Metrology of Additive Manufactured Lattice Structures

Additive Manufacturing (AM) has proved efficient in many medical, aerospace, and automotive applications. While most critical AM parts still require a case-by-case verification, medium size productions have been proven successful and future plans of mass customisation and quality inspection protocols are being drawn. AM is beneficial and cost effective to use in low volumes or when parts have highly functional complex features like topology optimised shapes or lattice structures. While AM has been existing for more than three decades, the usual high cost, especially of Powder Bed Fusion (PBF) means that the use of advanced design techniques, like the incorporation of functional lattice structures, is necessary to capitalise on the technology investment. However, while design and manufacturing capabilities has significantly increased in the last decade, especially with methods like design for AM (DfAM), the metrology side is still falling behind, especially when it comes to internal features or complex geometries like lattices. This challenge has led further academic and industrial research in metrology related to AM, which is sometimes referred to as “design for metrology”. This has been done by understanding the quality measurement tools, considering them from the beginning of AM process and also by using AM benchmark artefacts followed by adequate measurement strategies. The lack of standards related to AM and to non-destructive evaluation (NDE) tools like X-ray Computed Tomography (XCT) meant that further research still has to be done in this field. XCT still currently lacks from the challenge of being heavily relying on user experience, which increases chances of human error. Another current challenge in the AM field is the lack of tools allowing for considering or designing the expected manufacturing defects like dimensional deviation or surface topography in the design phase, making most current design simulations inaccurate as they are done on perfect computer-aided design (CAD). Finally, and since there is still no unique AM benchmark artefact that is standardised and can be used for all processes, multiple designs are currently suggested in literature, although, none of them being mainly focused on lattices with clear and appropriate measurement strategy. This thesis reports on development of novel protocol that can assist XCT users to optimise scan process settings in a cost effective and timely manner using 2D image analysis prior to reconstruction. This is especially critical when using lattice structures since XCT is the only tool that can give a holistic analysis as well as reach internal features or re-entrant ones. The technique has been initially tested on machined parts and further developed to work for lattice structures. This work has increased the efficiency and optimised the dimensional metrology process of lattices. After proposed method related to dimensional metrology, a method was developed to extract surface data of AM lattices using XCT alongside a script developed to allow the design of AM PBF like surfaces on any CAD using areal surface parameters as inputs. The method was then further optimised and adapted to work for the CAD of lattice structures which have different up skin and down skin surface values. Subsequent to proposed dimensional and surface research studies, an AM lattice benchmark artefact design and measurement strategy has also been developed, which was an ideal way to complete the overall research study. The novel design has a gradual strut diameter and is the first AM benchmark artefact suggested in literature that is solely made for lattices. The measurement strategy has used ISO/ASTM 52902:2019 as a guideline to develop lattice specific measurement methods using XCT. This sequence of connected research experiments has been designed to focus specifically on AM lattice structures, providing adequate and efficient methods in dimensional and surface metrology fields using XCT. The research is completed by a novel AM lattice benchmark artefact that is parametric and not process specific, which was printed in this research in both PBF and Fused Deposition Modelling (FDM) processes. The developed research has been chosen to be relevant in industrial scenarios where cost effectiveness is essential. The work presented in this thesis represents a milestone in research related to XCT dimensional and surface metrology linked to lattices as well as research related to AM benchmark artefacts. Further research in this field can accelerate the transition and use of efficient AM protocols and adoption of lightweight and highly functional lattice structures, accompanied by reliable processes from the design stage to metrology one.