Inclusion-induced fatigue crack initiation in powder bed fusion of Alloy 718

Abstract

Fatigue crack initiation of Alloy 718 additively manufactured via electron beam-powder bed fusion (EB-PBF) process was investigated. The melt parameters were chosen to achieve sufficient energy input and minimize process-induced defects. A line offset of 200 µm with enough line energy was used, leading to the formation of wide and deep melt pools. This strategy facilitated the formation of equiaxed grains at the melt pools bottom, and short columnar grains within the melt pools aligned parallel to the build direction. The mixed grain morphology and texture were retained after various thermal post-treatments, including heat treatment (HT), hot isostatic pressing (HIP), and HIP-HT. Micron-sized non-metallic inclusions in the feedstock powder, such as Al-rich oxide and titanium nitride clustered during the EB-PBF process, and remained intact during the post-treatments. Low cycle fatigue cracks mainly originated from the non-metallic inclusions found near the surface of the test specimens. HIPing was able to remove a portion of the internal defects, including round-shaped and shrinkage pores; therefore, a small fatigue life enhancement was observed in HIP-HT compared to HT.