HCF assessment of additively manufactured notched specimens in Inconel 718

Exploiting the potential of metal Additive Manufacturing (AM) mainly means the ability to produce complex geometries in high-performance materials. It implies that AMed components present several notches (holes, grooves, and shoulders), both on external and internal surfaces. A fatigue assessment of AMed components of industrial interest must deal with notch effects: multiaxial stresses and stress concentration. Inconel 718 is a widespread material for AMed components used in high-temperature applications, which typically feature complex geometries and notches. Criteria developed for dealing with notches in traditionally manufactured components, such as the Average Strain Energy Density (ASED), can be extended to AMed specimens including the effects introduced by the manufacturing process. In the present work, it is analyzed the room temperature HCF behavior of as-built notched and smooth cylindrical specimens in the framework of the criteria present in the literature. Three geometries of V-notches, resembling the ones typically present in industrial components and featuring a k_t in the range between 1.5 and 3.0, were considered. High cycle fatigue tests were carried out at room temperature in an axial load configuration with a stress ratio of 0.05 and a loading frequency of about 150 Hz, by using a resonant machine. It was analyzed the life region between 10^4 and 10^6 cycles. Fractographic analyses, carried out with a Scanning Electron Microscope (SEM), were used to identify the nucleation and crack propagation region, as well as the presence of the defects in proximity to the fracture onset. Microstructural investigations were also carried out to investigate grain dimension and growth direction, as well as the presence of internal porosity and hot tearing cracks. Optical microscope analyses were finally used to investigate the surface quality in proximity to the fracture nucleation regions.