Structural Component Development of Three-Layer Cylinders for Superconducting Generators, Part 2: Development of Three-Layer Welding Techniques to Extend Axial Length and Evaluation of the Welds

Abstract

The warm damper is one of the multi-cylindrical rotor components, which is located at the outermost position in a rotor of the superconducting generator. The main function of the warm damper, in addition to bear rotor torque, is to shut off the magnetic fields of the armature. While maximum stresses in the warm damper are caused by electromagnetic forces in the event of a short-circuit fault, which cause large bending stresses, so it is required to have high conductivity and sufficient strength against bending stresses. A promising design for it is a three-layer cylindrical structure composed of a highly conductive Cu-Cr cylinder sandwiched between inner and outer stiffeners made of a high-strength nonmagnetic material (A286). To realize this structure, it is necessary first to select a suitable layer bonding method and then to research and develop the procedures to construct this large structure. The previous paper described selection of diffusion bonding as the suitable layer bonding method. The first step to fabricate this large structure was to prove the structural integrity of a diffusion-bonded mock-up model, which was 885 mm in diameter and 2800 mm in axial length [1]. This paper is a study on circumferential three-layer welding to extend the axial length allowing for the fabrication of a ever larger structure. Two A286/Cu-Cr/A286 three-layer rings, 625.5 mm in O.D. and 300 mm in axial length, were prepared by diffusion bonding the layers of each rings simultaneously by HIP. Then, the axial ends of their three-layers were butt welded with the filler wires, which were selected for each layers in advance. Subsequently, in accordance with the results of a study on the condition for aging treatment following diffusion bonding (800°C × 4 h) to obtain the proper mechanical properties of the alloy A286, which is a material for the major load bearing component, the rings were age-treated at 700°C for 10 h. The tensile test results of the welded joint and weld metal specimens cut out from the welded rings proved that both alloys A286 and Cu-Cr welded components had adequate mechanical properties. That is these properties were equivalent to those of the base metals, with no reductions of layer bonding strength due to weld heat input.