Papers by Author: Bernard Billia

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Authors: Guillaume Reinhart, Henri Nguyen-Thi, Brice Sarpi, Aboul Aziz Bogno, Bernard Billia
Abstract: Temperature Gradient Zone Melting (TGZM) occurs when a liquidsolid zone is submitted to a temperature gradient and leads to the migration of liquid droplets or channels through the solid, up the temperature gradient. TGZM has a major influence on the preparation of the initial solid-liquid interface during the stabilization phase following the directional melting of an alloy and is at the origin of the diffusion of solute towards the top part of the mushy zone. TGZM is also causing the migration up the temperature gradient of dendrite secondary arms during directional solidification, which can have a significant impact on the micro-segregation pattern of the final microstructure. In this communication we report on a directional solidification experiment carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France) on Al4.0 wt.% Cu alloy to study the dynamics induced by the TGZM phenomenon on an equiaxed grain that nucleated in front of a columnar structure. Based on in situ experimental observations obtained by synchrotron X-ray radiography, the dissolution of the bottom part of the equiaxed grain is characterized and measurements are compared with predictions of the TGZM theory in diffusive regime.
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Authors: H.J. Jung, Nathalie Mangelinck-Noël, Henri Nguyen-Thi, Nathalie Bergeon, Bernard Billia, Adeline Buffet, J. Baruchel
Abstract: The columnar to equiaxed transition (CET) has been widely studied for many years [1] because this phenomenon is observed in metallurgical applications like castings. In non refined alloys, detachment of dendrite fragments is the most probable mechanism responsible for the formation of an equiaxed microstructure [1]. In this frame, melt convection influences the grain structure evolution by playing a role in the fragmentation phenomena [2].
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Authors: Charles-André Gandin, Bernard Billia, Gerhard Zimmermann, David J. Browne, M.D. Dupouy, G. Guillemot, Henri Nguyen-Thi, Nathalie Mangelinck-Noël, Guillaume Reinhart, Laszlo Sturz, Shaun McFadden, Jerzy Banaszek, Yves Fautrelle, K. Zaïdat, A. Ciobanas
Abstract: The main objective of the research project of the European Space Agency (ESA) - Microgravity Application Promotion (MAP) programme entitled Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL) is the investigation of the formation of the transition from columnar to equiaxed macrostructure that takes place in casting. Indeed, grain structures observed in most casting processes of metallic alloys are the result of a competition between the growth of several arrays of dendrites that develop under constrained and unconstrained conditions, leading to the CET. A dramatic effect of buoyancy-driven flow on the transport of equiaxed crystals on earth is acknowledged. This leads to difficulties in conducting precise investigations of the origin of the formation of the equiaxed crystals and their interaction with the development of the columnar grain structure. Consequently, critical benchmark data to test fundamental theories of grain structure formation are required, that would benefit from microgravity investigations. Accordingly, the ESA-MAP CETSOL project has gathered together European groups with complementary skills to carry out experiments and to model the processes, in particular with a view to utilization of the reduced-gravity environment that will be afforded by the International Space Station (ISS) to get benchmark data. The ultimate objective of the research program is to significantly contribute to the improvement of integrated modelling of grain structure in industrially important castings. To reach this goal, the approach is devised to deepen the quantitative understanding of the basic physical principles that, from the microscopic to the macroscopic scales, govern microstructure formation in solidification processing under diffusive conditions and with fluid flow in the melt. Pertinent questions are attacked by well-defined model experiments on technical alloys and/or on model transparent systems, physical modelling at microstructure and mesoscopic scales (e.g. large columnar front or equiaxed crystals) and numerical simulation at all scales, up to the macroscopic scales of casting with integrated numerical models.
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Authors: Cedric Weiss, Nathalie Bergeon, Nathalie Mangelinck-Noël, Bernard Billia
Abstract: The properties of structural materials are to a large extent determined by the solid microstructure so that the understanding of the fundamental physics of microstructure formation is critical in the field of materials engineering. A directional solidification facility dedicated to the characterization of solid-liquid interface morphology by means of optical methods has been developed by CNES in the frame of the DECLIC project. This device enables in situ and real time studies on bulk transparent materials. The aim of the project is to perform experiments in microgravity to eliminate the complex couplings between solidification and convection and to get reliable benchmark data to validate and calibrate theoretical modeling and numerical simulations. Presently, ground experiments are performed to finalize the design and the experimental procedures and to guarantee the accuracy of the measurements. These experiments also provide reference data for the study of solidification microstructure dynamics in the presence of buoyancy-driven natural convection. Recent progress is presented concerning the control of the interface shape (critical for pattern analysis), the selection of single crystal of defined orientation (critical for dendritic growth) and the analysis of the dendrite shape.
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Authors: Guillaume Reinhart, Henri Nguyen-Thi, J. Gastaldi, Bernard Billia, Nathalie Mangelinck-Noël, Thomas Schenk, J. Härtwig, J. Baruchel
Abstract: Solidification is a dynamic phenomena and, as a consequence, it is of major interest to be able to investigate this process by in situ and real time observation. With synchrotron sources, this can be achieved by applying X-ray Imaging techniques (Radiography and Topography). Hence it is possible to follow the dynamical selection of solidification pattern on metallic alloys and to observe strain effects during growth process. In this paper, we present results obtained by using separately the two imaging techniques for the study of the microstructure formation during Al – Ni alloys solidification.
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Authors: Henri Nguyen-Thi, Bing Hong Zhou, Guillaume Reinhart, Bernard Billia, Q.S. Liu, C.W. Lan, Tatyana Lyubimova, B. Roux
Abstract: This paper presents a summary of cellular and dendritic morphologies resulting from the upward directional solidification of Al – Ni alloys in a cylindrical crucible. We analysed the coupling of solid-liquid interface morphology with natural and forced convection. The influence of natural convection was first analyzed as a function of growth parameters (solute concentration, growth rate and thermal gradient). In a second step, the influence of axial vibrations on solidification microstructure was investigated by varying vibration parameters (amplitude and frequency). Experimental results were compared to preliminary numerical simulations and a good agreement is found for natural convection. In this study, the critical role of the mushy zone in the interaction between fluid flow and solidification microstructure is pointed out.
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Authors: Waldemar Wołczyński, Bernard Billia, K. Rabczak
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Authors: Adeline Buffet, Henri Nguyen-Thi, Aziz Bogno, Thomas Schenk, Nathalie Mangelinck-Noël, Guillaume Reinhart, Nathalie Bergeon, Bernard Billia, J. Baruchel
Abstract: In the present study, we report on an image analysis procedure, which enables to extract from synchrotron radiographs the long range solute profiles in the whole sample and in both phases (solid and liquid). This image analysis is based on the measurement of local density differences, and is applied to study the directional solidification of Al - 4wt% Cu alloy, from planar to onset of the initial instability. Dedicated experiments were carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). In order to validate this analysis the value of a key solidification parameter, namely the partition coefficient, was experimentally determined during the planar solidification, and a very good agreement was found with value found usually in the literature. On a further step, the evolution of the microstructure and solute profile during the initial transient of solidification was analysed in detail.
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