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Post-processing of additively manufactured Ti-6Al-4V: improving the mechanical properties of near-net shape parts

par De Formanoir De La Caze, Charlotte
Président du jury Hendrick, Patrick
Promoteur Godet, Stéphane
Publication Non publié, 2017-12-07

Thèse de doctorat

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Résumé :

The recent breakthroughs of Additive Manufacturing (AM) have shed light on the ever more versatile technologies this term encompasses. AM, popularly known as 3D printing, offers distinct benefits compared to traditional manufacturing, such as reduced design constraints, “complexity for free” and waste reduction. This “bottom-up” strategy differs from the more constraining “top-down” approach used in traditional manufacturing. Among the many AM processes developed for metals, Electron Beam Melting (EBM) and Selective Laser Melting (SLM) are powder-bed fusion processes allowing complex three-dimensional geometries to be produced from the selective melting of successive layers of metal powder. EBM and SLM are the two most widely used AM processes for the production of critical Ti-6Al-4V parts, particularly for the biomedical and aeronautic industries. Despite their many advantages, these technologies present severe limitations that remain to be addressed. These include the presence of build defects in the material and a high surface roughness, which is inherent to powder-bed fusion processes.Moreover, the microstructure of as-built EBM or SLM Ti-6Al-4V is far from being optimized. In order to improve the material properties of additively manufactured Ti-6Al-4V parts, postprocess treatments can be considered. This thesis aims at investigating the impact of such treatments on the microstructure and mechanical properties of Ti-6Al-4V produced by EBM. After characterizing the microstructure, texture, and tensile properties of as-built Ti-6Al-4V, the effect of standard post-treatments, such as Hot Isostatic Pressing (HIP) and surface machining, are quantified on simple geometries. The interest of HIP is clearly demonstrated, especially when combined to improvement of the surface finish via machining. The removal of critical defects from both the bulk and the surface results in a substantial increase in ductility. Removal of the rough surface via machining also increases the mechanical efficiency of the parts. Regarding microstructural optimization, considering the impossibility of applying hot working on near-net shape parts as a major limitation, innovative heat treatments have to be specifically developed for additively manufactured Ti-6Al-4V. In this thesis, dual-phase alpha+alpha’ microstructures are generated, by performing subtransus annealing followed by water quenching. Depending on the annealing temperature, a broad range of mechanical properties are obtained. For annealing temperatures of 900 to 920°C, a simultaneous increase in ultimate tensile strength and ductility is achieved. The existence of a mechanical contrast between the soft alpha’ martensite and the harder alpha phase is clearly demonstrated and partly explains the remarkable work-hardening behaviour of this heterogeneous material. Further annealing of this out-of equilibrium alpha+alpha’ microstructure generates various microstructures. In the continuous process of martensite decomposition, precipitation hardening strengthens the alpha’ phase. Eventually, bimodal microstructures consisting in coarse primary alpha and fine secondary alpha+alpha' can be engineered, without involving any hot working in the process. Post-processing is also performed on more complex structures, namely additively manufactured lattices. Since machining cannot be performed on such intricate geometries, a chemical etching procedure inducing a substantial and homogeneous decrease in surface roughness is developed. Dissolution of surface defects results in an improvement of the mechanical efficiency of the structure. As a result, when chemical etching is performed, the relative stiffness approaches that of an ideal structure. Performing subtransus annealing and water quenching in order to induce a dual-phase alpha+alpha’ microstructure substantially increases the ability of these structures to absorb energy during compression. This thesis demonstrates the interest of developing post-process treatments specifically for near-net shape additively manufactured parts. Such treatments partially address critical issues of powder bed AM, expanding the range of possible applications of additively manufactured Ti-6Al-4V.