HIP joining of Be/CuCrZr for fabrication of ITER first wall

ITER first wall was designed to consist of the plasma facing armor made of Be, the heat sink layer made of Cu alloys (CuCrZr) and the supporting plate made of the stainless steels. The joining of the three different materials has been the critical issue in the manufacturing of ITER first wall. The hot isostatic pressing (HIP) has been considered the most promising joining technology to realize a sufficient mechanical integrity of a joint under the anticipated high neutron and stress fields. However, the HIP joining of Be to Cu alloys still have several challenges to be addressed before starting the mass production of the ITER first wall whereas no particular issue was made on the HIP joining of Cu alloys and the stainless steel. Since the reactivity of Be with Cu alloy would increase and the strength of Cu alloys is degraded with increase of the HIP temperature, the HIP joining of Be to Cu alloys was proposed to be performed at lower temperatures than 600 oC with a help of the interlayer which can be formed by various method before HIP. In this study, the effects of the HIP temperature, the HIP pressure and the interlayer have been studied in order to find the optimum joining condition of Be to Cu alloys. The interlayer materials employed in this study were Cr, Cu, Ti and Al which were coated on the surface of Be before the HIP joining. The joint specimens consisting of Be and Cu alloys were fabricated by HIP at 100-150 MPa and 550-620 oC for 2 hours. The microstructure observation showed that no defect such as pores and cracks was formed in the interface of the joint specimens. The joining strengths of the joint specimens, which were investigated by use of the four-point bend test and the shear test, was significantly different depending on the interlayer type. The joint specimen with interlayer of Ti/Cu showed the highest joining strength among the joint specimens tested. The joining strength increased with increase of the HIP temperature and pressure with more remarkable increase with HIP temperature from 550 oC to 620 oC. However, the increase of the HIP temperature gave rise to the decrease of the strength of the Cu alloys. Based on the results obtained, the joining condition for Be/Cu joint was optimized by taking account of the joining strength, the properties of the base metal and the economy of the joining process. (author)