Thermomechanical Characterisation of W-Eurofer 97 Brazed Joints

Abstract

The preliminary conceptual design of both the blanket and the divertor of the future DEMO reactor envisages tungsten as plasma-facing material joined to Eurofer 97 steel as the structural element. Therefore, the development of new joining technologies to constitute these first wall components has become an essential task in the fusion community. In this regard, there is a lack of knowledge on the thermal stability and mechanical behaviour of these W-Eurofer 97 joints under the expected service temperatures, which will be situated around 400–600 °C. In this paper, successful diffusion brazing of Eurofer 97 steel with tungsten was achieved by using a copper interlayer. Its microstructure, hardness and flexural strength were investigated for different configurations and thermal histories in its predicted operating temperature range. Investigations on as-brazed specimens showed defect-free interfaces and a continuous metallic bond between W and Eurofer 97 was successfully accomplished; although the hardening of steel base material was detected after joining. Microstructure alterations were observed just at the W-Eurofer 97 interface, with the diffusion of Cu braze into the iron grain boundaries and the formation of a Fe-rich hard intermetallic layer, which was, in fact, the weakest part of the joint during thermo-mechanical testing. The flexural strength of the brazed specimens at room temperature was relatively low, with a strong dependence on the anisotropy of the W base material and little influence of the stress-relieving thermal treatment. However, testing at temperatures up to 600 °C showed high strength of the bonded specimens, with values as high as 470 MPa, comparable to those of the Eurofer 97 steel.