Numerical Simulation And Process Research On Aluminum Alloy Ultrasonic Vibration Assisted Friction Stir Welding

Friction stir welding(FSW)technology is a new type of solid phase joining technology,which is widely used in the welding of light alloys due to its low welding temperature,small welding deformation and high joint quality.Since the welding heat input is derived from friction heat generation,the strring head needs to input a large axial pressure during the welding process,which causes short life of the stirring head,difficulty in improving the welding speed,and large welding load.In order to solve the above shortcomings,a variety of external energy-assisted friction stir welding techniques have been developed.Ultrasonic vibration-assisted friction stir welding technology is used as a mechanical energy-assisted form to improve the mechanical energy of metal atoms,improve welding quality,reduce welding load and improve welding.At the same time,the speed avoids the shortage of other heat source auxiliary forms such as double thermal cycle and deteriorating working conditions,and gradually becomes the focus of research.However,the influence of ultrasonic vibration is complicated,involving many aspects such as welding heat generation and material flow.Simply relying on the process test does not fully explain the material change during the welding process.Through the numerical simulation method,the influence of ultrasonic vibration on the temperature field and plastic flow of the friction stir welding process is analyzed,and then the process parameters are optimized to achieve high speed,low load and high quality welding,which has important engineering application value.Through the analysis of the thermal and thermal stress involved in the friction stir welding process,the assumptions and simplification conditions involved in the numerical model are determined,and the temperature stress constitutive model and the physical property parameters of the material with temperature are selected to establish the contact betw een the stirring head and the workpiece.The hot friction heat source model and the body heat source model of the material plastic deformation heat generation determine the thermal boundary conditions,velocity boundary conditions and welding loads of the model according to the actual conditions of the working conditions.The FLUENT software was used to establish the heat flow coupling model of the conventional friction stir welding dwell phase and the welding phase.Based on the verified conventional friction stir welding numerical model,the ultrasonic vibration assisted friction stir welding numerical model was established based on the ultrasonic vibration mechanism.The dynamic mesh method is used to solve the problem of large stress and large deformation simulation during friction stir welding,and the model convergence is accelerated.The numerical simulation of conventional friction stir welding results in the temperature field and flow field data of conventional friction stir welding at different times of the staying and welding stages.The variation of heat generation and material plastic flow in different stages is analyzed,mainly involving stirring.The head shaft shoulder heat generation,the whole temperature field cloud diagram of friction stir welding at different times,the material flow velocity near the stirring needle and the flow trace diagram.At the same time,the temperature field and flow field data of ultrasonic vibration assisted friction stir welding at the same time were obtained,and the differences of heat generation,heat transfer and material plastic flow between ultrasonic vibration assisted friction stir welding and conventional friction stir welding were compared.Studies have shown that after applying ultrasonic vibration,the peak temperature in the vicinity of the stirring head is lowered,the area of the low temperature area is larger,the amount of plastic metal near the stirring head is increased,the fluidity of the metal is increased,and the area in which the material flows is larger.The variable-parameter method was used to carry out the friction stir welding experiment of 3mm thick 2024-T4 aluminum alloy.The mechanical parameters and microstructure analysis of the weldment were used to determine the optimum process parameters.The parameters were used for conventional friction stir welding and ultrasonic at different frequencies.Vibration assisted friction stir welding comparison test.Studies have shown that the welding speed and the stirring head speed affect the size of the heat input,which in turn affects the growth of the grain in the weld nugget.Under the same process parameters,compared with the conventional friction stir welding,applying a specific frequency ultrasonic vibration can effectively soften the surface metal of the weld,increase the amount of plastic metal,and improve the welding quality of the workpiece surface;After applying ultrasonic,the tensile fracture of the joint has larger and deeper equiaxed elliptical dimples,and the number of strengthening phases in the dimple is more,and better tensile properties are obtained;ultrasonic vibration can soften the material before the stirring head and reduce the welding temperature.,inhibit grain growth.The above results arc the same as the numerical simulation results of ultrasonic vibration assisted friction stir welding,and both indicate that the 25kHZ ultrasonic vibration has a better gain effect on the friction stir welding quality than the ultrasonic frequency of 20kHZ.