Study On Microstructure Transformation Of The Weld Joint Of Aluminum Alloy And Mechanism Of Ultrasonic Assisted TIG Welding

In the TIG welding of aluminum alloy,there are many insufficiencies,such as the coarse microstructure of weld grain,the softened of joint,the welding porosities,and the low-efficiency of welding.For these problems of aluminum alloy TIG welding,an ultrasonic assisted TIG welding method for aluminum alloys was proposed.In this study,the ultrasonic was acted on the solid base metal behind the molten pool during the welding processing.The distance between the ultrasonic output end and the molten pool is constant,which reduces the attenuation of the energy.The characteristics of weld formation under ultrasonic action were studied.The effects of the ultrasonic on the microstructure transformation and porosity defects of the weld were analyzed.The mechanism of ultrasonic on grain refinement was explored.Firstly,through the bead-on-plate welding of aluminum alloy,the characteristics of the weld formation of the ultrasonic assisted TIG welding for aluminum alloy were studied.It was found that the weld appearance is periodically convex due to the action of periodic input of the ultrasonic energy.Besides,the penetration depth,width and the fusion area of the weld increase.The effects of ultrasonic on the flow behaviors and the heat conduction of the melting metal were analyzed.It was found that under the ultrasonic vibration,the melting section of weld was changed from simple melting type to central melting type.Secondly,the microstructures and porosity defects in butt welds for 2219 aluminum alloy were studied,and the mechanical properties of the welded joints were tested.The results show that under the action of ultrasonic,the grain shape of the heat-affected zone changes and the grain size decreases.The coarse columnar crystals in the fusion zone are broken and the size decreases.The nucleation rate near the fusion zone increases,and the grains grow to be equiaxial.Periodic crystallization occurs in the weld zone,and equiaxial and columnar crystals alternately distribute.Ultrasonic changes the temperature gradient of the molten pool and the cooling rate of the weld.The coarse precipitated phases in the heat affected zone are broken,and the precipitated phases both in the fusion zone and in the weld are uniformly dispersed.Ultrasonic energy can promote the growth and the floating of the bubbles,which leads to the number of pores in the weld reduces.Compared to conventional TIG welded joints,the hardness of the 1000 W ultrasonic assisted TIG welded joint increases by 8.4%,and the tensile strength of the 800 W ultrasonic assisted TIG welded joint increases by 30.1%.Finally,the mechanism of action of ultrasonic on grains refinement was preliminarily analyzed.When the ultrasonic wave spreads in the liquid medium,it causes cavitation effect,vibration effect and sound flow effect.The mechanism of them on the promoting nucleation and the dendrites breaking is analyzed respectively.The two mechanisms for grain refinement are verified by spot welding test.