Effects of heat treatment on structure-property relationships of additively manufactured Inconel 718, The

Abstract

To date, standard heat treatments (SHTs) for cast and wrought (C&W) conditions have been utilized for additively manufactured (AM) materials. However, it is known that AM processes induce high thermal gradients and rapid cooling rates that produce fundamentally different microstructure consisting of hierarchical features, epitaxial grain growth, and strong crystallographic fiber texture with respect to the build direction. Since these microstructures differ from their C&W forms, their reactions to SHTs should be inherently different as well. Understanding these differences is vitally important to producing high-quality, superior AM parts. Hence, the overarching objective of this dissertation is to elucidate the structure-property relationships of a precipitation-hardenable alloys subject to post-processing heat treatments—the Ni-based superalloy, Inconel 718. Specifically, studies of Inconel 718 focused on the development of modified heat treatments to better suit the underlying AM microstructure. Detailed analysis of the microstructural and mechanical responses led to the design of a new heat treatment that enhanced the monotonic tensile properties of AM Inconel 718 over its C&W form through the balance of precipitation strengthening and preserving the AM process-induced cellular substructure. Post-mortem investigation of the variously heat-treated conditions of the initial study brought to light the influences the cellular substructure, microchemical landscapes, and strengthening precipitates on the deformation mechanisms observed. Lastly, the industry standard and modified heat treatments from the initial study were evaluated for their high temperature performances against monotonic and cyclic fatigue loading conditions.