Fatigue prediction through quantification of critical defects and crack growth behaviour in additively manufactured Ti-6Al-4V alloy

The study presents a methodology for predicting the fatigue life and identifying critical defects in additively manufactured (AM) Ti-6Al-4V alloy processed via laser powder bed fusion (L-PBF). Predictions were made on LPBF Ti-6Al-4V alloy in two conditions: as-built and heat-treated by using X-ray mu-Computed Tomography ( mu-CT) for the quantification of the defects and fatigue crack growth (FCG) data. For validation, fatigue life predictions were made on the same specimens on which mu-CT was conducted prior to fatigue testing. FCG and fatigue tests ( S- N ) highlighted differences in the performance between the as-built and heat-treated conditions: the as-built condition had a lower threshold stress intensity factor range ( Delta K t h ) of 2.16 MPa m 1/2 than that of the heattreated condition, 4.96 MPa m 1/2 . Differences in fatigue limit were attributed to differences in Delta K th in as-built and heat-treated specimens. To gain mechanistic understanding of these differences near Delta K th cracks were examined using Electron Backscatter Diffraction - Kernel Average Misorientation (EBSD-KAM). It was found that upon heat treatment, the dislocation density increased around near threshold fatigue cracks. Increases in Delta K th are attributed to the increases in beta -phase content and alpha-lath thickness caused by heat treatment.