Research On Pulse TIG Spot Welding And Repair Welding Technology For 0.5mm Thick Cd-2 Plate

The nuclear fuel has been used by the nuclear power station, which is called the spent fuel . The spent fuel will produce very strong nuclear radiation because of their mutual releasing neutrons bringing a chain nuclear fission reaction when they are stacked together. Cadmium as a good absorbing neutron materials is applied on the manufacture of stockpile frame, which is a main equipment used to dispose the spent fuel. The spent fuel frame sleeve is a three-layer structure ,which is composed of 2mm inner circular pipe, 0.8mm outer circular pipe and Cadmium sleeve welded by 0.5mm Cadmium plate (Cd-2) in the middle of inner and outer circular sleeve. Because of low melting point and oxidizable property of Cadmium, the welding of Cadmium plate (Cd-2) has been being a difficult problem in the process of manufacture of the spent fuel frame sleeve .This paper studies the welding qualities of pulse TIG spot welding and remelting repair welding technology of 0.5mm Cadmium(Cd-2) plate using different welding process parameters, including pulse current, welding time, arc length and shield gas flow rate, through welding test, mechanical test, micro-structure analysis and neutron flux test. The test results show welding time tp shall be reduced when pulse current Ip is higher, welding time tp shall be increased when pulse current Ip is lower, the tungstic electrode neck-in shall be 5mm～6mm, and argon flow rate shall be 3～4L/min. With the increase of the pulse current and welding time, the shearing resistant force and welding spot diameter D2 will also increase. The torsion form of fracture indicates the force the welding joint receives is pull-cut compound strength. The fracture analysis shows the morphology of the fracture is dimples + quasi-cleavage mixed fracture, and the fracture location is in the heat-affected zone. The grain micro-structure of 0.5mm Cadmium (Cd-2) base plate presents hexagonal shape, which conforms to the fact that cadmium shall be hexagonal lattice structure. The structure of welding joint of 0.5mm Cadmium (Cd-2) plate presents approximate columnate crystal grain organization. The increase of the pulse current and welding time will make crystal grain size increase, and Cadmium ratio value Rcd will increase along with the reduction of crystal grain size, so lower current and short welding time shall be selected. The following welding process parameters are recommended: Pulse current Ip=8A, weld time t2=3s, tungstic electrode neck-in 5mm, argon flow rate 3.5L/min.The test results show the welding process parameter used in the spot welding can also be applied to remelting repair welding. The micro-structure of weld seam indicates crystal grain grows coarser, but which is finer than spot welding. The analysis confirms the increase of nucleation rate and the slower of crystal grain growth velocity are the main reason of this phenomenon.