The toroidal field coil case (TFCC) is a large structure having a D-shape with a height of 16.5 m and a width of 9 m. The TFCC is composed of four subassemblies called as AU, BU, AP, and BP. Although the subassemblies are large welding structures, it is required for the interfaces to satisfy severe dimensional tolerances that are on the order of millimeters. The AU and BU each have U-shaped cross-sections and are assembled in two and three segments, respectively, by welding. In the case of butt welding for U-shaped cross-sections, larger angular welding deformation is commonly generated. In order to minimize the welding deformation, the authors developed a control method through welding trials using small and full scale mock-ups. For the AU subassembly, a combination method using manual TIG welding and monitoring the welding angular deformation was proposed. For the BU subassembly, a procedure using automatic TIG welding with a U-shaped groove shape and reinforced jigs while monitoring angular deformation during welding was developed. After the establishment of these welding technologies, the welding of AU and BU began. As the result of AU and BU welding in series production, welding deformation has been controlled well and machining duration rationalized, thereby decreasing amount of extra material based on manufacturing results. As of August 2020, a total of 19 AU and BU weld have been successfully completed.
