The Effect of Powder Characteristics on Build Quality of High-Purity Tungsten Produced via Laser Powder Bed Fusion (LPBF
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UCTION
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, 3695 K), high thermal conductivity (164 Wm1 K1), and high density (19250 kg m3).[1] 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 long-term waste storage does not need to be considered and recycling methods are possible after 75 years.[2]
A.C. FIELD is with the School of Metallurgy & Materials, University of Birmingham, Edgbaston, B15 2TT, UK and also with the UK Atomic Energy Authority, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK. L.N. CARTER, N.J.E. ADKINS, M.M. ATTALLAH, and M. STRANGWOOD are with the School of Metallurgy & Materials, University of Birmingham. Contact e-mail: [email protected] M.J. GORLEY is with the UK Atomic Energy Authority, Culham Science Centre. Manuscript submitted May 24, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
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, 673 K).[3] Conventionally, powder metallurgy methods including sintering have been used, but as final machining is challenging, the complexity of component geometries has been limited.[4] The current divertor monoblock design can be seen in Figure 1; its simple shape is largely dictated by manufacturing issues. Additive manufacturing techniques including Laser Powder Bed Fusion (LPBF) offer the potential to produce components with greater complexity, such as small internal cooling channels, and without the need for low melting point binders or sintering aids (e.g., Cu, Ni).[5] Early attempts at LPBF of tungsten-based materials investigated additions of these sintering aids which acted as a binder phase, lowering the melting point and increasing processability so that densities of around 80 pct with little cracking were achieved processing at 100 W.[6,7] The high activation of Cu and Ni makes this strategy unsuitable for fusion applications. Attempts at processing both pure tungsten and molybdenum yielded low densities (< 85 pct) due to the low laser powers
(< 200 W) used [8,9] resulting in incomplete consolidation. More recent work, conducted with 300- to 500-W systems have shown improved results with densification up to 96 pct of theoretical density (TD), but suggested that cracking may be present.[10,11] Densities of 96 pct are significantly higher than previous attempts, but are still lower than those achievable in LPBF of conventional materials such as 316L where densities in excess of 99.5 pct are possible.[12] Within radiation shielding applications, it is generally known that increasing the density of a material improves its shielding effect,[13] and as a result, improved densities are of importance to the successful implementa
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