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Evolutions microstructurales et mécaniques de l’AS7G03 modifié au strontium au cours de l’étape de forge du procédé COBAPRESSTM

Abstract : The COBAPRESSTM is a hybrid process consisting of forging a cast preform. In this thesis, we are interested in its application to make parts with an AS7G03 aluminum alloy modified with strontium. Microstructural evolutions in hot conditions and the mechanical properties resulted from are not well known for this cast alloy and constitute the present work. The first part of the study presents the results obtained by simulation with two finite elements analysis software Simufact and ABAQUS. We could see that the maximal deformation occurs in the parting surface with a deformation closed to 0.5 and a rate which cans reach 10 s-1. Deformation in the skin due to the preform surface roughness is considered. The second part exposes the microstructure in the parts after forging. These observations were made by optical micrography and also by EBSD. This microstructure is predominantly substructured but can be recrystallized for Zener-Hollomon parameters higher than 1012. The typical skin microstructure and the effect of the quenching were also studied.Finally, we studied the evolutions of the static and cyclic mechanical properties. We could see that the forging step allows the welding of the shrinkage porosities with mechanical characteristics good enough to resist fatigue tests. The yield stress, the ultimate tensile stress and the ductility are all improved. The optimal microstructure seems to be a substructure of the dendrites which is the best for a deformation of 0.5 and a rate of 5 s-1 with a preform temperature of 530°C. The good substructuration and the reduced roughness of the skin allow a greater improvement of the fatigue life of the samples after the forging step.
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Frédéric Perrier. Evolutions microstructurales et mécaniques de l’AS7G03 modifié au strontium au cours de l’étape de forge du procédé COBAPRESSTM. Autre. Ecole Nationale Supérieure des Mines de Saint-Etienne, 2013. Français. ⟨NNT : 2013EMSE0700⟩. ⟨tel-02003616⟩

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