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Refusion à l'arc sous vide du superalliage 718 : modélisation du procédé et étude de la formation des "white spots"

Abstract : Some very detrimental defects, called white spots, can form during the remelting of Alloy 718. Depleted in alloying elements, they are supposed to be generated by the fall of solid metallic precursors in the liquid pool. In a first step, a model aimed to simulate the remelting process, based on the SOLAR software, has been adapted to the remelting of Alloy 718, then validated by comparison between the simulation results and experimental observations of a remelted ingot. The characterization of electrodes and ingots has shown that large dendrites in the electrode could be the origin of dendritic white spots. This study also determines quantitatively the main characteristics of the three possible sources of discrete white spot precursors (electrode torus, shelf and crown) in terms of structure, composition and size. Dipping experiments of synthetic discrete white spots precursors allow a direct measurement of their melting kinetics. These results qualitatively validate a numerical model which describes the melting/solidification process of a spherical particle immerged in a liquid metal bath. Last, the behaviour of a discrete white spot in the liquid pool of a VAR ingot is determined through coupling the phase change model to a calculation of the precursor trajectory. The hydrodynamic and thermal behaviour of the liquid pool is calculated by SOLAR and the characteristics of each of the potential precursor origins are taken into account as input data. The overall model computes, for each origin, a precursor critical size leading to the formation of a white spot
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Submitted on : Thursday, March 29, 2018 - 1:50:46 PM
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Aude Soller. Refusion à l'arc sous vide du superalliage 718 : modélisation du procédé et étude de la formation des "white spots". Autre. Institut National Polytechnique de Lorraine, 2006. Français. ⟨NNT : 2006INPL019N⟩. ⟨tel-01752547⟩



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