent. This is accounted for by a source term in the hydrogen species conservation equation, and growth rate of hydrogen bubbles is calculated on the basis of diffusive flux at the interface. The level-set method is used for tracking the bubble-liquid interface growth, and the macroscopic enthalpy model is used for obtaining heat transfer and solid front position. The model is validated with analytical solution by comparing the front position and the solidification time. The variation of foam density with a transient hydrogen generation source is studied and qualitatively compared with results reported in literature. The modeling strategies proposed in this work are generic and therefore have potential in simulating a variety of complex multi-phase problems.

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)

Scanning Laser Epitaxy (SLE) is a new layer-by-layer additive manufacturing process being developed in the Direct Digital Manufacturing Laboratory at Georgia Tech. SLE allows for the fabrication of three-dimensional objects with specified microstructure through the controlled melting and re-solidification of a metal powder placed atop a base substrate. This dissertation discusses the work done to date on assessing the feasibility of using SLE to both repair single crystal (SX) turbine airfoils and manufacture functionally graded turbine components. Current processes such as selective laser melting (SLM) are not able to create structures with defined microstructure and often have issues with warping of underlying layers due to the high temperature gradients present when scanning a high power laser beam. Additionally, other methods of repair and buildup have typically been plagued by crack formation, equiaxed grains, stray grains, and grain multiplication that can occur when dendrite arms are separated from their main dendrites due to remelting. In this work, it is shown that the SLE process is capable of creating fully dense, crack-free equiaxed, directionally-solidified, and SX structures. The SLE process, though, is found to be currently constrained by the cumbersome method of choosing proper parameters and a relative lack of repeatability. Therefore, it is hypothesized that a real-time feedback control scheme based upon a robust offline model will be necessary both to create specified defect-free microstructures and to improve the repeatability of the process enough to allow for multi-layer growth. The proposed control schemes are based upon temperature data feedback provided at high frame rate by a thermal imaging camera. This data is used in both PID and model reference adaptive control (MRAC) schemes and drives the melt pool temperature during processing towards a reference melt pool temperature that has been found to give a desired microstructure in the robust offline model of the process. The real-time control schemes will enable the ground breaking capabilities of the SLE process to create engine-ready net shape turbine components from raw powder material.

Une étude expérimentale systématique de la formation des microstructures de solidification d’alliages métalliques (Al-Cu) a été effectuée par application de la radiographie X synchrotron. La radiographie X-Synchrotron nous a donné accès à des observations in situ et en temps réel qui nous ont permis d’analyser de manière quantitative les phénomènes physiques impliqués au cours de la solidification (vitesse de croissance, redistribution du soluté, interaction entre grains équiaxes etc.). Elle nous a également permis de mettre en évidence l’influence de la convection naturelle et de la gravité sur ces différents paramètres physiques et par conséquent sur la formation de la microstructure de solidification. Nous avons comparé nos résultats expérimentaux avec des modèles de prédiction de la croissance dendritique et ensuite avons montré l’intérêt des expériences en microgravité. Nous avons enfin effectué des séries de tests du dispositif expérimental conçu et développé par SSC (Swedish Space Corporation) dans le cadre du projet XRMON (In situ X-Ray MONitoring of advanced metallurgical processes under microgravity and terrestrial conditions) de l'ESA-MAP en vue d’une expérience in situ et en temps réel de solidification en microgravité à bord d’une fusée sonde Maser12. Cette expérience prévue en Novembre 2011. Les résultats obtenus lors des séries de tests valident le dit dispositif en termes de comportement thermique et d’imagerie X par radiographie. / A systematic experimental study of the formation of solidification microstructures of metallic alloys (Al-Cu) was carried out by the application of synchrotron X-ray radiography. Synchrotron X-ray radiography gave access to in situ and real time observations which allowed us to quantitatively analyze the dynamical physical parameters involved in the solidification process (growth rate, solute redistribution, equiaxed grain interaction etc). It also allowed to show the influence of natural convection and gravity on these various physical parameters and consequently on the formation of the solidification microstructures. Our experimental results were compared with models predictions of the dendritic growth and the necessity of microgravity solidification experiments was evidenced. Finally series of tests were carried out on the experimental setup designed by the Swedish Space Corporation (SSC) in the framework of the project XRMON (In situ X-Ray MONitoring of advanced metallurgical processes under microgravity and terrestrial conditions) of ESA-MAP for in situ and real time solidification experiments under microgravity conditions on board a sounding rocket Maser12. Maser12 mission is scheduled for November 2011. The tests results validated the experimental setup in term of imaging and thermal behaviour.

La radiographie X synchrotron ou avec une source microfocus a été appliquée pour étudier différents phénomènes dépendants du temps en relation avec la solidification directionnelle d'alliages Al-Cu. Les effets de la gravité ont été étudiés par comparaison d'expériences sur Terre et en microgravité dans le cadre du projet ESA-MAP XRMON. Les mouvements des fragments sont le sujet majeur de notre étude. Sur Terre, le mouvement des fragments est imposé par la poussée d'Archimède, avec une forte influence des effets de paroi et de la morphologie du fragment, alors qu'en microgravité, la force motrice pour le mouvement des fragments est l'écoulement du fluide interdendritique induit par la contraction du solide. L'effet d'un champ magnétique permanent sur la solidification des grains équiaxes dans un gradient de température a été également étudié. Nous avons montré qu'un couplage entre le gradient de température et le champ magnétique donne naissance à une force Thermo-électromagnétique qui agit sur les grains solides. Une bonne description a été obtenue en utilisant un modèle analytique pour une particule sphérique. Enfin, nous avons étudié l'évolution d'une zone pâteuse dans un gradient de température fixe. Trois régimes successifs ont été identifiés, suivant l'intensité de la diffusion du soluté dans la zone pâteuse et dans le bain fondu. L'analyse quantitative des radiographies par traitement d'image a clarifié le rôle de chaque phénomène de diffusion (TGZM, fermeture des canaux, murissement et diffusion du soluté dans le bain fondu). / X-ray radiography with synchrotron and microfocus sources was applied to investigate various time-dependent phenomena related to directional solidification of Al-Cu alloys. Gravity effects were investigated by a comparative study of ground and microgravity experiments in the framework of ESA-MAP XRMON project. Fragment motion was the major subject of our investigation. On Earth, fragmentation motion was imposed by buoyancy, with a strong dependency on wall influence and fragment morphology, whereas in microgravity conditions, the driving force for fragment motion is the interdendritic fluid flow induced by the solid shrinkage. The effect of a permanent magnetic field on the solidification of equiaxed grains in a temperature gradient was also studied. We have shown that a coupling between the temperature gradient and the magnetic field gives birth to a Thermo-Electro-Magnetic force that acts on the solid grains. A good description was obtained by using an analytical model for a spherical particle. Finally we studied the evolution of the mushy zone in a fixed temperature gradient. Three successive regimes were identified, depending on the relative magnitude of solute diffusion in the mushy zone and in the bulk liquid. Quantitative analysis of radiographs by image processing enlightened the role of each diffusion phenomena (TGZM, channel closure, coarsening and solute diffusion in the bulk liquid).

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