Mechanical Properties Of Electric Assisted Friction Stir Welded 2219 Aluminum Alloy

With the progress of society,more efficient,energy-saving and environmentally friendly processing technology is required for industrialization.Friction Stir Welding is widely used in aerospace,high-speed rail,automobile and other fields.Because of the limited heat input,the welding of medium and heavy plates and high melting point alloys is limited.The electrically assisted friction stir welding technology was studied.In order to improve heat input and material fluidity,a electrically assisted friction stir welding system was designed.In the electrically assisted welding process,the structure design,high-speed operation,current flow mode,insulation,current control,cooling and gas protection of the equipment are required to meet the welding process requirements.In order to solve the problems of non-uniform heating,low heat input and poor structure and properties of high melting point alloys in conventional friction stir welding,2219 T86 aluminum alloy was used as research material.Two different current forms,direct current and pulse current,were used to load different currents(0-600A)in the welding process.The mechanical properties and microstructures of the joints after welding were studied by temperature measurement and analysis on the advancing and retreating sides of the welds at different stages.The effects of current on the properties of 2219 T87 aluminum alloy joints were revealed through the macro-morphology of the weld fracture,the internal morphology of different regions of the weld and the distribution trend of hardness in different regions.The results showed that the temperature of the advancing side of the weld is higher than that of the retreating side in the electrically assisted friction stir welding process.The temperature field of the original heat input was redistributed due to the influence of current.With the load of current,the original weld area gradually increases,and the area of the heat-affected zone increases the most.Aluminum alloy,as a metal with good conductivity and low resistivity,can significantly improve the performance of welded joints when the current exceeds the average current of more than 500 A.With the same average current,the dynamic recrystallization zone of grains increases with the increase of heat input,and the tensile strength and elongation of the joints increase more with the increase of pulse current,because the peak value of pulse current is higher than the average value.The DC tensile specimens fractured at the stir zone,and the pulsed specimens fractured at the junction of the advancing stir zone and the heat affected zone.By studying the microstructure of electrically assisted fracture,it was found that the dimples are deeper and larger,and fine equiaxed dimples are uniformly distributed around the dimples.The pulse electrically assisted joint has better properties,which enhances the ductile fracture of the joint.The cross-sectional hardness of welded joints was studied,and the hardness distribution has a "V" distribution,in which hardness of the welded jonits increased in the the joint assisted by pulse current.The effect of pin and heat distribution are different in different areas of weld,which leads to different distribution of precipitated phases in weld.Among them,aggregation phenomenon of the precipitated phase was found at the junction of the advancing stirring zone and the heat affected zone,which affected the hardness of the joint and improved the mechanical properties of the welded joints.