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Catchpole-Smith, Sam (2020) Laser powder bed fusion of lattice structures for thermomechanical applications. PhD thesis, University of Nottingham.

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Abstract

Laser powder bed fusion (LPBF) is increasingly being adopted in high-value industries such as aerospace because of its design freedoms. An example of LPBF design freedom is incorporation of lattice structures within components. These structures exist in forms including strut-type, consisting of beam elements connected at nodes, and mathematically defined surface-based geometries, where triply periodic minimal surface (TPMS) lattices are found. Strut-type lattices have already found application in end-use components where they are typically used to reduce component mass due to their high specific strength properties. Lattice structures also show promise in heat transfer applications, but there is a lack of research in evaluating performance in this area.

This thesis explores the path taken to manufacture and evaluate lattice structures manufactured via LPBF, including process development for nickel-based superalloys that are desirable in the gas turbine industry. Materials for LPBF are limited by high thermal gradients that are experienced during the process. Fractal scan strategies aiming to reduce thermal gradients were demonstrated in LPBF. This achieved a reduction in cracking of the nickel-based superalloy CM247LC. Although not successful in complete crack mitigation, the study suggested that thermal gradients in LPBF can be altered using alternative scan strategies. This finding may be extended to other alloys that are currently not processable via LPBF.

A range of lattice structures manufactured in Hastelloy-X (HX) via LPBF were evaluated for compressive performance in the as-LPBF and heat-treated conditions. It was demonstrated that TPMS lattice structures exceed the strength and modulus of strut-type geometries in every case. Further, heat treatment resulted in up to 20 % decrease in lattice modulus and 50 % decrease in lattice yield strength. This was linked to the increase in grain size observed after heat treatment. The HX lattices exhibited ductile structural compression throughout, except for a single geometry in which fracture failure of the lattice elements was observed. The failure sites were investigated using X-ray computed tomography, where shear was identified as the primary failure mode.

Custom thermal evaluation apparatus were developed, providing a capability for determining thermal conductivity and convective heat transfer performance of lattice structures. Experimentation deduced that the bulk material thermal conductivity has the primary effect on lattice thermal conductivity, with some secondary effects from natural convection that are related to the sample geometry. Design equations were developed for lattice thermal conductivity and demonstrated through manufacture and testing of samples with a specific thermal conductivity. Further, volume fraction grading was used to manufacture a lattice structure sample with a custom thermal profile in 2-dimensions.

The convective heat transfer apparatus was used to evaluate TPMS lattice structures for pressure drop and convective performance. TPMS geometries showed enhanced convective heat transfer over a pin-fin benchmark in every flow condition, particularly for applications where high pressure drop is acceptable if heat transfer is maximised. Therefore, such geometries are suitable for gas turbine component applications such as turbine blades or nozzle guide vanes, where a coolant fluid pressure drop of 3 – 5 bar is available. Calculation of thermal performance curves for each geometry provides the data in a format that is required by design engineers to implement lattice structures.

This investigation has, therefore, advanced the understanding and applicability of lattice structures under mechanical, thermal or thermomechanical applications across a range of industries. Case studies were used to demonstrate this enhanced performance through use of lattice structures in a real use application.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Clare, Adam Thomas Tuck, Christopher
Keywords:	Laser powder bed fusion; Lattice structures; Minimal surfaces; Thermal conductivity;
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Faculties/Schools:	UK Campuses > Faculty of Engineering
Item ID:	60244
Depositing User:	Catchpole-Smith, Sam
Date Deposited:	31 Jan 2023 08:54
Last Modified:	31 Jan 2023 08:55
URI:	https://eprints.nottingham.ac.uk/id/eprint/60244

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