Spirrett, Fiona
(2021)
Investigations into glass additive manufacturing by selective laser melting and directed energy deposition.
PhD thesis, University of Nottingham.
Abstract
Glass is a desirable material for many industrial applications, offering unique properties such as transparency, chemical durability, and high thermal resistance. Current production of complex glass shapes is typically achieved through the use of moulds. Customisation of glass geometries therefore often increases the production cost of bespoke glass pieces. Additive Manufacturing methods are capable of fabricating complex geometries at efficient cost for small production volumes, making customisation cost-effective. The opportunities that AM presents for glass manufacturing could be exploited for a number of applications, including fabrication of customised microfluidic devices, and bespoke décor for high value glass packaging. The potential applications for additively manufactured glass parts have driven research and industry to face the associated challenges, such as thermal stresses from high temperature gradients causing parts to crack or fracture, high transmittance in the near infrared (NIR) range reducing laser absorption at certain wavelengths, and porosities and cracking compromising transparency.
In this thesis, research into glass processing by two AM techniques is presented: Selective Laser Melting (SLM), and powder-fed Directed Energy Deposition (DED). Investigations were carried out to define suitable processing parameters for SLM and DED of a common glass composition, soda lime silica. Investigations evaluated the effect of laser parameters and processing set ups on glass processing.
For SLM, soda lime silica was processed onto two substrates: soda lime silica glass and alumina. Different geometries were fabricated, including single walls, cubes, hollow cylinders, lattices, and text structures. Channel structures were fabricated to demonstrate the potential for customised glass continuous flow reactor (CFR) production by SLM. For on-glass processing, adhesion of parts to substrates was inspected, highlighting the impact of SLM processing on crack formation in glass substrates for the first time. The effect of substrate heating on glass SLM was also investigated, showing promising results on transparency and porosity in glass SLM. For applications requiring removal of parts from substrates, alumina discs provided suitable adhesion to glass powders during processing, and easy removal of parts post process. Energy densities between 80-110 J/mm3 are recommended for processing 3D structures on alumina substrates, and for on-glass processing energy densities above 28 J/mm2 were found necessary to achieve glass consolidation.
Novel glass processing by a powder-fed DED method is presented in this work, demonstrating customisation of glass bottle packaging. Process maps are presented for powder-fed DED of soda lime silica glass onto glass substrates for the first time, evaluating the effect of laser power and scan speed on glass powder consolidation and substrate cracking. Suitable processing parameters were identified, with cracking found to associate with laser power, and consolidation of glass correlating with energy density. Parameters of laser power below 115 W and energy density above 11 J/mm2 are recommended. Challenges including the transmission of laser energy through transparent feedstocks and substrates and delivery of glass powder through nozzle systems were evaluated and overcome. Darkened base plates are recommended below transparent substrates to reduce laser reflection, and a single layer of cellophane tape was used to improve glass melting and adhesion to substrates by acting as a heat source during processing.
Also highlighted in this research was the flowability of glass powder feedstocks for AM methods, and the effect of flowability on forming homogenous powder beds for SLM and achieving consistent powder delivery for DED. A case study on glass powder spheroidisation is presented, comparing methods of altering angular glass powder morphologies for improved flowability. Flame spheroidisation and plasma spheroidisation are presented as promising techniques for improving flowability of glass materials for AM, and their limitations are evaluated.
The work done during the course of this PhD contributes to understanding of glass processing by SLM and through powder-fed DED, demonstrating the potential for glass processing by these AM methods. Recommendations are made for future work to further develop these methods of glass processing, with the hopes of establishing AM as a valid technique for forming customised, complex geometries for high value applications.
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