Influence of post-processing on very high cycle fatigue resistance of Inconel 718 obtained with laser powder bed fusion

Abstract

The pore defects and surface roughness derived from additive manufacturing restrict applications in the production of ultra-long service time parts. Due to the high sensitivity of very high cycle fatigue (VHCF) resistance to defects, smaller defects may also become potential crack initiation sites. To remove these defects and consequently improve fatigue strength, three different post-processing routes, hot isostatic pressing (HIP, 1160 °C/1500 bar/3h ± 30 min), machining, and their combinations, are employed on samples in this research. Three-dimensional X-ray tomography (3D-XRT) shows that pores larger than 25 μm in diameter are mainly located in the “macropore-rich layer” with 300 μm thickness below the surface. HIP can effectively close the pores, but this does not translate into an increase in fatigue performance, which is attributed to the increased roughness. Machining with a certain depth can significantly reduce the surface roughness and remove the “macropore-rich layer.” Therefore, it can improve fatigue resistance. The co-effect of HIP and machining is confirmed to be the most effective solution to improve fatigue resistance. Taking the depth-width ratio of the micro-depression as the characteristic size of the surface roughness, it found that the fatigue life is closely related to the characteristic size of the micro depression. For the pore-induced fatigue failure, the fatigue resistance is related to the pore size and the location. The area of the crack initiation (rough area, abbreviation as RA) is introduced and Areapore/AreaRA is calculated to reflect the pore characterization including the size and location, and the ratio of the pore area to the RA area is found to be have a linear relationship to the fatigue life in single logarithmic coordinates.