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

Abstract

International Thermonuclear Experimental Reactor (ITER) Correction Coil (CC) case,
\nmade of ultra-low carbon austenitic stainless steel 316LN with 20 mm thickness, was
\ndesigned in the outmost layer of the CC, and was used to protect the internal
\nsuperconducting coil, resist the deformation effect caused by the powerful
\nelectromagnetic force and thermal stress during the operation process. These cases have
\ncharacteristics of small cross-section, large dimension, and complex structure. They
\nwere divided into two parts for the convenience of the internal superconducting coil
\nbeing inserted into the case, and they will be closure welded together after insertion.
\nThus, weld seam is located the whole perimeter of the case, which will decide the large
\nquantity weld with the specific distribution caused by the geometric profile of the case.
\nThe distribution requires a specific welding system, aim at the CC case closure welding
\ncharacteristic, should be developed to realize the field welding. The strict deformation
\nrequirement, temperature control of internal coil and full penetration with high welding
\nquality brings the technology challenges to this closure welding. The welding system
\nmust be constructed and these key technologies must be solved using this welding
\ntooling before the offcial production of CC.
\nA fibre laser robotic welding system was developed for the case closure welding based
\non the research, scheme design and analysis in this thesis. The specific qualification
\nexperiments were carried out on this laser welding system according to technology
\nchallenges on the welding quality, temperature control and deformation.
\nFirstly, according to the distribution of weld seam, the preliminary scheme designs of
\nwelding systems based on several different welding methods were carried out. It was
\nfound that the laser welding is identified as the most suitable welding method. Based
\non above, reasonable arrangement of robots and external rail, and simulation of robotic
\nwelding motion process were carried out in order to further optimize design and
\nanalysis. The simulation results show that there exist the dead zone where the robot arm
\ncan 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,
\nthe robot was successful in covering all weld seams in all cases.
\nSecondly, to complete this welding system, the welding fixture should be developed to
\nsupport the welding platform, adjust the assembly tolerance and provide the rigid
\nconstrain to control the welding deformation, and electric control should be developed
\nto integrate the welding system and increase its security and stability. A number of
\nground supports match with the C-type clamps were designed and special distributed
\naim at the geometric profile of case. The rotatable ground supports, which can provide
\ntwo types of welding platform, were developed to meet the two different welding
\npositions with arched and concave shape of SCC case. In order to provide the turn over
\nfunction for SCC case, a welding tilter with tilter framework, single central axis,
\nload-bearing jig was designed base on the good finite element analysis (FEA) result. In
\naddition, the design requirements of the control system of the CC case laser welding
\nwere analyzed and the integrated control system was developed based on SIEMENS
\nPLC system S7-300. Thirdly, narrow gap multi-pass laser welding with hot wire was developed as the
\nwelding process for the case closure welding. The welded jointed with defects-free and
\ngood mechanical properties was achieved based on good optical system, reasonable
\ngroove structure and optimized welding procedure. In order to