Fatigue behavior and microstructural evolution of additively manufactured Inconel 718 under cyclic loading at elevated temperature

Abstract

This study investigates the fatigue behavior of Inconel 718 (IN718) fabricated through laser beam directed energy deposition (LB-DED) process. Fully-reversed (Rε = −1) strain-controlled fatigue tests are conducted on LB-DED IN718 specimens at 650 °C and the results are compared with those of LB-DED IN718 specimens tested at room temperature as well as the ones obtained from wrought counterparts at both room temperature and 650 °C. The microstructure is characterized after fabrication, heat treatment, and fatigue tests at elevated temperature. Microstructure characterizations reveal that as-built LB-DED IN718 has a dendritic structure. The heat treatment applied could not eliminate the dendritic structure, and needle-like δ phases are formed on the grain boundaries. The comparison of LB-DED and wrought fatigue results show that the LB-DED IN718 possesses slightly lower fatigue resistance at room temperature, while exhibits a comparable fatigue strength at elevated temperature. In the low cycle regime, the fatigue resistance of LB-DED IN718 tested at elevated temperature is somewhat inferior to the ones tested at room temperature, which can be explained by the formation and growth of brittle needle-like δ and Laves phases on the grain boundaries during fatigue testing at elevated temperature and their effects on increasing the fatigue crack growth rate. In the high cycle regime, however, LB-DED IN718 specimens tested at elevated temperature had somewhat similar fatigue lives to the ones tested at room temperature. This may be partially explained by the surface oxidation during fatigue testing at elevated temperature, which can retard the microstructurally short cracks to grow.