Analysis of factors affecting fatigue performance of HIP'd laser-based powder bed fusion Ti–6Al–4V coupons

Abstract

To achieve widespread adoption of AM, industry must understand the factors that lead to intermittent incidents of low fatigue performance. To explore this, 150 fatigue coupons were constructed across six builds on L-PBF systems from two different machine manufacturers using both virgin and reused Ti–6Al–4V powder feedstock. X-ray computed tomography (XCT) was used to identify flaws in both the pre- and post-HIP condition. After HIP, machined and polished fatigue coupons were tested at a common loading condition to identify the factors that led to outliers in fatigue performance. Registration of the XCT data to fractographic failure origins revealed statistically significant reductions in fatigue lifetimes were attributable to the presence of pre-HIP lack-of-fusion flaws that were not visible in post-HIP CT scans. Post-fatigue fractography revealed that contaminants identified at the failure origin, were responsible for the largest debit on fatigue lifetimes. Finally, statistically significant differences in the fatigue performance between two L-PBF systems were believed to be attributable to differences in prior β grain texturing and interstitial oxygen levels. These findings may help guide strategies to enable broader adoption of L-PBF AM Ti–6Al–4V components in fatigue-limited applications.