Influence of heat treatment on the fatigue resistance of Inconel 718 fabricated by selective laser melting (SLM)

Abstract

In this research work, the influence of heat treatment on the fatigue resistance of Inconel 718 (IN718) fabricated by selective laser melting (SLM) and cast and wrought (C&W) process route was investigated. The optical image of the as-built SLM sample shows melt pool layers, within the melt pool layers both cellular and columnar dendritic structures are visible. The hardness value of the SLM sample under as-built condition is around 34% less compared to the reference (C&W) IN718. The as-built Inconel 718 samples were heat treated under Hot Isostatic Pressing (HIP) and Aeronautic Heat Treatment (AHT) conditions. During heat treatment the tensile residual stresses accumulated in the as-built sample was reduced close to zero. The hardness value has increased nearly equal to the hardness value of the reference sample which is due to the precipitate formation and reduced porosity during heat treatment. To evaluate the fatigue limit, the four point bending fatigue tests were carried out. Results showed fatigue limit of the C&W samples is high compared to the SLM samples due to the differences in their microstructure. Within the SLM samples, the HIP + AHT SLM has survived more fatigue cycles at high stress levels compared to the SLM HIP samples. This is due to the presence of more porosity which acted as the crack initiation site and the presence of δ phase in the SLM HIP samples.