FATIGUE STUDY OF TI-6AL-4V FABRICATED BY LASER POWDER BED FUSION: ROLE OF DEFECT STRUCTURE AND RESIDUAL STRESS

Since the start of the second wave of digital manufacturing, the emergence of additive manufacturing (AM), there have been numerous efforts to explore and establish the linkages between process parameters and the mechanical behavior of additively manufactured metals. Among mechanical properties, fatigue life has received much more attention because of its importance in load-bearing applications. On the other hand, decisive factors like residual stress and defect structure hinder the applicability of AM components from being used in mission-critical load-bearing applications. This paper studies defect structure and residual stress's impact on fatigue life as a necessary step to understanding the criticality of these factors on fatigue performance of Ti-6Al-4V components manufactured via laser powder bed fusion (LPBF). X-ray computed tomography (XCT) and X-ray diffraction (XRD) were performed to characterize the residual stress and defect structure of AMed Ti-6Al-4V specimens. Next, the samples underwent fatigue testing to correlate the XCT and XRD data to fatigue life. The residual surface stress showed a substantial effect on fatigue life. An obvious reduction was observed in fatigue life with the increase of residual surface stress in both scan and build directions. Additionally, average residual stress tends to reduce the fatigue life, but with a scattered behavior. Furthermore, a significant relationship between defect size and fatigue performance was observed. Maximum defect size showed a strong linear impact on fatigue life reduction. Also, a gradual decrease of fatigue life with average defect size was realized.