| International Thermonuclear Experimental Reactor (ITER) known as a "artificial sun" on earth will entirely solve the energy problem puzzled mankind forever. Thermal anchor, one of the key components of ITER magnetic gravity system, functions as a heat exchanger to keep temperature balance of the whole support system. However, the thin-wall cooling pipe and the thick flexible plate of thermal anchor are a difficult combination to form a good welding joint, for the thin-wall pipe being easily broken. The arc braze-welding, a low heat-input welding method, is a good choice to attach the thermal anchor in responsible for its ability of controlling deformation of the long pipe and huge plate, especially of avoiding the thin-wall pipe broken.In this paper, ITER thermal anchor was welded by TIG braze-welding. Joint macro-and microstructure observation, SEM and EDS analysis, tensile strength and microhardness tests were adopted to systematically investigate the effects of heat input on wetting and spreading ability of brazing filler and butt-joint welding quality, and effects of welding angle, location and using fluxes, or not, on the quality of thermal anchor joint. Finally, the copper penetrating crack generated near the interfacial zone was analyzed and its formation mechanism was illustrated.The results obtained indicate that the higher TIG heat input is, the better is the wetting and spreading ability of braze filler. Compared with CuSi3and CuSi3Mn filler metal, Ag45CuSnNi braze filler has a best wetting and spreading ability, yet that of the CuSi3is relatively lowest. The butt-joint characteristic varies form a brazing joint to a fusing-brazing mixed joint as heat input increases. When a moderate heat input is produced by a50A welding current, a good joint quality combined with a fine wetting and spreading ability of filler is obtained, and tensile strength of the butt joint sample is451MPa.Without fluxes, a fusing welding thennal anchor joint welding by CuSi3Mn filler can be obtained when welding angle θ is60。 and welding locationy is1.5mm and x is10mm. With fluxes, a good thermal anchor joint combined with characteristics of both fusing and brazing is obtained. The fusing zone in flexible plate has a higher dissolution of base metal and therefore generates a wider interfacial zone, while the brazing zone has a less dissolution and narrower interfacial zone. The interfacial zone is a zone that elements change drastically between weld seam and base metal.The microstructure of weld seam and interfacial zone of thermal anchor joint is consisted of a-Cu matrix and Fe based solid solution. The existence of fine particle-shape Fe solid solution in weld seam significantly improves its comprehensive quality. Owing to a high solid solute content of Cu, Si, and Mn in Fe based solid solution in interfacial zone, the microhardness is greatly increased. The microhardness of the wider interfacial zone in fusing zone of the joint is412HV0.2, and that of weld seam and base metal is209HV0.2and147.2HV0.2, respectively. The higher microhardness of the narrower interfacial zone in brazing zone may contribute to a higher solid solute content of element of brazing filler, which reaches487.5HV0.2.The copper penetrating cracks have been detected near the interfacial zone in all thermal anchor joints TIG brazed by CuSi3, CuSi3Mn, and Ag45CuSnNi filler metal. The necessary condition of generating copper penetrating crack is the intimate contact of molten copper liquid and base metal, simultaneously under local stress concentration, followed by crack propagation under the effect of decreasing grain boundary surface energy and binding force of atoms near crack tip. Using316L filler and pure Ni filler to TIG welding thermal anchor, or using Cu based filler to vacuum brazing the joint, any copper penetrating cracks in the HAZ zone of the joint was not detected. |
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