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Additive Manufacturing of Tungsten via Selective Laser Melting and Electron Beam Melting

Wright, Jonathan (2019) Additive Manufacturing of Tungsten via Selective Laser Melting and Electron Beam Melting. PhD thesis, University of Sheffield.

Available under License Creative Commons Attribution-Noncommercial-No Derivative Works 2.0 UK: England & Wales.

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This thesis explores powder bed Additive Layer Manufacturing (ALM) of pure tungsten. Two ALM processes were investigated; Selective Laser Melting (SLM) and Electron Beam Melting (EBM). An optimal experimental design approach was adopted to investigate the effect of process parameters on parts produced. Analysis of the SLM process was carried out using a Response Surface Methodology (RSM). The beam power in SLM significantly effected porosity; a laser power of at least 400 Watts was required to produce near dense parts (0.23% porosity). A valid response model could not be fitted to the SLM experimental data. Cracks propagated throughout SLM components as the porosity was reduced. This was attributed to stress imparted during processing and an operating temperature below the Ductile to Brittle Transition Temperature (DBTT). An Arcam EBM system was modified in order to reduce the build volume, allowing small volume materials development. A RSM was adopted to model the effects of EBM process parameters on defects within parts. Specifically, hatch spacing, beam current, and beam speed were investigated and shown to all have a significant effect on porosity and geometric accuracy. Second order models were generated to fit the experiential data, representing the response well (R^2 = 99% and R^2 = 93%) a minimum porosity of 0.04% was achieved. The properties of EBM tungsten were characterised; three point bending and Weibull analysis was used to determine characteristic strength (340MPa). Hardness and modulus was measured via nanoidentation was found to vary as a function of the position within samples. This was attributed to residual stress imparted during processing. EBSD revealed a strong [111]texture. This was attributed to the angle of thermal gradients in the melt pool.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield)
The University of Sheffield > Faculty of Engineering (Sheffield) > Materials Science and Engineering (Sheffield)
Identification Number/EthosID: uk.bl.ethos.800558
Depositing User: Mr Jonathan Wright
Date Deposited: 02 Mar 2020 10:08
Last Modified: 01 Apr 2020 09:53
URI: http://etheses.whiterose.ac.uk/id/eprint/26247

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