Structural Component Development of Three-Layer Cylinders for Superconducting Generators, Part 1: Selection of Layer-Bonding Method and Development of Long Large-Gauge Three-Layer Cylindrical Structure

The warm damper, a component located outermost in a rotor of the superconducting generator, is intended to shut off magnetic fields of the armature and bear large bending stresses due to electromagnetic force. So it is required to have both high conductivity and sufficient strength against bending stresses. Meeting these requirements is a promising three-layer cylindrical structure composed of a highly conductive cylinder sandwiched between cylindrical high-strength nonmagnetic stiffeners, so that only middle cylinder bear high conductivity and outer and inner ones bear most of the bending stresses. Candidate materials for the middle is Cu-Cr, and for the outer and inner is A286, a iron-base superalloy. To realize this three-layer cylindrical structure, the first step is to select a method of bonding the three layers and the second step is to research and develop large structures. Using test rings (300 mm in O.D. × 500 mm in axial length) made to a reduced scale, layer bonding tests were carried out in the present study to investigate three bonding methods: explosion bonding, brazing, and diffusion bonding. The two methods other than brazing were selected because they involved few defects and provided high bonding strength. When manufacturing long large-gauge warm dampers, deformations due to bonding must be uniform and bonding strength must be thermally stable so as to join them axially by three-layer welding. The two methods were compared for these requirements and finally diffusion bonding was chosen. As the last step, diffusion bonding tests were carried out with a mock-up warm damper model, which was the largest diffusion-bonded cylinders that ever made. The size of the model was 885 mm in O.D. and 2800 mm in axial length, and the layer bonding strength was evaluated, proving that sufficient layer bonding strength could be obtained together with uniform deformations.