Fracture toughness of laser-based powder bed fusion produced Ti-6Al-4V

Abstract

The rapid solidification and cooling rates, directional cooling, and the line-by-line, layer-by-layer consolidation inherent in laser-based powder bed fusion (LPBF) create unique microstructures, often leading to high strength but limited ductility and toughness. In load-bearing applications, where strength and toughness are critical, fracture toughness is a fundamental property and is pivotal in structural design. This study examines the relationship between these unique microstructural features, the LPBF process, post-processing heat treatments, and the fracture toughness of Ti-6Al-4V. First, elongated prior-β grains induce anisotropy in fracture toughness, which can be altered by heat treatment above the β-transus temperature. Second, a below β-transus temperature heat treatment that coarsens α laths improves fracture toughness due to a combination of lower yield strength and increased ductility. This increased ductility is attributed to a reduced strength difference between larger primary and smaller secondary and tertiary laths. Third, anisotropy in the rising J-R curve behaviour is linked to a dominant ∼45° lath orientation relative to the dominant ⟨001⟩ prior-β grain texture aligned with the build direction (Z-axis). Notably, a fracture toughness of 90 MPa $\sqrt{m}$, yield strength of 964 MPa, ultimate tensile strength of 1010 MPa, and 18 % elongation after the break is achieved, which compare favourably with the properties of the wrought counterpart.