Microstructure And Properties Of Ultrasonic Assisted 7075 Aluminum Alloy CMT Welded Joint

7075 aluminum alloy has the advantages of low density,high strength and excellent fracture toughness.It is widely used in military and civil industries such as aviation,aerospace,transportation and so on.The common welding methods of aluminum alloy sheet include gas metal arc welding and argon tungsten arc welding.In recent years,cold metal transition technology（CMT）has become the main technology of sheet metal welding.Due to the high thermal conductivity and thermal expansion coefficient of7-series aluminum alloy,it is easy to produce welding pores,resulting in the decline of bearing stability of welded components.At present,there are metallurgical measures and process measures to eliminate or reduce welding pores and improve weld quality.Among them,ultrasonic assisted method is a clean,environmental friendly and relatively convenient process measure.Therefore,this paper aims to improve the microstructure and mechanical properties of 7075 aluminum alloy CMT welded joints by ultrasonic assistance.Taking 7075 aluminum alloy as the research object,by changing the welding current and welding speed,the effects of different welding heat input on the microstructure and mechanical properties of 7075 aluminum alloy welded joints before and after ultrasonic assisted application were studied,and the better welding parameters were obtained.Through EBSD analysis and residual stress test methods and means,the mechanism of ultrasonic assisted welding improving joint structure and properties is further clarified.Before applying ultrasonic,when the welding current is 135 A and the welding speed is 8 mm/s,the tensile property of the welded joint is better,the tensile strength is 305 MPa and the elongation after fracture is 1.3%;The fracture of welded joint is in the weld area,and the fracture presents a large dimple.The fracture mode is ductile fracture,and the joint plasticity is good.Under this welding parameter,the average value of weld microhardness is 96.89 HV;The burning loss of Zn and Mg elements is not serious,and the content of strengthening phase is high.With the increase of ultrasonic power,the number of welding pores decreases and the weld width increases gradually.When the ultrasonic power is more than 1200W,no welding pores have been observed.When the ultrasonic power is 1200 W,the welding current is 135 A and the welding speed is 8 mm/s,the tensile strength and elongation after fracture of the welded joint,and the average microhardness of the central area of the weld are 390 MPa,2.7%and 101.31 HV respectively;The distribution of elements in the welded joint tends to be uniform,and there is no aggregation of any element;The peak value of Mg Zn₂in the X-ray diffraction pattern of welded joint has been significantly improved and more strengthening phases have been formed.The ultrasonic applied on the base metal causes the welding pool to produce cavitation effect and acoustic flow effect,which increases the fluidity of the pool.The edge of the pool is between liquid and solid phases.Ultrasonic vibration will break the columnar dendrite,and the broken dendrite will be trapped into the molten pool with the flow of the molten pool.It will become a new nucleation particle in the center of the molten pool and refine the weld grain.Therefore,applying ultrasonic can inhibit the columnar crystal and promote the growth of equiaxed crystal.When the ultrasonic power is 1200 W,the grain size in the weld center is 36.89μm.Compared with no ultrasonic,the grain is obviously refined;The distribution density of oriented grains decreased under different ultrasonic power.The transverse and longitudinal residual stresses of welded joints are reduced compared with those without vibration.