Selective laser melting-fabricated Ti-6Al-4V alloy: Microstructural inhomogeneity, consequent variations in elastic modulus and implications

Abstract

Additively manufactured Ti-6Al-4V implants represent a premier application of metal additive manufacturing (AM) since 2007. Microstructural inhomogeneity can noticeably affect one or more aspects of their mechanical or biomedical performance. This paper investigates the microstructural inhomogeneity in selective laser melting (SLM)-fabricated Ti-6Al-4V using micro X-ray diffraction (μ-XRD) and the consequent changes in elastic modulus (concerns are trivial with other mechanical properties of SLM Ti-6Al-4V implants). Cylindrical specimens of ϕ12 mm × 20 mm (few Ti-6Al-4V implants are thicker than 20 mm) were manufactured by SLM for detailed charaterization. μ-XRD (beam size: 500 μm) was applied to 40 uniformly spaced regions at different build heights, assisted with microscopic examination. The distribution of lattice parameters for each phase was mapped out based on a detailed analysis of the μ-XRD two-dimensional (2D) diffraction data (ring patterns). The bottom half of the cylindrical specimen consisted of essentially α and β while α′-martensite was prominent above the mid-height, with the top 3.5-mm thick region being fully martensitic. In relation to this, the elastic modulus was found to decrease almost linearly from the mid-build height to the top. It is further shown that the μ-XRD 2D ring patterns offered an improved approach to characterising the β-phase in SLM Ti-6Al-4V. The implications of these findings are discussed explicitly.