| 304 stainless steel is widely used in industry.Because of its low thermal conductivity,low melting point and high coefficient of linear expansion,it is easy to appear deformation,hot crack,stress and burn through defects in sheet metal welding.With the development of welding technology,low heat input and high efficiency CMT welding has become the main method of sheet metal welding.However,low heat input is easy to form columnar crystals deep into the weld center,which will damage the mechanical properties of the joint.Therefore,ultrasonic was applied to 304 stainless steel CMT welding process to carry out ultrasonic assisted CMT welding.The principle is that the ultrasonic horn is applied to the base metal sheet,and the ultrasonic energy is introduced into the molten pool through the solid base metal to improve the microstructure and mechanical properties of the welded joint.The experiment was carried out in three stepsFirstly,304 stainless steel sheet was welded by CMT welding method,and the effects of different welding parameters on the microstructure and mechanical properties of the welded joint were studied.The results showed that the weld grain increased with the increase of welding current,and the tensile strength of the welded joint was the highest at 160 A,690.3MPa,and the fracture mode is ductile fracture;with the increase of welding speed,the grain growth direction is more perpendicular to the weld centerline,and the tensile strength of the welded joint reaches the highest at 7mm/s.Through the welding test,the results showed that the performance of the joint is better when the welding current is 160 A and the welding speed was 7mm/s.Due to the columnar crystal in the central region of the welded joint obtained under better parameters,it is not conducive to the mechanical properties of the joint.In order to obtain good welding joint,ultrasonic is applied in the welding process to carry out ultrasonic assisted CMT welding.The influence of ultrasonic power on Microstructure and mechanical properties was analyzed.The results showed that ultrasonic vibration could reduce the proportion of large grains,increase the proportion of small and medium grains and make the distribution of grains more uniform.The results showed that the main orientation of weld grain is still <001> orientation,and the orientation strength decreased from 4.1 to 1.7;the(001)<110> and(001)<100> texture strength decreases in the weld,which leads to the increase of texture types;the increased of ultrasonic power makes the austenite content increase,and when the ultrasonic power was 2000 W,the austenite content is the most,accounting for about 98.9% of the weld zone;with the increase of ultrasonic power,the performance of welded joint is improved,and the austenite content in the weld zone was increased,when the ultrasonic power was 1200 W,the average microhardness of the welded joint increased from 219 HV to 234 HV,and the tensile strength increased from 690.3MPa to734MPaBy analyzing the mechanism of ultrasonic assisted welding,it is concluded that the vibration of welding pool will be caused when ultrasonic is applied to the base metal.Especially when the ultrasonic power exceeds 600 W,the cavitation of the weld pool will be intensified,accompanied by the generation and disappearance of cavitation bubbles,which would lead to the increase of undercooling of the weld pool,the enhancement of heterogeneous point nucleation,and the intensification of dendrite fragmentation,resulting in the decrease of orientation strength of weld grains,the increase and refinement of the number of grains,thus improving the mechanical properties of the welded joint. |
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