Fatigue crack growth parameters of Laser Powder Bed Fusion produced Ti-6Al-4V

The value of fatigue predictive NASGRO models hinges on the quality of established parameters to describe the behaviour of the subject material. To establish these parameters for laser powder bed fusion (LPBF) produced Ti6Al-4V the influence of process inherent microstructure, residual stress, and orientational dependant mesostructure needs to be considered. Examining near-threshold in combination with steady-state fatigue crack growth rates (FCGR's) offers insight into the impact of crack closure in the NASGRO formulation. This study investigates the influence of microstructure, residual stress and orientation of LPBF produced Ti-6Al-4V on establishing parameters for NASGRO models from a foundational perspective. Near-threshold crack growth rate data from Compact Tension (CT) coupons built in three orientations and as-fabricated (AF), stress-relieved (SR) and duplex annealed (DA) state are considered. Optical and scanning electron micrographs are used to consider morphological features. Results show anisotropy in material constraint, used in describing crack closure, between orientations in AF and SR state, lending to unique consideration for each orientation. DA is shown to relieve this effect. Overall, this study establishes and demonstrates the importance of uniquely considering NASGRO parameters for LPBF produced Ti-6Al-4V.