Role of defects and microstructure on the fatigue durability of Inconel 718 obtained by additive layer manufacturing route

A Laser Powder Bed Fusion Inconel 718 produced with intentionally non-optimized parameters is investigated to understand the role of defects and microstructure on the high-cycle fatigue life. Samples from three built directions and three heat treatments were used for fatigue tests at room and 650 degrees C under R=0.1$$ R=0.1 $$. Printing orientation has a minor effect on fatigue life at both temperatures. Most samples failed from lack of fusion defects, carefully analyzed by scanning electron microscope (SEM) observations. Surface defect leads to transgranular fracture surface, while subsurface initiation leads to crystallographic fracture surface. S-N$$ S-N $$ curves related to hot isostatic pressure condition shown that defects have higher influence at 20 degrees C$$ {20}<^>{{}<^>{\circ}}\mathrm{C} $$ than at 650 degrees C. Kitagawa diagrams are plotted for both temperatures to quantify the influence of defect size on fatigue limit. Natural cracks are observed at the surface in order to discuss the relative part of initiation life in the whole fatigue life. Fatigue is governed by lack of fusion (LoF) ranging from 200 to 600 mu mThe fatigue limit is higher at high temperature compared with ambient temperatureHot isostatic pressure (HIP) treatment improves the fatigue resistanceNatural crack initiated from LoF propagates in a very similar way compared with long crack