Design and additive manufacturing of TPMS-like cellular structures

Additive manufacturing (AM) is an emerging technology which has enabled unprecedented design freedom, allowing fabrication of intricate designs which were not feasible or possible under traditional methods. This design freedom has enabled the fabrication of specifically architected cellular structures, such as lattices or triply periodic minimal surfaces (TPMS). These cellular structures have many potential applications including lightweighting and/or creating a tuneable response to mechanical loading. This thesis contributes to the body of knowledge on additively manufactured cellular structures in three key areas related to the design and additive manufacturing of TPMS-like cellular structures. The first key area of this thesis focuses on assessing the interaction between geometry and defects for a commonly used AM technology known as Laser Powder Bed Fusion (L-PBF). L-PBF has presented significant opportunities in designing and producing high-value technical products, however commercial implementation of L-PBF requires that the inherent interaction between the manufacturing process and the designed geometry be better understood. For L-PBF, data describing this interaction is sporadically available, and is limited to assessing qualitative metrics, designed for strut-based lattices, flat surfaces, or discrete features such as the diameter of a strut or cylindrical hole. To address this need, the first part of this thesis presents a novel approach to predicting manufacturing outcomes for thin-walled structures produced in Al-10Si-Mg alloy by assessing the local inclination angle, curvature, and thickness of a surface. For L-PBF systems, inclination angle was observed to have the strongest effect on manufacturability, with the shape (defined by the Gaussian curvature) and thickness becoming important in regions of low inclination. The model was applied to a more complex geometry, a thin-walled gyroid-surface (a TPMS), successfully predict some key manufacturing outcomes and defects. This approach provides previously unavailable design guidance for additively manufacture structures, and robust experimental procedures for generating and analysing data for parts produced using AM technology. The second key area focuses on creating a repeatable, parametric approach to designing, generating and assessing various properties of the generated TPMS-like structure. TPMS present a starting point for designing an infinite number of unique cellular solids, creating the need for a structured approach to their design and evaluation. In this section a design space for TPMS-like structures is formalised by defining a set of inputs. Additionally, a set of computationally inexpensive methods for analysing cellular solids are presented and applied in a full factorial design of experiments, defining interactions between the design parameters and a set of metrics that indicate mechanical performance of a resulting design. Individual interactions are also explored in depth, to give a more complete understanding of how the input parameters can be manipulated to reach a more optimal design. As part of this study a software tool, TPMS Designer, was developed. TPMS Designer is a tool for rapidly generating, visualizing and analysing implicitly defined structures. The software allows users to parametrically modify the size, aspect ratio, rotation, and resolution of a structure. Cellular volume fraction can be adjusted using surface type and isovalue options. TPMS Designer includes tools for importing and exporting to traditional computer aided design programs. Properties of a surface such as orientation and curvature can be analysed using in-built visualisation techniques. These computational experiments are supported by a physical case study, which demonstrates how the manufacturability of a structure can be predicted and altered using the input design parameters. The third key area of investigation begins to explore the possibilities of utilising AM to produce multi-material cellular structures. Many naturally occurring materials exhibit repeating hierarchical structures, comprising of multiple constituent materials that combine to form a composite material. In this work, AM is used to fabricate periodic multimaterial structures on the millimetre scale using materials with distinctly different mechanical properties. The hybrid material architecture was developed based on the gyroid and assessed for elastic modulus and energy absorption using a combination of analytical, finite element and experimental methods. The finite element method provides the best results for predicting the effect of shape, while an analytical power-law model provided the best fit to the experimental data. Thin surface ‘double-gyroids’ have higher stiffness, strength and a distinct buckling-yield behaviour, while network type ‘single-gyroid’ provide a lower stiffness, strength and has a more gradual yielding behaviour. Using fewer unit cell repetitions causes a reduction in mechanical performance and greater error between modelling and physical experiments, caused by edge effects which occur at the open boundaries of the sample. Finally, the research knowledge is applied in case study chapters, which look to bridge the gap between and translate the results from academia to the automotive industry. Two case studies were conducted, the first is an investigation into manufacturability considerations for an AM fabricated turbocharger compressor wheel. It was shown how the analytical modelling of surface roughness (a critical property for turbo-machinery) was able to successfully predict the roughness on the as-built part. Additionally, low-computational cost analysis techniques were applied to the compressor wheel geometry, allowing "high-risk" geometric features to be identified, enabling near instant feedback on suitability of a design to AM. The second case study is an investigation into applications for using cellular structures produced using L-PBF as a scaffold for overmoulding to produce enhanced doorcheck levers. This study identified potential benefits in overmoulding of AM cellular structures, with the ability to create geometry which is more suited to the overmoulding process, and to create a finely tuneable mechanical response. These case studies were carried out in collaboration with local and international industry partners MtM and Ford from the automotive sector.