Laser Powderbed Fusion (LPBF) of tungsten and tungsten alloys for nuclear fusion applications

Field, Amanda Catherine (2020). Laser Powderbed Fusion (LPBF) of tungsten and tungsten alloys for nuclear fusion applications. University of Birmingham. Ph.D.

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Tungsten is a candidate material for the plasma facing components (PFCs) within a nuclear fusion reactor as a result of its high melting point (3420°C), high thermal conductivity (170 Wm−1 K−1), and high density (19.4 gcm−3). These allow the components to survive the operating temperatures as well as providing effective radiation shielding and conduction of heat through the components. The comparatively low activation of tungsten means that longterm waste storage does not need to be considered and recycling methods are possible after 75 years. There are difficulties associated with the processing of tungsten, however, as a result of its high melting point and intrinsic brittleness (Ductile–Brittle Transition Temperature (DBTT) ~ 400 °C). Conventionally, powder metallurgy methods including sintering have been used, but as final machining is challenging, the complexity of component geometries has been limited. The simple shape of the current divertor monoblock design is largely dictated by manufacturing issues. Interstitial impurity elements had been shown to significantly worsen the mechanical behaviour of tungsten.

It was considered likely that cracking would be an issue for LPBF tungsten due to the high cooling rate, the residual oxygen content and its sensitivity to thermal shock. Thus, this study was designed as a feasibility study to investigate the potential of LPBF processing of tungsten. While high densities (98 % theoretical density) were achieved, there was a significant issue with cracking. The cracking was found to be hot cracking of the grain boundaries and was caused by segregation of oxygen causing embrittlement. The effect of raw material quality on LPBF processability was also investigated and it was found building with spherical powder resulted in improved part quality with 8.8 % higher densities achieved. This was determined to be due to a higher effective laser absorptivity determined through penetration depth measurements into the build plate.

As it had been shown that LPBF processing of unalloyed tungsten resulted in significant cracking, methods to improve the manufacturability were investigated; the use of bed heating at 400 °C and 600 °C and alloying with 10 w.t.% Ta, through elemental powder blending were trialled. The parts produced with bed heating were found to have a modified cracking structure with a central region which had finer cracking. However, they suffered from extensive oxidation, which resulted in an increase in porosity. The single scan tracks showed significant issues with surface tension which prevented accurate measurement. Alloying with tantalum was found to be more successful with a significant reduction in cracking. It was shown that within the W-Ta alloy oxygen did not segregate to the grain boundaries and instead formed discrete clusters through the material. This was found to have a maximum load three times higher than that of unalloyed tungsten during small punch testing but was still significantly weaker than conventionally produced pure tungsten.

While improvements were made to LPBF processing of tungsten and its alloys, significant improvement would be needed to have a process sufficiently robust for fusion components particularly with regard to oxygen embrittlement. It is likely specialised hardware would need to be designed in order to effectively solve this issue.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Licence: All rights reserved
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Metallurgy and Materials
Funders: Engineering and Physical Sciences Research Council, Other
Other Funders: United Kingdom Atomic Energy Authority
Subjects: T Technology > TN Mining engineering. Metallurgy


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