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International audience; The influence of hot-topping conditions on macrosegregation in a large industrial steel ingot is experimentally and numerically studied. Two types of insulating refractory materials are considered in the hot top - isothermic (classical hot topping) and exothermic. Unlike the isothermic material, the exothermic material releases heat into the solidifying ingot by an exothermic chemical reaction taking place during a certain period. Experimental macrosegregation and grain size at the central axis in a 6.2-ton industrial steel ingot are presented for the two hot top conditions. Thereafter, numerical studies using a multiscale model and confrontation between the experimental and numerical results are presented. In the multiscale model, the grain growth model is fully coupled with a volume-averaged two-phase macroscopic solidification model that takes into account macroscopic fluid flow, grain transport, heat transfer, and solute transport. The effects of the two hot top scenarios on the heat transfer behaviour, macrostructure and the macrosegregation are discussed. We have found that the insulating refractory material in the hot top plays an important role in the grain formation with exothermic refractory material resulting in a finer axial grain structure with more globular grain morphology and more macrosegregation. The model results well explain the experimental results for the two hot top scenarios. The exothermic chemical reaction can increase the number of nuclei, which in turn influences the grain structure and the macrosegregation in the ingot.

International audience; Although a significant amount of work has been already devoted to the prediction of the macrosegration in steel ingots, the majority of studies considered the solid phase as fixed. It has been shown that with such an assumption it is not possible to predict the macrosegregation in the centre of heavy steel ingots. The motion of the equiaxed grains is generally suspected to be the cause of this macrosegregation. We developed a multiphase and multi-scale model, which is able to describe the evolution of the equiaxed crystals and to take into account their motion. Fragments of dendrites are assumed to be at the origin of the equiaxed grains. The flow of the interdendritic liquid when the grains pile up and are packed is also described. The model was numerically implemented using a 2D finite-volume formulation. We applied the model to study the development of the macrosegregation and the macrostructure in a 65 ton steel ingot that was experimentally analyzed. The ingot was cast with particular conditions. A metallographic study was performed and the macrosegregation pattern was characterized. The macrosegregation pattern predicted by the model is presented and compared to experimental results. It is shown that in the case of such a heavy ingot, the motion of the equiaxed grains is indeed the main cause of the negative macrosegregation in the centre of the product

International audience; Since the quality of Vacuum Arc Remelted zirconium ingots is directly linked to their chemical homogeneity and metallurgical structure after solidification, it is important to relate these factors to the operating conditions. Therefore, a modeling study of the solidification process during VAR has been undertaken. To this purpose, a numerical model has been used, based on the SOLAR software developed at Nancy during the past 20 years. Solidification mechanisms recently implemented in the model include a full coupling between energy and solute transport in the mushy zone, and accounting for finite diffusion of solutes at the microscopic scale, in both solid and liquid phases. This modeling can be applied to actual multi-component alloys. In this paper, the macrosegregation in Zircaloy 4 ingots is investigated. To validate the model results, one single melt of a chemically homogeneous electrode has been specifically performed. The results show the importance of accounting for solutal convection to forecast properly the macrosegregation in zirconium ingots.

International audience; Les alliages biphasés α2/γ de base TiAl sont des candidats prometteurs pour des applications à hautes températures dans les domaines de l'aéronautiques. Leur développement reste limité du fait de la faible robustesse de la réponse aux traitements thermiques en particulier en présence de macroségrégation ou de la réactivité à haute température, limitant la surchauffe du métal liquide et donc la coulabilité de ces alliages. Les procédés de coulée centrifuge permettent un remplissage des moules à grande vitesse ce qui permet de pallier la faible coulabilité. La caractérisation de ce procédé passe par une meilleure compréhension de la formation des macroségrégations en présence de force centrifuge. Dans cette étude nous présentons les résultats de modélisation de macroségrégation prenant en compte la force centrifuge et le mouvement des grains pour un alliage de base Ti-Al-Nb.

International audience; Since the quality of Vacuum Arc Remelted ingots is linked to their chemical homogeneity and metallurgical structure, a modeling study of the solidification during VAR has been undertaken. It is based on the solution of the coupled transient heat, momentum and solute transport equations during the remelting of a cylindrical ingot. Solidification mechanisms implemented in the model include a full coupling between energy and solute transport in the mushy zone, and accounting for nucleation and finite diffusion of solutes at the microscopic scale, in both solid and liquid phases. This modeling can be applied to actual multi-component alloys. In this paper, the macrosegregation in Zircaloy 4 ingots is investigated. To validate the model, one single melt of a homogeneous electrode has been specifically performed. The comparison between the predicted macrosegregation and the experimental results shows the importance of accounting for solutal convection to forecast properly the macrosegregation in remelted ingots.

International audience; The present paper deals with the numerical simulation of a multiphase system. The solidification of a binary alloy in a rectangular domain is considered. Coupled volume averaged governing equations for mass, energy, momentum and species transfer are introduced as a governing system. The phase properties are resolved from the Lever solidification rule, the fluid dynamics in the porous mushy zone is modeled by a Darcy force and the liquid phase is assumed to behave like an incompressible Newtonian fluid. Double diffusive effects in the melt are modeled by the thermosolutal Boussinesq hypothesis. The governing equations are solved by the novel Local Radial Basis Function Collocation Method (LRBFCM). The fields of all physical quantities are represented on overlapping 5-noded sub-domains through collocation by using multiquadrics Radial Basis Functions (RBF). The involved first and second derivatives of the fields are calculated from the respective derivatives of the RBFs. The equations are solved through explicit temporal discretization and the pressure-velocity coupling is calculated through a local pressure correction scheme. The advantages of the represented meshless approach are its simplicity, accuracy, similar coding in 2D and 3D, and straightforward applicability in nonuniform node arrangements and adaptive node arrangements. The presented spatial discretization technique shows several convenient properties and possibilities for upgrade, whereas stays simple and extremely CPU efficient. On the other hand, the completely local solution procedure makes the implementation and parallelization straightforward (without solving big systems of equations).

International audience; The phenomena responsible for the formation of macrosegregations, and grain structures during solidification are closely intertwined. We present a model study of the formation of macrosegregation and grain structure in an industrial sized (350 mm thick) direct chill (DC) cast aluminum alloy extrusion ingot. The modeling of these phenomena in DC casting is a challenging problem mainly due to the size of the products, the variety of the phenomena to be accounted for, and the nonlinearities. We used a volume-averaged multiscale model that describes nucleation on grain refiner particles and grain growth, coupled with macroscopic transport: fluid flow driven by natural convection and shrinkage, transport of free-floating globular equiaxed grains, heat transfer, and solute transport. We analyze the heat and mass transfer in the slurry moving-grain zone that is a result of the coupling of the fluid flow and the grain nucleation, growth and motion. We discuss the impact of the flow structure in the slurry zone and of the grain packing fraction on the macrosegregation.