Papers by Author: Yves Fautrelle

Abstract: A comparison of the results of RANS k-ε and LES turbulence models was done via the simulation of the electromagnetic stirring of liquid 75,5%Ga-24,5%In alloy (in a 10 mm diameter & 30 mm high crucible) using Ansys Fluent. Each velocity component, the distribution of eddies inside the melt and other flow parameters were compared respectively. The accuracy was checked with measured angular velocity data of A. Rónaföldi. The turbulent energy spectra were also produced to see the validity of the LES models.

Abstract: In solidification processes the fluid flow occurs almost at every scale from the bulk, near the interfaces and deeply in the mushy zone. Numerical modeling is a valuable tool for understanding and master the solidification processes, however, macro-scale models are not always able to predict in detail the random behavior of the solidification process whereas models for micro scales are not capable to take into account a complex structure of flows which enter into the mushy zone. In the present paper the variety of the flows and imprints they left on solidification structure are discussed and illustrated with experimental data which naturally comprise every flow occurring in the process.

Abstract: Recently several experiments on directional solidification of Al-6.5wt.Si-0.93wt.%Fe (AlSi7Fe1) alloy were performed under terrestrial conditions and onboard the International Space Station (ISS) in the Materials Science Lab (MSL) with use of electromagnetic stirring and without it. Analysis of the samples showed that stirring with a rotating magnetic field lead to the accumulation of iron-rich intermetallics in the center of the sample and influenced the primary dendrite spacing while the secondary dendrite arm spacing were not affected. In the present paper the accumulation of the intermetallics b-Al₅SiFe in the center of the samples due to RMF stirring is demonstrated numerically and the evolution of primary and secondary dendrite arm spacing is discussed.

Abstract: It is well known that the application of a magnetic field during the growth process can have pronounced effects on cast material structures and their properties, so that magnetic fields have been widely applied since the 1950s. In the case of a permanent magnetic field, some recent results revealed a dual effect on the liquid metal flow. 1: the magnetic field has a selective damping action on the flow at the scale of the crucible, due to the Lorentz force; 2: the interaction of thermo-electro-magnetic (TEM) currents in the close vicinity of the solid-liquid interface with the applied magnetic field leads to the generation of electromagnetic forces, which act both on the liquid and on the solid at the scale of the microstructure. We present an experimental investigation of the TEM forces induced by a permanent magnetic field during columnar and equiaxed solidification of Al-4wt%Cu. In situ visualization was carried out by means of synchrotron X-ray radiography, which is a method of choice for studying dynamic phenomena. It was shown that the TEM forces were at the origin of a motion of dendritic particles, perpendicular to the direction of gravity. A heuristic analysis allowed us to estimate the fluid velocities and the velocities of the solid particles, and a good agreement was achieved with the experimental data. Similar observations were also made during equiaxed growth in a temperature gradient. The in situ observation of the grain trajectories for various values of the temperature gradient demonstrated that gravity and TEM forces were the driving forces which controlled the grain motion.

Abstract: The results of running a quasi two-dimensional experimental benchmark for horizontal directional solidification of binary Sn-3 wt.%Pb alloy in a rectangular region are presented. The computation was performed using the GIGAN software and two models to simulate the mushy zone were compared, namely, the one based on the equilibrium approach and the non-equilibrium one.

Abstract: A numerical investigation of directional solidification of Al-7wt.%Si alloy stirred by a rotating magnetic field is compared with experimental results. Experimental study of such process has revealed periodical macrosegregation in axial direction of the samples in the shape of a “Christmas tree”. Similar macrosegregation pattern is obtained in simulations for the two values of magnetic field. Numerical simulations have shown also that formation of the periodical structure depends not only on the external conditions but on the permeability of the mushy zone.

Abstract: A three-step combined analytical and numerical approach to thermal modelling of a two-heater power-down furnace for controlled directional solidification of an intermetallic alloy is proposed. An analytical sensitivity analysis of the thermal model is carried out to show the effect of adiabatic zone length, and both hot-zone and cold-zone heater temperatures, on the initial thermal gradient in the sample and on the length of melt in the adiabatic zone. The subsequent axisymmetric front tracking method (FTM) simulations of directional solidification of a binary intermetallic Ti-46at.%Al alloy show that temperature gradient in the melt declines and velocity of the solid-liquid front increases with time, thus promoting good conditions for a columnar to equiaxed transition. The proposed analytical calculations combined with full-scale numerical FTM simulations provide a convenient and predictive optimization tool for the two-heater power-down furnace design and growth conditions for the future microgravity experiments.

Abstract: Simulations of several laboratory experiments developed for the study of structure and segregation in casting are presented. Interaction between the development of dendritic grain structure and segregation due to the transport of heat and mass by diffusion and convection is modeled using a Cellular Automaton - Finite Element model. The model includes a detailed treatment of diffusion of species in both the solid and liquid phases as presented elsewhere in this volume [1]. Applications deal with prediction of columnar and equiaxed grain structures, as well as inter-dendritic and inter-granular segregations induced by diffusion and macrosegregation induced by thermosolutal buoyancy forces.

Abstract: The authors are members of the integrated project Intermetallic Materials Processing in Relation to Earth and Space Solidification (IMPRESS), funded within the European Framework (FP6). One of the aims of IMPRESS is to develop new alloys and processes for the casting of TiAl-based turbine blades for the next generation of aero and industrial gas turbine engines. Within IMPRESS, two related issues have been identified during the primary solidification stage, namely, segregation and the columnar-to-equiaxed transition (CET). The authors have set out to isolate the effects of thermo-solutal convection, by designing a microgravity experiment to be performed on a European Space Agency platform. This experiment will investigate the CET formation during solidification. It is planned to use a sounding rocket providing a microgravity time of approximately twelve minutes. The results of this microgravity solidification experiment will be used as unique benchmark data for development and validation of new computational models of TiAl solidification. This in turn will produce accurate models and ultimately new robust industrial processes by project partners in the aerospace industry. The evolution of the design of the microgravity experiment is discussed and the results of preliminary ground reference experiments are presented. Future plans and objectives for the project are also highlighted.

Abstract: This paper presents a method aimed at controlling free surface flow and stirring melt via a magnetic field induced by the permanent magnets. The rotating magnetic field (RMF) can realize the free surface shape control and the melt stirring simultaneously. Numerical model was built to analyse the magnetic field distribution. Two drivers that have the same structure were analysed and optimised. Quasi-steady-state free surface was obtained by regulating the rotating velocity of the magnetic drivers, which is proportional to the magnetic force. Solidification experiment was preformed on a platform of a mini-continuous caster. The solidifying front was observed via addition of a small quantity of Sn-wt.43%Pb into the continuous casting alloy Sn-wt.3.5%Pb, it was found that the solidifying shell grows uniformly under the condition of a proper imposition of the double-permanent-magnet-driver.