Influence of crystal microstructure and defects on fatigue property of additively manufactured Alloy 718 via electron-beam powder bed fusion

In this study, the influences of multiple crystal microstructures and defects on the fatigue properties of Ni-based superalloy 718 additively manufactured by the electron-beam powder bed fusion (EB-PBF) method were investigated. To investigate the influence of crystal microstructure and defects, three types of crystal microstructure specimens, i.e., columnar, equiaxed, and mixed, were built by adjusting the process parameters. Some specimens were treated with hot isostatic pressing (HIP) to reduce defects. Fatigue tests were conducted on these specimens at 25°C, and the loading direction was parallel to the building direction. After the tests, the fatigue crack initiation and propagation processes were investigated. The results showed that most of the fatigue cracks initiated from clustered inclusions formed perpendicular to the building direction. These were composed mainly of Al-rich oxides, which remained and affected the fatigue crack initiation even after HIP treatment. Fatigue crack propagation characteristics differed among the microstructures, with fine equiaxed grains exhibiting higher resistance to propagation. However, the area where the defects initiated fatigue cracks was considered the most significant factor influencing fatigue life. The clustered inclusions were related to melting and solidification phenomena, which may lead to microstructural inhomogeneity, including lack of fusion defects, and thus affected crack initiation during fatigue. Since grain boundaries influence crack propagation, the morphology of the crystal grains is also considered to affect crack propagation.