Study On The Technology And Characteristics Of TIG Welding For Molybdenum Alloy

The Mo-La alloy is being widely used in agriculture,chemical industry,metallurgy,electricity,environmental protection,nuclear power,aerospace and other fields because of its low thermal expansion coefficient,eximious electrical conductivity and thermal conductivity and excellent normal atmospheric temperature and high temperature strength.So far molybdenum alloy has better physical and chemical properties compared with zirconium alloy.Therefore,there is a strong sense of practical significance that the use of molybdenum alloy instead of zirconium alloy as the nuclear fuel rod cladding materials can evade the metal-water reaction to avoid nuclear leakage.The TIG welding is used to weld refractory metals such as molybdenum,niobium and zirconium,so as to avoid the contamination of active metals by gas impurities,which not only reduces the equipment cost,and do not limit the size of work piece.Moreover,the open-arc welding seam formation can be seen visually,and the welding technological parameters are easy to adjust.In this study,an investigation was carried out for the TIG welding process of the Mo-La alloy plate lap-structure and tubing end-plug structure.And compared with different welding parameters,the optimum parameters were that welding current of was 210 A,welding speed was 200mm/min for plate lap-structure;welding current was120 A,preheat current was 50 A,welding speed was 25s/r and adding auxiliary pulse for tubing end-plug structure.The back protection gas and the argon-filled tank both have a gas flow rate of 5L/min,and the gas of the torch is 8L/min of helium mixed with 1L/min of argon.The tungsten has a length of 5mm,from the work-piece 2~3mm.We characterized and compared the microstructure of the welded joints the Mo-La alloy plate lap-structure and revealed that the pores and cracks in the weld were improved by appropriately increasing the current.There were many large pores distributed along the fusion line in the weld,and the maximum size reached 708.23?m when the current was low.The number of large pores in the weld and its size was greatly reduced,the maximum size was 506?m,but the cracks in the weld tended to be biased when the medium current is adopted.Large pores in the weld disappeared when the current was large,but the crack was more serious,the welding residual stress was larger.However,the crack defects was eliminated in a large extent by preheatingbefore welding,so as to achieve the welding joints with good surface forming and less weld defects.The weld zone structure can be divided into columnar crystals growing vertically along the fusion line and equiaxed grains distributed in the center of the weld.A large degree of recrystallization and the aggregation of La2O3 particles occurs in the heat affected zone.The average micro-hardness(165HV)of the weld zone was lower than that of the heat affected zone(190HV),but the hardness of the welded joint was fluctuating.We characterized and compared the microstructure of the welded joints the Mo-La alloy tubing end-plug structure and revealed that the pores and cracks in the weld zone and heat affected zone were improved by adding pulse assistance,preheating before welding and appropriately increasing the welding current.There were a lot of crystal,multilateral and liquefied cracks in the weld area and the heat affected area of the weld joints with poor mechanical properties.The microstructure of the weld zone and heat affected zone were composed of dark ?-Mo based grain dissolved more La2O3 particles and light ?-Mo based grain dissolved less La2O3 particles.The maximum tensile strength of welded joint reached 530 MPa,which was 80% of the base metal strength,but showed obvious brittleness.Obvious fracture characteristics such as slip system and dissociation step were found on the fracture surface.The PWHT involved a recrystallization treatment(1200? for 30min)to inprove the mechanical properties.And it revealed that the crack defects disappeared in the weld,and the microstructure of the grain was more uniform.The phenomenon of fiber broadening was obvious in the heat affected zone.There were many fine grains and some sub grains in the grains,and the recrystallization occured in the heat affected zone and the weld zone.It showed obvious plasticity and toughness on the whole joint,but it also exhibited brittle fracture characteristics.The tensile strength reached607.1MPa,which was 90% of the base metal strength.