Effect of Heat Treatment on Creep Deformation and Fracture Properties for a Coarse-Grained Inconel 718 Manufactured by Directed Energy Deposition
Abstract
The creep properties of a laser-directed energy deposition (L-DED) technique manufactured Inconel 718 (IN718) was investigated at 650 degrees C/700 MPa. Microstructure and creep properties of L-DED IN718 samples were tailored by various post heat treatments involving homogenization heat treatment with temperature ranging from 1080 to 1180 degrees C + double aging and hot isostatic pressing (HIP).
Microstructural changes and their influence on the creep behavior and fracture mechanism were observed and discussed. The results show that L-DED sample heat treated by a simple double aging exhibits a 49% increase in creep lifetime t(r) and a comparable creep elongation e(f) when compared to the wrought material, due to the reserved coarse dislocation cell substructure from the L-DED process.
The loss of dislocation cell structure and the coarsening of grains at higher temperature of heat treatments contributes to a shorter t(r,) epsilon(f), but faster epsilon(min) (minimum creep rate). The present work demonstrates that a simultaneous improvement of creep strength and creep elongation can be achieved in the case of a coarse-grained L-DED IN718 by a double aging treatment which can preserve both the strengthening precipitates and an appropriate size of dislocation cells.