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Étude expérimentale de la tenue en fatigue de l’alliage AlSi10Mg élaboré par fusion laser de lit de poudre

Abstract : This work shows the impact of defects and microstructure on the fatigue limit of AlSi10Mg produced by Additive Layer Manufacturing (ALM). Samples are produced according to three orientations with respect to the construction plate (0 °, 45 ° and 90 °); the studied surfaces are machined or left as-built (AB) in the gauge section. The specimens are studied with or without T6 heat treatment. The study surfaces are machined or as built. Some specimens are subjected to T6 heat treatment. Before any others study, the material is characterized in connection with the process parameters through several techniques (microscopes and 3D X-ray microtomography). Regarding the fatigue, the S-N curves are established before and after T6, mainly at R = -1 under uniaxial loading. For all the fatigue test specimens, fracture surfaces analysis shows that it is always a defect that cause fatigue failure. Thus, a criterion is applied to define these critical defects (type, size morphology and position) and the fatigue limit is analyzed through the Kitagawa type diagrams. The role of the building direction on the fatigue strength is studied, before and after T6 heat treatment, for both machined and as-built surfaces. For this purpose, a sketch based on the characteristic grain size is proposed to explain post-T6 orientation effects. The contribution of the precipitation structure is also studied; as well as the role of defects (type, size, morphology and position) on the fatigue limit at different microstructural states: before and after T6. In order to understand the surface fatigue damage mechanisms, the replica method is used on a polished specimen. In this context, a propagation law of natural cracks, that is to say due to a defect inherited from the process, is identified. It makes it possible to separate the initiation and propagation phases, thus feeding the discussions on the phenomena of priming in the presence of defects. In addition, some fatigue criteria are also discussed and the Defect Stress Gradient (DSG) approach is adapted to the studied material, by taking into account the size of the crystallographic grains. In the specific case of specimens with as-built useful sections, the role of the process of suppression of the building supports on the initiation of fatigue cracks is studied; the definition of the concept of defect size in the presence of roughness, at the scale of the surface undulation, is discussed. Knowing that initiation can occurs on a surface undulation or on an isolated defect (porosity or lack-of-fusion), an experimental method is proposed to analyze the competition between these factors. In a context of industrial development, the influence on the fatigue limit of the process parameters, related to the laser (scanning speed, power and hatching distance), or powder bed (chemical composition, particle size, bed thickness) is studied, in order to feed the discussions towards the process optimization regarding the fatigue strength.
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Julius-Noël Domfang Ngnekou. Étude expérimentale de la tenue en fatigue de l’alliage AlSi10Mg élaboré par fusion laser de lit de poudre. Autre. ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2019. Français. ⟨NNT : 2019ESMA0015⟩. ⟨tel-02483178⟩

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