ITER-International Thermonuclear Experimental Reactor: joining technologies for ITER components

This work is framed in the joining technologies for fusion reactor applications, more specifically in ITER tungsten-to-steel joints. The main problem for W/steel joints is the mismatch between the coefficients of thermal expansion of tungsten and steel that after joining process causes residual stresses at the interface, reducing the strength of the whole joint. Moreover, the deeper diffusion of the metallic filler (in case of brazing) leads to a softening in the seam area and consequently has a detrimental effect on mechanical properties. Finally, the direct contact between copper and steel causes the formation of a reaction layer constituted of brittle compounds which weaken the whole joint. This thesis is divided in two parts, one regarding bibliographic research in which different kinds of nuclear fusion joints are described, and a second one which is experimental and was carried out at Politecnico di Torino within the Glance Group (Glasses Ceramics and Composites). The joints have been manufactured using a brazing process, based on pure Cu or commercial Cu-based filler (Gemco ®); analysis have been performed on W/Cu/steel and W/Gemco®/steel joints, and on a possible surface modification of the steel by laser, in order to improve the joint performance. Following a complete bibliographic research, different brazing conditions were analysed, to find the optimal parameters to achieve sound W/steel joints. The brazing technique seems to be one of the most suitable for ITER joints, thanks to its easier applicability and well-known technology. The first activity is focused on brazed joints, and two studies are carried out. First, a W/Gemco®/steel joint is produced, in order to investigate if the presence of a Cu-based alloy as filler material (87.75 wt% Cu, 12 wt% Ge and 0.25 wt% Ni) reacts well with the adjacent base materials, considering the ductility of Cu and low-activation of Ge. W/Cu/steel joint has been investigated as well. Moreover, we proposed a way to reduce the diffusion of the copper inside the steel, adding a thin Cr layer on the surface steel substrate. The layer was deposited on the steel samples by magnetron sputtering technique, in order to create a barrier for copper diffusion.For the same reasons, a coating of Cr was also deposited on the steel surface of the W/Gemco®/steel joint. Joints manufactured with and without Cr coating have been compared.The microstructures and the chemical compositions were characterised by Field Emission Scanning Electron Microscopy (FESEM) analysis and with Energy Dispersive Spectrometry (EDS) technology. Finally, the produced joints will be subjected to High Heat-Flux tests at Forschungszentrum Jülich Research Centre (FZJ- Jülich, Germany) and eventually to mechanical tests. The second activity is centred on a modification of the steel surface, in order to prepare the steel face for direct bonding with tungsten improving its mechanical and tribological properties. Three steel samples were characterised and textured with a nanosecond laser, varying the laser power and fluence. The aim of this thesis is to produce a good starting point for the manufacturing of W/steel joints, with improved performance if compared to joints manufactured by direct bonding, and to demonstrate that the presence of Cr on the steel surfaces of brazed joints may represents a promising solution for fusion nuclear joints. However, further investigations are needed, and a thermo-mechanical analysis must be explored.