Direct Tension and Fatigue Characterization of Additively Manufactured Ti-6Al-4V Defects: A Microsample Approach

Abstract

Additive Manufacturing (AM) has the potential to improve readiness and increase the speed of aircraft components to the fleet with on-demand part production. In fatigue-critical applications, the lack of understanding of the AM process-structure-property relationship limits AM parts' widespread use and presents challenges for certification. It is essential to investigate the relationship between manufacturing process parameters, the resulting defects, and the defect-dependent mechanical performance of these materials, to understand the impact of manufacturing and defect formation on structural performance. In this study, Selective Laser Melting AM Ti-6Al-4V is mechanically tested using MicroTensile and MicroFatigue testing techniques. The results from microsample testing were compared to standard-size samples. By varying the processing parameters away from standard processing levels, defects were intentionally induced, creating both keyhole and lack-of-fusion type defects. The effects of defect morphology, porosity, microstructure, and heat treatment are discussed. Results show that the differences in mechanical performance are not solely due to increased porosity. While it is known that high porosity levels are unfavorable, this study shows that, at similar porosity levels, it is preferable to operate in the keyhole domain than in the lack of fusion domain. This is attributed not only to the spherical nature of keyhole defects but also to the increased crystallographic texture of keyhole material. The characterization of AM material using microsamples and standard techniques also highlighted the importance of mechanical testing across length scales. By testing the material at the same length scale as AM features, microsamples show that the results from standard size samples overlook the localized effects of AM defects. These findings illustrate that the characterization of localized mechanical behavior of AM components is critical for ensuring optimal structural performance.