Effect of deposit thickness on microstructure and mechanical properties at ambient and elevated temperatures for Inconel 718 superalloy fabricated by directed energy deposition

The effect of deposit thickness on the microstructure and the mechanical properties of Inconel 718 (IN718) fabricated by directed energy deposition was systematically investigated in both as-deposited (AD) and homogenization + solution + aging (HSA) treated conditions. Results indicate that deposit thickness for thin parts has a more significant impact on the microstructure and subsequent room (RT) and elevated tem-perature (650 degrees C) tensile properties compared to thick parts. Applying HSA treatment can effectively homogenize the microstructure of thick builds with a microstructure transition from columnar-dendritic grain structure presented in the AD condition to recrystallized equiaxted grain structure, which results in almost similar yield strength, ultimate tensile strength and ductility of ~ 970 MPa, ~ 1237 MPa, ~ 20% at RT and ~ 816 MPa, ~ 954 MPa, ~ 12% at 650 degrees C, respectively. In contrast, thin parts maintaining columnar grain features during HSA treatment yield a higher yield strength of 1112 MPa (increased by 15%) at RT and 936 MPa (increased by 15%) at 650 degrees C, but much lower ductility of 9% (decreased by 54%) at RT and 6% MPa (decreased by 51%) at 650 degrees C. The remaining large amount of residual stress primarily contribute to the enhancement of yield strength, as well as large amount of delta phase responsible for the much worse ductility for thin parts after HSA treatment. In addition, the underlying thermal history evolution in various deposit thickness is elucidated by the thermal simulations with the use of Finite Element Method (FEM) method within 3DExperience software. The results present in the current study show the capability of the directed energy deposition process to manufacture homogeneous components with varying thickness for high temperature application after a proper heat treatment, with regard to their initial as-deposited materials.(c) 2022 Elsevier B.V. All rights reserved.