3D Printing of Refractory Alloys and Role of Oxygen Uptake

3D printing can revolutionise manufacturing by increasing speed at reduced energy input and efficiently using resource-intensive materials. But it is most commonly applied to commodity metals based upon iron (Fe) or aluminium (Al) thus compromising its displacing of more traditional manufacturing routes. Our strategy – underpinning the context of the proposed research – is to target it instead on refractory alloys for which conventional processing – involving casting, forging and subtractive manufacturing – is hampered by their very high (>2000oC) melting points. Refractory metals e.g. tungsten, tantalum and niobium can withstand the challenging environments of ultra-high temperature, the human body or outer space. Here, superior metallic products are needed to support the advanced economies of the UK and Japan, where high-value manufacturing is critical for competitiveness. Examples are plasma-facing components for nuclear fusion reactors, hot sections of thrusters for rockets and human implants. Through the design of hollow structures, it is even possible to embed further functionalities, such as thermal management, light-weighting, and biocompatibility. Nevertheless, processing-related defects, e.g. cracks and pores, threaten the application of this transformative technology. The proposed research will improve the quality of refractory metal parts by elucidating the fundamental causes of defects and propose pragmatic strategies to mitigate their formation. The key challenge: due to their high affinity for oxygen, 3D printing of refractory alloys can cause embrittlement and hence hamper properties e.g. strength, fracture toughness and ductility. Oxygen pick-up can occur during (i) the powder-making process – its surface area is large – and (ii) additionally the subsequent printing. How to mitigate this risk? Ingenious approaches will be attempted, using insights into the chemical processes at play. Controlling the oxygen level is crucial to enable advanced application of these alloys, since otherwise manufacturing defects such as fissures and cracks arise, caused by large cooling rates and stress build-up by differential thermal contraction. This project’s broad aim is the successful 3D printing of refractory alloys, promoting it as the accepted method for production. We will do this via the specific objectives of (i) elucidating the risk factors for oxygen uptake, (ii) mitigation of them by modelling-inspired approaches and (iii) demonstration of proof-of-concept via the execution of defect-free 3D printing. We are proposing three coherent work packages spanning manufacturing, characterisation and mitigation which utilise unique expertise from UK and Japan partners. We have three major goals: (1) to quantify oxygen intake by different manufacturing routes, including laser-powder bed, electron beam melting, hot isostatic pressing and spark plasma sintering; (2) to understand the oxygen ingress mechanisms and kinetics by multi-length-scale characterisation and computer simulations; (3) to mitigate oxygen by in-situ alloying using getter (sacrificial) materials, nano-particles and as volatile substance, with interaction rationalised by computational fluid dynamics modelling. This work will emphasise consistent powder feedstock, oxidation reactions at crystal defects and effectiveness of mitigation strategies. The project will assist the UK and Japan to increase its competitiveness in the materials sector. This is playing an ever-greater role in low-volume high-end critical applications, promoting widespread societal benefits. The advances made in this project are transformative to a wide range of applications including energy production (nuclear fusion), healthcare (biomedical implants) and telecommunication (satellites). The proposal is timely given the recent energy crisis, ageing society and space technology development. The envisaged translational impact echoes strongly with the UN sustainable development goals.