Study on Characteristics of Dissimilar Material Joints for ITER First Wall

The first wall (FW) of ITER blanket includes beryllium (Be) armor tiles joined to CuCrZr heat sink with stainless steel (SS) cooling tube and backing plate in order to improve plasma performance and reduce thermal stress. Since these joints must withstand thermal and mechanical loads caused by the plasma and electromagnetic force, it is important to evaluate the strength and thermal fatigue life of the joints. Qualification tests of the FW fabrication technology are performed in order to qualify the joining technologies. The Japan Domestic Agency (JADA) has fabricated the qualification mock-up with a hot isostatic pressing (HIPing) technique and carried out the materials testing and the non-destructive examinations. When the dissimilar materials joints are subjected to external force and thermal load, the stress distribution shows singularity and large stress concentration at the interface edges caused by discontinuity of the elastic properties. The characteristics of stress singularity were analyzed for the Be/CuCrZr and SS/CuCrZr joints. The results of tensile test showed that the Be/CuCrZr joint fractured at the interface because of large stress concentration at the interface edges. For the SS/CuCrZr joint, however, the joint fractured at CuCrZr apart from the interface. There was insignificant stress concentration at the interface edges, because those elastic properties are almost same. Since Charpy impact strength of the SS/CuCrZr joint was inferior to the CuCrZr, elasto-plastic finite element analyses were also carried out to clarify the deformation characteristics and fracture of the tensile and Charpy specimens. Those fracture behavior were caused from the plastic deformation properties of both materials.