Investigation On The Fabrication And Properties Of Copper/Steel "Tube-to-tube" Junctions For Fusion Applications

From the viewpoint of the structural design of the divertor components,the copper/steel "tube-to-tube" joining is a necessary process for the manufacturing of WEST,ITER and DEMO divertor components.The junctions are the pressure boundaries of the cooling water.Once cracked,it will result in the leakage of the cooling water,which greatly threatens the safety of the devices.Firstly,based on the actual requirements of the WEST project,we investigated the microstructures and mechanical properties of electron beam welded CuCrZr/Inconel625/316 L junctions.As the welding of ending tubes is performed after the hot isostatic pressing for the real components,in this study the CuCrZr tubes were annealed with identical parameters of HIP before welding in order to simulate the real situations of component manufacturing.The junctions were free of defects such as micro-cracks or incomplete penetration.The mechanisms of macro-segregation features,such as beaches,peninsulas,and irregular islands,and of micro-segregation features,such as small spheres,dendrites,Cu-rich particles,and Nb/Mo-rich networks have been discussed.Mechanical testing shows that post-weld aging nearly does not improve the tensile strength of the junctions because it just shifts the weakest point of CuCrZr slightly away from the Inconel/CuCrZr weld.Meanwhile,post-weld aging decreases the hardness of CuCrZr in regions out of the heat affected zone.Cold helium testing performed after rotary fatigue testing indicated that post-weld aging does not change the fatigue resistance of the junctions,at least under the current testing conditions.Significant increments in hardness of irregular Inconel-rich islands induced by aging may be attributable to the formation of Nb/Mo-rich networks.Secondly,ITER sized CuCrZr tubes and 316 L tubes,as reference materials,were tube-to-tube joined by rotary friction welding.The microstructure,hardness and tensile properties of the junctions were investigated systematically.The mechanisms of microdefects and fine Cr bands in CuCrZr were discussed.Post-weld aging treatment somewhat improved the microhardness and tensile strength of the junctions,but its effect on room temperature ductility is negligible.The demonstrated feasibility of RFW implies its potential application for the joining of advanced tubular materials.Finally,based on the above investigations,tube-to-tube joining between T91 and ODS-Cu(reference materials),which is difficult to accomplish by traditional fusion welding,has been successfully realized with rotary friction welding.The junctions are free of defects such as microcracks.Asymmetric diffusion of elements has been observed.Very narrow soft regions were found in ODS-Cu at the vicinity of the interfaces in all specimens.All the specimens have appreciable tensile strengths(290to 330 MPa)but exhibit brittle characteristics.Both two as-received specimens were not broken after the scheduled rotary bending fatigue testing.Relationships among tensile properties,microhardness and microstructures have been tried to establish,and the mechanisms of asymmetric diffusion,low-hardness region and brittleness also have been discussed.In summary,the microstructures and reliability of the junctions fabricated with electron beam welding have been systematically evaluates from the actual requirements of the WEST project,and the tube-to-tube joining between advanced copper alloys and steels has been successfully realized with rotary friction welding,which is technically mature.