4.19.5.1.1.1 Background information

The main problem of bonding Be to Cu alloys is that Be reacts with almost all possible metals (except Al, Si, Ag, and Ge) and forms brittle intermetallic phases.157-159 Such bond joints have poor mechanical integrity. More robust joints use metal interlayers to act as either diffusion barriers and/or strain accom­modating compliant layers to avoid the formation of deleterious phases and to assist in the accommoda­tion of thermal cycling-induced strains.160

R&D has been performed over a number of years to develop the design and manufacturing techniques required to meet the demanding design require­ments. Significant experience has been gained with these manufacturing techniques and the associated inspection techniques. It must be noted that in the 1990s the best joining technology developed for manufacturing the Be/Cu actively cooled compo­nents was brazing with Ag-base alloys (e. g., InCuSil with ~41.75% Ag) which was successfully used in JET. However, the use of Ag base brazing alloy was not allowed in ITER mainly because of the transmu­tation by neutron irradiation to Cd (~5 wt% Cd will be produced in Ag-Cu eutectic alloy at 1 MW am~ ) whose presence would (1) reduce the melting tem­perature of the braze; (2) lead to the formation of highly radioactive isotopes; and (3) affect the pump­ing system in case of Cd release to the vacuum chamber and codeposition in the cryopumps panels.

During the early stage of the ITER first-wall design development, dispersion-strengthened copper (DS-Cu) alloys (e. g., Glidcop Al25) were considered as the first option because (1) the stresses were within the design allowable, and (2) they had better thermal stability under the manufacturing route, which required a first wall to be integrated with a 4 t shield. The main developments for fabrication of joints between Be and DS-Cu alloys are reported in ITER MAR129 and Lorenzetto et al.161 However, as a result of a design change that took place from an integrated first-wall panel to a separated first-wall panel design, a precipitation-hardened copper-chromium-zirconium alloy (CuCrZr), was subsequently chosen. This was because the fracture toughness of DS-Cu is very low above 200 °C even for unirradiated material. Fracture toughness of the unirradiated and irradiated CuCrZr alloy decreases with increasing temperature, but it remains at a rela­tively high level in the ITER working temperature range and it is significantly higher than fracture tough­ness of DS-Cu. The use of separable first-wall panels makes it possible to perform heat treatments with fast cooling rates, which are mandatory to adequately retain the mechanical properties of precipitation — hardened materials.

Thus, extensive studies were then performed during the last 10 years to develop reliable silver free Be/CuCrZr alloy joining techniques and to modify the joining conditions to minimize the mechanical strength loss of the CuCrZr alloy. Dif­ferent methods have been considered and investi­gated. Some of these were eliminated because of bad results (e. g., explosive bonding, inertial welding, joint rolling, and some types of brazing). Two meth­ods gave good results and were kept for further investigations: HIPping and brazing. Good results were achieved with the HIP joining technique by lowering the HIP temperature as close as possible to the CuCrZr alloy ageing temperature (about 480 °C) and with the brazing technique in the development of a fast brazing method to minimize the holding time at high temperature. The latter was achieved by induction brazing in Europe and by fast heating and cooling using an e-beam test facility in the Russian Federation.