Beryllium-copper joining techniques for use on plasma-facing components

摘要

Several technologies are being evaluated as methods of joining beryllium to copper for plasma facing components in fusion reactors. The mechanical and microstructural properties of these bonds are reviewed and compared with the requirements for the application. The prime candidate for the plasma facing material is S-65C grade beryllium. At present three copper alloys are being considered for the structural substrate and include an oxide dispersion strengthened copper (Cu-Al/sub 2/O/sub 3/) and two precipitation strengthened copper alloys (CuCrZr and CuNiBe). The three joining technologies presented in this study include inertia welding, electroplating and brazing. In the case of the brazed bond, the braze alloy is selected based on compatibility with the reactor design. Several brazing candidates have been discounted because of their transmutation products in the high neutron flux environment. The braze alloys used in this study were aluminum base. Of paramount concern in all methods of joining was the elimination of a reaction between beryllium and copper to form an intermetallic phase, the presence of which would severely degrade bond ductility. In the case of inertia welding, short times at temperature (5 seconds) were used to reduce the time for a reaction between the beryllium and copper. In the case of the electroplating technique, the temperature was kept low to eliminate any intermetallic formation during the plating process. In the case of the brazing techniques, a diffusion barrier was used to inhibit the diffusion of copper to the beryllium surface. Evaluation of the inertia welded bond showed evidence of intermetallic phases suggesting that little time is needed to form the intermetallic. Limited mechanical tests indicated that fracture occurred at the intermetallic phase. Evaluation of the electroplated bond showed good bonding characteristics; failure in a ring shear test occurred in the electroplated copper with shear strengths in excess of 250 MPa. Evaluation of the brazed bonds is currently in progress, although preliminary results using several diffusion barrier materials appears promising.