| Author: | Fu, Jin |
| Title: | Investigating the mechanical performance of the triply periodic minimal surface based metallic lattices made by micro laser powder bed fusion |
| Advisors: | Fu, Mingwang (ME) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Minimal surfaces Additive manufacturing Manufacturing processes Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xix, 176 pages : color illustrations |
| Language: | English |
| Abstract: | Triply Periodic Minimal Surfaces (TPMS) based cellular structures, with distinct geometrical characteristics and superior mechanical performance, show a great potential for various applications in different industries. Benefiting from the development of micro LPBF (µLPBF) technology, one of the additive manufacturing (AM) technologies, TPMS metallic lattice structures with low relative density can be readily fabricated for lightweight application. However, current understanding of the LPBF process and the TPMS structures is focused on a larger scale. There is a lack of exploration of the µLPBF and the µLPBFed low-density TPMS structures. This thesis aims to fully understand the nature of µLPBF and the mechanical behavior of µLPBFed TPMS structures. Furthermore, design of TPMS-based components (i.e., conformal lattices) for AM and their mechanical responses are studied, aiming to explore the real-world application potential of TPMS-filled structures and the µLPBF technique. The µLPBF is different from the conventional LPBF (cLPBF) due to the use of finer beam size, smaller powder size and layer thickness. To understand the physical fundamentals of µLPBF in fabricating metals and alloys, this work provides a comparative study of the µLPBF and cLPBF of the well-known material, stainless steel 316L. Results show that µLPBF creates lower surface roughness, smaller grains and a cellular microstructure with smaller cell size and cell wall thickness compared with cLPBF. The yield strength of µLPBFed samples is marginally lower than cLPBFed ones, which is dominated by the difference of compositional microsegregation in the cellular structures. The cantilever distortion induced by macroscopic residual stresses is lower in the µLPBFed samples due to the smaller molten pool and more thermal cycles. To know more the mechanical response and light-weighting potential of TPMS structures fabricated by µLPBF, metallic TPMS sheet structures with different relative densities and cell orientations were fabricated by µLPBF. Low-density TPMS structures with a shell thickness as small as ~100 µm and a relative density ~5% were realized. Quasi-static compression tests and finite element modelling were conducted to study their compressive responses. Their light-weighting potential related to the scaling behavior of mechanical properties as a function of relative density was analyzed. Results show the deformation mechanisms and mechanical properties of TPMS structures are highly dependent on the relative density and cell orientation. Compared with the cLPBF, the µLPBF TPMS structures demonstrate higher mechanical properties and superior light-weighting potential. To design conformal TPMS-based components, the method based on isoparametric transformation was implemented. The effects of key design factors, including unit cell orientation, shape transformation and boundary between cells, on the mechanical properties of conformal structures were experimentally and numerically evaluated. The results show that the mechanical properties and deformation mechanism of the shape transformed structures are highly influenced by the tilting angle of the side walls and the type of shape transformation. The boundary between neighboring shape transformed TPMS does not deteriorate the mechanical properties. Finally, a case study of conformal TPMS-filled tubes was conducted to verify the effectiveness of the conformal design for mechanical applications. It is found that the conformal TPMS-filled tubes not only have the desired manufacturability by LPBF, but also show the better mechanical performance as compared with the uniform TPMS-filled counterparts. Overall, this thesis presents that better surface finish, finer microstructure, more desirable mechanical properties and smaller residual stress-induced distortion can be obtained by µLPBF. The superiority of the µLPBF technology in fabricating lightweight TPMS structures for mechanical applications is also highlighted. In addition, this work also provides quantitative correlation between the design factors and the mechanical properties of the shape transformed structures, and the potential of TPMS-based conformal design for real-world applications in various industrial sectors. |
| Rights: | All rights reserved |
| Access: | open access |
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