Design and additive manufacturing of mechanical metamaterials

• Main Author: Zeng, Zhuohong
• Other Authors: Zhou Kun
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Mechanical engineering
• Online Access: https://hdl.handle.net/10356/173284
• Institution: Nanyang Technological University

Mechanical metamaterials represent a new class of architected materials that exhibit exceptional mechanical properties and unique responses such as high stiffness, extreme strength-to-weight ratios, and enhanced energy absorption capacity, achieved through rationally designed geometries instead of compositions. However, conventional manufacturing methods, such as casting, forging, and machining, are not feasible for producing such metamaterials because of their internal intricate features. Additive manufacturing (AM) materialises three-dimensional (3D) digital models into actual parts by adding materials in a layer-by-layer process. The layer-by-layer fabrication mode of the emergent additive manufacturing (AM) technology enables the creation of parts displaying intricate geometries with exceptional accuracy. Metallic metamaterials have been successfully produced using AM at different length scales, all of which demonstrate excellent mechanical properties. Moreover, owing to the fast cooling rate, additively manufactured metals possess non-equilibrium microstructural features that are sensitive to process parameters, resulting in superior intrinsic material properties. This Ph.D. research is proposed to investigate the mechanical properties of mechanical metamaterials fabricated by metal AM processes and explore design approaches in improving the mechanical performance of auxetic metamaterials. Firstly, this study investigated a specific type of metamaterial, namely, auxetic metamaterials with a two-dimensional (2D) hexachiral design. Hexachiral auxetic metamaterials can exhibit unconventional properties, such as a negative Poisson’s ratio that can achieve superior mechanical properties, including energy absorption and indentation resistance. The thickness ratio between the wavy ligament and centre ring, which is denoted by t∗, has a critical influence on the isotropic mechanical properties of the hexachiral structures. This work aims to investigate the effects of t∗ on the isotropic auxeticity of wavy hexachiral structures under tensile and compressive loadings through simulations and experiments. A parametric study on hexachiral unit cells with t∗ values ranging from 0.25 to 1.5 was conducted through finite element analysis. With a decrease in t∗ values, the deformation mechanism showed improved coiling of the wavy hexachiral structure with thinner ligaments and thicker centre rings, which exhibit better bending and rotational deformations, respectively. The wavy hexachiral structure with a t∗ value of 0.33 achieved the lowest average effective Poisson’s ratio of −0.94 and anisotropic factor of 0.00023. The tuneable isotropic auxeticity is validated by experimental values of the Poisson’s ratio and deformations of the wavy hexachiral structures, which were fabricated by laser additive manufacturing up to 3.33% tensile strain and 20% compressive strain. These findings provide insights on the design of mechanical isotropy of chiral metamaterials for additive manufacturing. Secondly, three-dimensional re-entrant lattice structures, made of stainless steel 316L, were fabricated by two mainstream powder bed fusion (PBF) techniques, namely, electron beam PBF (EB-PBF) and laser PBF (L-PBF). Different grain morphologies and crystallographic textures were found in the EB-PBF and L-PBF samples, which significantly influence their mechanical properties through microscopic deformation. The EB-PBF and L-PBF samples achieved energy absorption capacities of 627.4 MJ/m3 and 834.8 MJ/m3, respectively, at a lattice relative density of ~24%. The EB-PBF sample exhibited equiaxed and elongated grains, while elongated grains were primarily observed in the L-PBF sample. The dominant deformation mechanism of the EB-PBF sample was obtained through dislocation. In contrast, dislocations trapped inside the solidification cellular walls and deformation-induced twinning were the dominant deformation mechanisms for the L-PBF sample, which contributed to its superior compressive strength and energy absorption capacities. This work provides insights into the enhancement of the mechanical properties of additively manufactured metallic lattice structures through microstructural control. Thirdly, cubic plate-lattices with spherical holes were fabricated via L-PBF. This fabrication approach combined tailored lattice topologies with a simple process strategy inspired by crystallography—tilting the build orientation. Compared to the normal build orientation, the tilted build orientation converts the printed microstructure of the plate-lattices from (100)-dominated to (111)- and (110)-dominated crystallographic textures and significantly refines the grain size, dictating the intrinsic properties of the printed metals. The optimised plate-lattice achieves an energy absorption capacity above all reported lattices of the same material with a simultaneous increase of 31.7% and 10.7% in compressive strength and strain. The proposed printing strategy is straightforward and applicable to other lattices with customised topologies to enhance their mechanical properties. The development of additively manufactured metallic metamaterials via L-PBF technology will provide insights on the design of mechanical metamaterials for additive manufacturing. This study opens a new path to further improving the performance of mechanical metamaterials fabricated by additive manufacturing.