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

Abstract

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 α GB 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 o0014 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 ductilitydue to the removal of critical surface defectsas well as a significant increase of the apparent yield strengthcaused by the removal of a $ 150 mm 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.

Simulation of temperature, stress and microstructure fields during laser deposition of Ti–6Al–4V

“…The size of the β phase is extremely fine in our simulations, ranging between 0.2 µm and 0.3 µm and the predicted yield stress for these λ β values ranges between 500 MPa and 650 MPa. These σ y values are consistent with previous studies [3,8,31].…”

“…[3], and the yield strength values approximated from our finite element simulations are reasonable with the experiments reported in Refs. [3,8,31]. Motivated by our present simulations, we plan on experimental…”

Sliding Contact Wear Damage of EBM built Ti6Al4V: Influence of Process Induced Anisotropic Microstructure

“…This is the typical morphology of prior β grains in cross-sectional planes perpendicular to the EBM layers [19]. In both cases, the microstructure morphology is similar and represents a typical Widmanstätten or basket-weave morphology, reported in literature for Ti6Al4V alloys [22,23]. The microstructure of both EBM-X and EBM-Z consists of alternating layers of acicular α phase separated by thin layers of retained β phase.…”

Additive Manufacturing of Titanium Alloys by Electron Beam Melting: A Review

“…In this way, the microstructure obtained in an EBM‐produced specimen is different from the microstructure observed in other AM‐produced parts, such as the α ′ martensitic phase transformed from β in a SLM‐produced part due to extremely fast cooling rates in production in SLM. The texture of the parent β phase in an EBM‐produced part exhibits a strong <001> β pole along the build direction …”