Electron beam melted Ti-6Al-4V: Microstructure, texture and mechanical behavior of the as-built and heat-treated material

Electron Beam Melting (EBM), a powder bed additive layer manufacturing process, was used to produce Ti-6Al-4V specimens, whose microstructure, texture, and tensile properties were fully characterized. The microstructure, analyzed by optical microscopy, SEM/EBSD and X-ray diffraction, consists in fine α lamellae. Numerical reconstruction of the parent β phase highlighted the columnar morphology of the prior β grains, growing along the build direction upon solidification of the melt pool. The presence of grain boundary αGB along the boundaries of these prior β grains is indicative of the diffusive nature of the β→α phase transformation. Texture analysis of the reconstructed high temperature β phase revealed a strong <001> pole in the build direction. For mechanical characterization, tensile specimens were produced using two different build themes and along several build orientations, revealing that vertically built specimens exhibit a lower yield strength than those built horizontally. The effect of post processing, either mechanical or thermal, was extensively investigated. The influence of surface finish on tensile properties was clearly highlighted. Indeed, mechanical polishing induced an increase in ductility - due to the removal of critical surface defects - as well as a significant increase of the apparent yield strength - caused by the removal of a ~150 µm rough surface layer that can be considered as mechanically inefficient and not supporting any tensile load. Thermal post-treatments were performed on electron beam melted specimens, revealing that subtransus treatments induce very moderate microstructural changes, whereas supertransus treatments generate a considerably different type of microstructure, due to the fast β grain growth occurring above the transus. The heat treatments investigated in this work had a relatively moderate impact on the mechanical properties of the parts.