| Because of its excellent corrosion resistance,weldability and plasticity,304 stainless steel was widely used in offshore platforms,nuclear power,chemical petroleum and other fields,and plays an important role in the development of national defense and high-quality economic development in China.Conventional TIG welding of 304 stainless steel has some problems,such as coarse grain,poor mechanical properties and low welding efficiency.The introduction of ultrasonic-assisted technology in TIG welding can not only obtain high-quality TIG welding joints,but also improve welding efficiency.However,at present,scholars mainly focus on flat butt welding,but less on thick plate narrow gap TIG welding.In this paper,ultrasonic vibration assisted TIG welding,arc ultrasonic TIG welding and ultrasonic-assisted arc hybrid TIG welding were adopted.Three ultrasonic-assisted welding processes were used to weld 304 austenitic stainless steel with 10 mm thickness in U-groove.The effects of three ultrasonic-assisted welding processes on 304 stainless steel narrow-gap TIG welding head were compared,and the mechanism of ultrasonic-assisted welding was explored.Ultrasonic vibration was introduced into narrow gap welding of 304 stainless steel,and the effects of different ultrasonic amplitudes on welding physical properties,weld formation,joint microstructure and mechanical properties were studied.The results show that the maximum horizontal width of ultrasonic vibration-assisted welding arc was widened by 2.65 mm,the welding heat input was unchanged,the weld penetration is increased,the crystal form of weld pool was mainly equiaxed crystal,the microhardness of weld zone was improved,which is close to the base metal,and the joint fracture is ductile fracture.The tensile strength of welded joint was increased from 619 MPa to 674 MPa,the yield strength was increased from 359 MPa to 376 MPa,and the elongation after fracture was increased from 23% to 25%.304 stainless steel was welded in narrow gap by arc ultrasonic TIG welding,and the effects of different ultrasonic excitation frequencies on welding physical properties,weld formation,joint microstructure and mechanical properties were studied.The results show that the introduction of ultrasonic makes the bottom of the arc shrink,and the heat at the bottom decreases,which leads to the shallow weld penetration.When the excitation frequency was 25 k Hz and 35 k Hz,the welding heat input changes,and the microstructure was still dominated by columnar crystals,and the joint fracture was cleavage fracture.When it reaches 70 k Hz,the microstructure becomes uniform,and the corresponding microhardness increases.The fracture form of the joint becomes ductile fracture,and the tensile strength of the welded joint increases by 13 MPa;The yield strength only increased by 9 MPa,and the elongation at break of welded joints increased from 23% to 25.5%.By studying the effects of different ultrasonic amplitudes on the physical properties,weld formation,joint microstructure and mechanical properties of 304 stainless steel during ultrasonic-assisted arc hybrid welding,the following conclusions were obtained: the introduction of ultrasonic heat input does not change,when the ultrasonic amplitudes were 80μm and 100 μm,the weld penetration increases,the bottom sidewall fuses well,and there was no arc climbing phenomenon,and the proportion of equiaxed crystals in the weld zone increases;When the ultrasonic amplitude was 60 μm,the average hardness of the weld zone is close to that of the base metal,and it is much higher than that of the base metal at higher amplitude.The fracture form of the joint was ductile fracture,and the maximum tensile strength can reach 782 MPa,which is about 29 MPa higher than that of conventional TIG welding,and the maximum yield strength is 107 MPa,which is about 31% higher,and the corresponding elongation was increased from 32% to 36%. |
PDF Full Text Request