No abstract

The manufacturing of refractory-metals components presents some limitations induced by the materials' characteristic low-temperature brittleness and high susceptibility to oxidation. Powder metallurgy is typically the manufacturing process of choice. Recently, Wire + Arc Additive Manufacturing has proven capable to produce fully-dense large-scale metal parts at relatively low cost, by using highquality wire as feedstock. In this study, this technique has been used for the production of large-scale tungsten linear structures. The orientation of the wire feeding has been studied and optimised to obtain defect-free tungsten deposits. In particular, front wire feeding eliminated the occurrence of pores and micro-cracks, when compared to side wire feeding. The microstructure, the occurrence of defects and their relationship with the deposition process have also been discussed. Despite the repetitive thermal cycles and the inherent brittleness of the material, the asdeposited structures were free from internal cracks and the layer dimensions were stable during the entire deposition process. This enabled the production of a relatively large-scale component, with the dimension of 210 x 75 x 12 mm. This study has demonstrated that Wire + Arc Additive Manufacture can be used to produce large-scale parts in unalloyed tungsten by complete fusion, presenting a potential alternative to the powder metallurgy manufacturing route.

Components manufactured via Wire + Arc Additive Manufacturing are usually characterised by large columnar grains. This can be mitigated by introducing in-process cold-rolling; in fact, the associated local plastic deformation leads to a reduction of distortion and residual stresses, and to microstructural refinement. In this research, interpass rolling was applied with a load of 50 kN to a tantalum linear structure to assess its effectiveness in changing the grain structure from columnar to equiaxed, as well as in refining the grain size. An average grain size of 650 µm has been obtained after five cycles of inter-pass rolling and deposition. When the deformed layer was reheated during the subsequent deposition, recrystallisation occurred, leading to the growth of new strain-free finer equiaxed grains. The depth of the refined region has been characterised and correlated to the hardness profile developed after rolling. A reduction of porosity was also registered. Furthermore, a random texture was formed after rolling, which should result in isotropic mechanical properties. Wire + Arc Additive Manufacturing process demonstrated the ability to deposit sound refractory metal components and the possibility to improve the microstructure when coupled with cold inter-pass rolling.

Functionally graded components are usually preferred for severe and critical service conditions, thanks to the possibility of achieving different complimentary material properties within the same structure. Wire + Arc Additive Manufacturing is an emerging technology which lends itself well to the production of sound graded structures. In this study, an integral structure of two functional gradients, namely tantalum to molybdenum, and molybdenum to tungsten, was successfully deposited. A linear gradient was observed in both composition and hardness. Microstructure, elemental composition and hardness were characterised as a function of position, and discussed. The study demonstrates that WAAM has the potential to successfully deposit functionallygraded structures of refractory metals, obtaining controlled properties.

Tungsten is considered as one of the most promising materials for nuclear fusion reactor chamber applications. Wire + Arc Additive Manufacturing has already demonstrated the ability to deposit defect-free large-scale tungsten structures, with considerable deposition rates. In this study, the microstructure of the asdeposited and heat-treated material has been characterised; it featured mainly large elongated grains for both conditions. The heat treatment at 1273 K for 6 hours had a negligible effect on microstructure and on thermal diffusivity. Furthermore, the linear coefficient of thermal expansion was in the range of 4.5x10 -6 µm m -1 K -1 to 6.8x10 -6 µm m -1 K -1 ; the density of the deposit was as high as 99.4% of the theoretical tungsten density; the thermal diffusivity and the thermal conductivity were measured and calculated, respectively, and seen to decrease considerably in the temperature range between 300 K to 1300 K, for both testing conditions. These results showed that Wire + Arc Additive Manufacturing can be considered as a suitable technology for the production of tungsten components for the nuclear sector.