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Title: Structural integrity of additive manufactured cellular lattice structures
Author: Ibrahim, Youssef Raouf Hashem A. K.
ISNI:       0000 0004 7969 8797
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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Additive manufacturing (AM) is a digital manufacturing process used to fabricate component geometries by the successive addition of material to a substrate, in contradiction to the successive removal of material as is the case with conventional machining operations. Advantages of using AM are efficient material consumption, minimal need for purpose-built tooling, relatively high portability facilitating on-site manufacturing, and unprecedented design freedom with the capability to reproduce very complex features. An application exploiting the design freedom offered by AM is the concept of cellular lattice structures, allowing for the implementation of very effective weight optimisation and low material usage into component design. The mechanical properties of cellular lattice structures can be tailored to fit the application simply by the manipulation of the cell geometry. The laser based powder bed fusion (LPBF) AM process, consuming 316L stainless steel powder, was used during this study to build and test material samples to investigate the influence of the build process parameters on the processed material properties. The parameter settings necessary to produce the best performing material samples were identified and used to model and predict the behaviour of cellular lattice structures built using the same process. An analytical model was developed in parallel to compute the elastic response and the relative density of lattice structures with specified dimensional cell parameters. The suitability of the STL file format, the most commonly used format to communicate geometry data to AM machines, to carry and process lattice structure geometry data was assessed and improvements were suggested to overcome some of the identified limitations. Finally, the acoustic resonance (AR) characterisation method was applied to measure the elastic response of AM lattice structure samples, with comparison to quasi-static compression characterisation and finite element (FE) modelling to evaluate the suitability of AR to characterise lattice structures. AR testing exhibited sensitivity to loose powder adhesion, an anomalous phenomenon where additional material fuses to the manufactured component surface during the AM build process. This limits the use of AR testing alone quantitatively while supplying a qualitative metric for assessing the quality of an AM build. A modification to the standard AR method was suggested which allows the extraction of multiple elastic stiffness measurements from a single test by utilising the detected overtone frequencies as well as the fundamental.
Supervisor: Dear, John ; Davies, Catrin ; Hooper, Paul Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral