Doctor of Philosophy (PhD)
Civil & Environmental Engineering
A current focus in the additive manufacturing (AM) research community is the production of complex shaped metallic parts and cellular structures. Many researchers went on analyzing and simulating these structures without considering the effect of the element thickness and heterogeneous printing parameters on the resulting material behavior. To advance the application of laser powder bed fusion (LPBF), an AM process, in producing Inconel718 (IN718) complex shaped parts and cellular structures, this study aimed to investigate the effect of thin-walled elements thickness on the mechanical characteristics of both the individual elements and overall cellular structure. A multiscale experimental evaluation of the microstructure and mechanical properties of LPBF-ed IN718 thin elements and hexagonal honeycomb lattice structures was performed in this work. Through nanoindentation tests and electron backscatter diffraction (EBSD) Analysis, distinct zones of mechanical hardness and microstructure were observed across the element thickness. This inhomogeneity was attributed to the variations in the energy density input between the interior area and the borders of the cross-section during the LPBF process. When the element thickness was reduced to 0.4 mm, the entire cross-section is scanned using border-scanning energy, the microstructure morphology and texture exhibited significant differences compared to the typical morphology and texture of LPBF-ed larger ingots and parts. The temperature and rate indentation size effect model TRISE by Voyiadjis et el.  was modified to include the effect of the extrinsic element size and used to evaluate the material intrinsic variable length scale. Furthermore, the performance of the LPBF-ed IN718 honeycomb structures was evaluated through quasi-static compression test. The results showed that IN718 cellular structures demonstrated excellent energy absorbing characteristics comparable to other additively manufactured alloys. The coupling between the chosen geometry and the resulting v microstructure/parent material properties was evaluated and demonstrated to be the reason behind many famous analytical models failing to predict the mechanical performance of additively manufactured structures. A modification to the Gibson-Ashby  model for stochastic foams was proposed and shown to predict the current results. Finally, a comprehensive evaluation of the effect of the element thickness, orientation, and testing temperature on the mechanical behavior of thin-walled elements was performed.
Abo Znemah, Reem Abdelkarim, "Size Effects in Additively Manufactured Inconel-718 Thin Elements and Cellular Structures" (2023). LSU Doctoral Dissertations. 6251.
Voyiadjis, Geroge Z.
Available for download on Sunday, August 11, 2024