Date of Award

3-2020

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Aeronautics and Astronautics

First Advisor

Ryan P. O'Hara, PhD

Second Advisor

Anthony N. Palazotto, PhD

Abstract

The nickel-based superalloy Inconel 718 (IN718) is an excellent candidate among aerospace alloys for laser powder-bed fusion (LPBF) manufacturing. As-built LPBF IN718 has a vertically aligned columnar (001) microstructure which translates into orthotropic mechanical behavior. The post-process heat treatments for IN718 were developed 60 years ago for wrought and cast processes and do not mitigate the columnar microstructure of the LPBF process. Recrystallization is required to remove the columnar microstructure, which would allow for parts to be fabricated on different machines or in different orientations but still achieve the same properties. This research investigated the microstructure of LPBF IN718 as it evolved under a solution treatment of 1160 °C. It was shown that this higher solution temperature mitigated the scan strategy effects and anisotropy resulting from the fabrication process. The grain size, shape, and recrystallization were measured and compared throughout the evolution. Additionally, the X–Y and X–Z planes were compared to find the point in time at which the annealing process resulted in equiaxed, isotropic grains. An equiaxed microstructure was successfully achieved through recrystallization and grain growth. Isotropic tensile properties were achieved following a modified solution treatment at 1160 °C for 4 hours and validated via nanoindentation and tensile testing. Rupture life was not improved by the equiaxed microstructure. Microstructural evolution was simulated in a kinetic Monte Carlo simulation using a novel approach of combining the stored energy of the as-built LPBF IN718 with the boundary energy and pinning particles within SPPARKS. The resulting models accurately approximated the experimental results of recrystallized area and JMAK model constants.

AFIT Designator

AFIT-ENY-DS-20-M-274

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