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Fusion Laser Selective de poudres de TA6V : microstructure et mécanismes de formation des porosités en lien avec les paramètres du procédé SLM et les propriétés structurales

Abstract : Selective Laser Melting (SLM) is an additive manufacturing process of metallic parts based on powder bed fusion (PBF). SLM is part of the industry 4.0 concept and is opposed to conventional manufacturing processes where the material is either subtracted, deformed or molded. From a numerical model, parts are built by material addition layer by layer. The SLM process has incontestable advantages as it offers the possibility of making small parts with complex shapes and reducing transportation costs. It also promotes mass customization and relocation of factories close to customers. However, the SLM process suffers of a lack of repeatability and control of manufactured parts which hinders its development on a larger scale. Among the possible reasons, the control of part properties is made difficult since it depends on many complex interactions between the input powders, the process parameters and microstructural features.In this work, we propose to investigate the relationships between manufacturing process parameters of Ti-6Al-4V titanium alloy parts, the mechanisms of formation of their voids, their microstructure and their structural properties. In this purpose, a parametric study was conducted to evaluate the influence of laser power, scanning speed and hatch-spacing parameters. Three energy densities were considered (50, 75 and 100J/mm3), and a rotative scanning strategy was chosen.From an original approach based on 2D fracture surface and 3D-Xray tomography analysis of parts, two mechanisms of pore formation were identified and investigated. For 50J/mm3, the spatial distribution of voids is shown to be dependent to the scanning strategy. The volume fraction and density of pores depend on the laser power and the scanning speed. For higher energy densities (75 and 100 J/mm3), two new mechanisms of pore formation were identified corresponding to over-melting and gas trapping.Concerning the microstructure, the whole characterized specimens have shown a weakly textured α’ martensitic structure. For a same energy density, prior-β grain morphology and α’ needle size were revealed to be dependent to the process parameters. Fine investigations carried out by Transmission Electron Microscopy underscored the presence of un-identified twins in literature. They are first, twins of type {01̅11} <2̅111̅>a’ or {01̅11} <21̅1̅1>’ with a rotation of 57° around the [21̅1̅0]a’ zone axis and a twinning shear of 0,542, and secondly, twins of type {011̅1}<7̅ 21 14 11>a’ or {1̅011}<11 14 1̅ 0>a’ with a rotation of 63° around the [12̅2̅1]a’ zone axis and a twinning shear of 0,963. These twins would indicate that a stress relaxation phenomenon takes place, induced by the high cooling rates and the martensitic transformation.Concerning mechanical properties, porosity affects the material ductility. Its behavior is more brittle even when the volume fraction of voids is small (<1%). 3D reconstruction of a tensile broken specimen attests a one to one relationship between the spatial distribution of pores and the crack path. Finally, pores formed by a lack of fusion and denudation are more detrimental for mechanical properties than over melting ones. This analyze is also performed by comparison with the volume fraction and the morphology of voids corresponding to each mechanism of formation.
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Jonathan Stef. Fusion Laser Selective de poudres de TA6V : microstructure et mécanismes de formation des porosités en lien avec les paramètres du procédé SLM et les propriétés structurales. Matériaux. Université de Bordeaux, 2018. Français. ⟨NNT : 2018BORD0433⟩. ⟨tel-02063477⟩

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