Effect of Pre-HIP Defects, Part Position, and Powder Quality on the Fatigue Performance of Additively Manufactured Ti-6Al-4V

A lack of pedigreed fatigue data remains one of the largest inhibitors to widespread adoption of additive manufacturing (AM) in industry. Hot Isostatic Pressing (HIP) is known to reduce variability and increase fatigue life through flaw closure and microstructural coarsening. However, little is known about the effects of the pre-HIP flaw populations on fatigue performance, and it has been demonstrated that some flaws can remain in post-HIP material. Furthermore, the effects of part position and feedstock quality have yet to be fully defined in HIP’d AM material. To this end, 75 fatigue coupons were constructed across three builds on a Renishaw AM250, a pulsed, laser powder bed fusion (L-PBF) system, with nominal processing parameters. Two of the builds were completed with virgin, Grade 23 Ti-6Al-4V powder, while the final build used heavily reused powder conforming to Grade 5 specifications as the feedstock. All coupons were scanned using x-ray computed tomography (XCT) with a voxel size of 10.5 µm, HIP’d, machined and polished, and fatigue tested at a max stress of 120 ksi with a stress ratio of 0.1. To quantify the effect of the pre-HIP flaw population, a custom, automated defect recognition (ADR) algorithm was used to identify flaws in the pre-HIP CT scans, and the effect of the flaws on the fatigue life was estimated using a fracture mechanics-inspired failure criteria. Our results indicate that both the pre-HIP flaw population and the part location on the build plate have little impact on the fatigue life of HIP’d coupons. However, specimens built using heavily reused powder show greater variability in fatigue life, highlighting the importance of feedstock quality on final part performance.