Production of Ti-6Al-4V alloy by 3D electron beam melting technique and development of itspost treatments

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2019
Doğu, Merve Nur
In this thesis, the effects of production variables in electron beam melting (EBM) and post-processing on the materials’ and mechanical properties of Ti-6Al-4V alloy were investigated by considering the microstructural change. Ti-alloy samples were produced at 0o, 45o and 90o with respect to building plate. Surface polishing with HF solution was applied to reduce the surface roughness. On the other hand, post-heat treatments, namely, 2-step thermo-hydrogen processing (THP) and annealing, were conducted to improve the mechanical properties, i.e. hardness and tensile strength, and to increase fatigue life. Production at different angles had no influence on texture formation such that in all samples the texture of α-phase was commonly concentrated in (101 ̅0) plane, which was parallel to building direction. The samples produced at 0o had the coarsest and lowest microstructure and hardness value, respectively, possibly due to higher heat input during production; however, they possessed comparatively higher density and lower surface roughness. Therefore, the samples displayed high tensile strength and ductility comparable to standards. Likewise, fatigue life improved by production at 0o due to lower surface roughness and defects. THP refined the microstructure and slightly increased the hardness by altering the texture. Although tensile strength increased, ductility reduced possibly due to oxidation. Annealing coarsened the microstructure and degraded the mechanical properties. On the other hand, both post heat treatment processes did not influence the fatigue life due to predominant effect of high surface roughness. Despite the reduced surface roughness by surface polishing, fatigue life decreased because of dissolved hydrogen in samples.