Le développement des structures de grains se formant durant les procédés de soudage par fusion a un large impact sur les propriétés et la résistance mécaniques des assemblages. Des défauts, tels que la fissuration à chaud, sont aussi liés à la texturation de grains propre à l'étape de solidification. La simulation directe du développement tri-dimensionnelle (3D) des structures de grains dans ces procédés, à l'échelle industrielle, est rarement proposée. Dans ce travail, une modélisation couplée 3D Automate Cellulaire (CA) – Eléments Finis (FE) est proposée pour prédire la formation des structures de grains dans les procédés de soudage multipasses GTAW (Gas Metal Arc Welding) et GMAW (Gas Metal Arc Welding). A l'échelle macroscopique, la modélisation FE permet la résolution des équations de conservation de la masse, de l'énergie et de la quantité de mouvement pour l'ensemble du domaine en s'appuyant sur un maillage adaptatif. Pour le procédé GMAW avec apport de matière, le modèle FE est enrichi et développé dans une approche level set (LS) afin de modéliser l'évolution de l'interface métal / air due au développement du cordon de soudure. Le domaine FE contient ainsi la pièce étudiée et l'air environnant dans lequel le cordon se développe. Les calculs FE sont couplés avec l'approche CA utilisée pour modéliser le développement de la structure de grains. Un maillage fixe (‘maillage CA') est superposé au maillage adaptatif FE (‘maillage FE'). Les champs macroscopiques propres au maillage FE sont ainsi interpolés entre le maillage adaptatif FE et le maillage fixe CA. Une nouvelle stratégie d'allocation / désallocation de la grille de cellules CA est ensuite utilisée basée sur l'allocation / désallocation des éléments du maillage CA. La grille CA est constituée d'un ensemble régulier de cellules cubiques superposées au domaine soudée. A l'échelle micro-, la grille est utilisée afin de simuler les étapes de fusion et solidification, à la frontière entre le domaine pâteux et le bain liquide, durant le processus de soudage. Les évolutions de températures des cellules sont définies par interpolation du maillage CA. Un couplage du modèle avec les chemins de solidification et les évolutions enthalpiques tabulés est aussi implémenté, permettant de suivre la thermique et les évolutions de fractions de phase propre à l'évolution du procédé. Avec de réduire les temps de calcul et la quantité de mémoire informatique nécessaire à ces simulations, une optimisation des maillages FE/CA et des tailles de cellules CA est proposée pour les deux approches FE et CA. La modélisation 3D proposée est appliquée à la simulation de la formation des structures de solidification formées durant le soudage GTAW et GMAW multipasses de pièces d'acier inoxydables de nuances UR 2202. Dans le procédé GTAW, l'influence de l'évolution des structures de grains selon les paramètres procédés est étudiée. L'orientation normale des grains avec l'augmentation de la vitesse de soudage est montrée. Dans le procédé GMAW, la modélisation permet de simuler la refusion et la croissance des grains des couches successives. De manière générale, les structures de grains prédites montrent qualitativement les évolutions attendues présentées dans la littérature. / Grain structure formation during fusion welding processes has a significant impact on the mechanical strength of the joint. Defects such as hot cracking are also linked to the crystallographic texture formed during the solidification step. Direct simulation of three-dimensional (3D) grain structure at industrial scale for welding processes is rarely modeled. In this work, a 3D coupled Cellular Automaton (CA) – Finite Element (FE) model is proposed to predict the grain structure formation during multiple passes Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding (GMAW). At the macroscopic scale, the FE model solves the mass, energy and momentum conservation equations for the whole system based on an adaptive mesh. For GMAW with metal addition, the FE model is enriched and established in a level set (LS) approach in order to model the evolution of the metal/air interface due to the weld bead development. The FE domain then contains the workpiece and the surrounding air where the weld bead forms. FE computations are coupled with the CA approach used to model the grain structure evolution. A fixed mesh, referred to as CA mesh, is superimposed to the adaptive FE mesh. FE fields are interpolated between the adaptive FE mesh and the fixed CA mesh. A new dynamic allocation/deallocation strategy of a CA grid of cells is then used based on the dynamic activation/deactivation of the elements of the CA mesh. The CA grid is made of a regular lattice of cubic cells superimposed onto the welded domain. At the micro scale, this grid is used in order to simulate the melting and solidification steps at the boundaries between the mushy domain and the liquid pool during the welding process. The temperature evolutions of the cells are computed by interpolation from the CA mesh. Coupling with tabulated transformation paths and phase enthalpy is also implemented, which permits to track the phase amount and latent heat release during the process. In order to master the resolution time and memory cost of the simulations, a management of the FE/CA mesh dimensions and CA cell size is considered for both FE and CA models. The 3D CAFE model is applied to simulate the formation of solidification structures during multiple passes GTAW and GMAW processes on a duplex stainless steel UR 2202. In GTAW, the evolution of the grain structures with respect to the welding process parameters is considered. The normal orientation of the grains with the increase of the heat source velocity is shown. In GMAW, the model is shown to compute the remelting and growth of successively deposited layers. Overall, the predicted structures qualitatively reveal the expected evolutions presented in the literature.

Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β.