Additive Manufacturing of Inconel 718 Cellular Structures

Abstract

Additive Manufacturing (AM) is getting adopted in various industries like automobile, aerospace, medical, nuclear, defense, and space, amongst many others, in the last five to ten years due to the tremendous advantages of cost and flexibility. However, AM adoption rate in the aerospace industry is relatively lower than a few other sectors. The current work focuses on significant aspects of AM that impact the aerospace industry. The thesis explores AM technology's historical development at the outset, focusing on various processes specific to metals. Among the multiple benefits of AM, the aspect of light-weighting the aerospace components using cellular structure stands out. Nickel superalloy Inconel 718 is one of the critical materials used to address the high-temperature, high-corrosion, high-strength, and high-toughness requirements of aerospace gas turbine engines, constituting more than 30% of the weight. More than a thousand manufacturing process parameters impact the microstructure. The repeatability and reproducibility of mechanical properties of AM Inconel 718 components are widely studied topics. The thin sections of cellular structures add additional complexities, such as geometrical nonlinearities, which are not well explored in the literature. The application of AM Inconel 718 cellular structures is the primary subject of this thesis. The details related to the proper process parameters are presented for the powder bed fusion AM process. Specimens are designed to evaluate the process capability to print planar, cylindrical, and spherical cellular structures. The Inconel 718 AM printed planar cellular specimens are then evaluated through quasi-static tensile and compression tests. Thin-walled cellular structures along with disk and ring specimens are subjected to Split Hopkinson Bar (SHPB) impact loads to understand the high strain rate 'material' properties w.r.t to the cellular geometry. Impact test on Inconel AM printed hemispherical cellular dome structure using a high-speed steel ball projectile is performed to understand the energy absorption characteristics. Light-weighting structures using lattice geometries is still an art. The Topology Optimization technique to be used in conjunction with lattice structures is explored. Homogenization is a key enabler for computationally efficient and practically meaningful topology optimization. Thus, the Variational Asymptotic Method (VAM) is therefore proposed to render a simultaneously accurate and efficient homogenization of thin-walled lattice structures. Finally, the thesis presents inferences from various design and experiments conducted as part of this work. It proposes a road map for the design and development of lightweight turbine disk containment rings for gas turbine engines. It suggests topics for potentially rewarding future research in this area.