Research On Ultrasonic Assisted Friction Stir Welding Technology Of Aluminum Alloy Car Body

With the rapid development of aluminum alloy technology,aluminum alloy materials are widely used in aviation,ships,rail transportation and other fields.Welding technology as the key technology of aluminum alloy car body manufacturing of rail vehicles,with the development of manufacturing industry and constantly updated.Traditional friction stir welding is prone to defects such as holes and grooves,which cannot meet the needs of high quality and high efficiency welding.Ultrasonic assisted friction stir welding can effectively improve welding efficiency and reduce welding defects by applying axial ultrasonic vibration.Therefore,it is necessary to explore the ultrasonic assisted friction stir welding process of aluminum alloy body materials.In this thesis,the ultrasonic assisted friction stir welding process of aluminum alloy is studied.Through the existing test platform,the ultrasonic assisted friction stir welding process test of aluminum alloy is carried out.The influence of process parameters on the welding quality of ultrasonic assisted friction stir welding is studied,and the process parameters are optimized.The main research contents and conclusions of this paper are as follows:(1)The influence of ultrasonic assisted friction stir welding process parameters on welding temperature and welding axial force was studied by single factor experiment.The test results show that the welding temperature increases with the increase of spindle speed and shoulder pressure,and decreases with the increase of welding feed speed and ultrasonic amplitude.The welding axial force increases with the increase of welding feed rate and shoulder pressure,and decreases with the increase of spindle speed and ultrasonic amplitude.(2)The influence of ultrasonic assisted friction stir welding process parameters on weld surface roughness,weld center microhardness,joint tensile strength and microstructure of the forward side transition zone was studied by single factor experiment.The test results show that the weld surface roughness and joint tensile strength decrease with the increase of shoulder pressure and ultrasonic amplitude,increase with the increase of welding feed rate,and decrease first and then increase with the increase of spindle speed.The microhardness of weld center increases with the increase of spindle speed,shoulder pressure and ultrasonic amplitude,and decreases with the increase of welding feed rate.With the increase of shoulder pressure and ultrasonic amplitude,the microstructure defects in the transition zone of the forward side are reduced,and the boundary line is more obvious.The influence of welding feed speed on the transition zone of the forward side is opposite.With the increase of spindle speed,it shows a trend of first decrease and then increase.(3)The orthogonal experiment of ultrasonic assisted friction stir welding was carried out for aluminum alloy materials,and the process parameters were optimized by signal-to-noise ratio and grey correlation analysis method.The test results show that with the tensile strength of the joint as the process index,through the signal-to-noise ratio and variance analysis,the optimal combination of parameters is obtained as follows: spindle speed 950r/min,welding feed rate 47.5mm/min,shoulder pressure 0.2mm,and ultrasonic amplitude 30μm.Under this process parameter,the welding test is carried out,and the tensile strength of the joint is251.78 MPa,which reaches 91.22% of the base metal.When the tensile strength and welding feed rate of the joint are taken as the process indexes,the optimal combination of process parameters is as follows: spindle speed 950r/min,welding feed rate 75mm/min,shoulder pressure 0.1mm,and ultrasonic amplitude 30μm.