Study on system design and key technologies of case closure welding for ITER Correction Coil

摘要

International Thermonuclear Experimental Reactor (ITER) Correction Coil (CC) case,made of ultra-low carbon austenitic stainless steel 316LN with 20 mm thickness, wasdesigned in the outmost layer of the CC, and was used to protect the internalsuperconducting coil, resist the deformation effect caused by the powerfulelectromagnetic force and thermal stress during the operation process. These cases havecharacteristics of small cross-section, large dimension, and complex structure. Theywere divided into two parts for the convenience of the internal superconducting coilbeing inserted into the case, and they will be closure welded together after insertion.Thus, weld seam is located the whole perimeter of the case, which will decide the largequantity weld with the specific distribution caused by the geometric profile of the case.The distribution requires a specific welding system, aim at the CC case closure weldingcharacteristic, should be developed to realize the field welding. The strict deformationrequirement, temperature control of internal coil and full penetration with high weldingquality brings the technology challenges to this closure welding. The welding systemmust be constructed and these key technologies must be solved using this weldingtooling before the offcial production of CC.A fibre laser robotic welding system was developed for the case closure welding basedon the research, scheme design and analysis in this thesis. The specific qualificationexperiments were carried out on this laser welding system according to technologychallenges on the welding quality, temperature control and deformation.Firstly, according to the distribution of weld seam, the preliminary scheme designs ofwelding systems based on several different welding methods were carried out. It wasfound that the laser welding is identified as the most suitable welding method. Basedon above, reasonable arrangement of robots and external rail, and simulation of roboticwelding motion process were carried out in order to further optimize design andanalysis. The simulation results show that there exist the dead zone where the robot armcan not doing the effective welding and the robotic welding workspace should be increased. A slip plate module for the welding robot was added to assist travel, after that,the robot was successful in covering all weld seams in all cases.Secondly, to complete this welding system, the welding fixture should be developed tosupport the welding platform, adjust the assembly tolerance and provide the rigidconstrain to control the welding deformation, and electric control should be developedto integrate the welding system and increase its security and stability. A number ofground supports match with the C-type clamps were designed and special distributedaim at the geometric profile of case. The rotatable ground supports, which can providetwo types of welding platform, were developed to meet the two different weldingpositions with arched and concave shape of SCC case. In order to provide the turn overfunction for SCC case, a welding tilter with tilter framework, single central axis,load-bearing jig was designed base on the good finite element analysis (FEA) result. Inaddition, the design requirements of the control system of the CC case laser weldingwere analyzed and the integrated control system was developed based on SIEMENSPLC system S7-300. Thirdly, narrow gap multi-pass laser welding with hot wire was developed as thewelding process for the case closure welding. The welded jointed with defects-free andgood mechanical properties was achieved based on good optical system, reasonablegroove structure and optimized welding procedure. In order to protect the internalsuperconducting coil, which is inside of the case, the backing strip was designed andwelded by laser welding behind the case in the actual welding structure of the CC case.The temperature distribution of the welding process was simulated by FEA andmeasured by the thermocouples on a short sample. The FEA temperature distributionshows good agreement with the experimental measurement, the highest temperature ofthe inner face of case was 255℃, and the highest temperature record of surface of theinternal coil was 59℃. The result shows the laser welding process will not harm thesuperconducting coil, and the welding process, based on the laser welding system, canmeet the temperature control requirement. To study the welding deformation, a SCCmodel case was designed, fabricated and welded by the laser welding system. Theprinciple of positioning welding before continuous welding, as well as segmented, skip,symmetry and repeated turn over, was developed to keep uniform heating of the modelcase. According to the detected results of welding deformation of the model case, theoverall welding deformation was controlled below ±2mm. Satisfactory results of thewelding deformation of the model case certify the reasonability of the welding structure,system and process of the CC case closure welding, and also provide technical supportfor the full scale CC case.In conclusion, this thesis study to develop a special robotic laser welding system tosolve the CC case closure welding, and some key technologies were studied aim at the related engineering challenges.