High cycle fatigue in selective laser melted Ti-6Al-4V

Abstract

A major drawback of additively manufactured metallic components is their poor high cycle fatigue (HCF) resistance, which is primarily due to the presence of porosity in them. Keeping this in view, the effect of process parameters such as laser power (w), layer thickness (t), and scan rotation (ϕ) on pore size, shape and distribution in selectively laser melted (SLM) Ti-6Al-4V alloy specimens and the influence of such pore characteristics on the HCF life under rotating bending fatigue conditions were investigated in this work. X-ray tomography was used to characterize the porosity in coupons produced using four different w-t-ϕ combinations. The possibility of enhancing the fatigue strength (σf) of the as-fabricated alloy through microstructural modification, via a post-fabrication heat-treatment that substantially improves the threshold for fatigue crack initiation, and subsequent shot peening were explored. Results show that the pore sizes and distribution are sensitive to the process parameters utilized; pores align in the build direction for ϕ = 90° whereas they are randomly distributed for ϕ = 67°, and a higher t results in denser parts. These observations are rationalized by recourse to the combined effect of the Marangoni convection and the Rayleigh instability in adjoining melt-pools. Only a marginal improvement in σf upon heat treatment was noted, whereas shot peening enhances it substantially such that σf is up to 55% of the tensile strength. These results are analyzed using the fracture mechanics-based K-T (Kitagawa-Takahashi) approach that is based on the El-Haddad formula.