Numerical Simulation Of Microstructure Evolution In Ultrasonic Vibration Enhanced Friction Stir Welding Of 2195 Al-Li Alloy

As a new generation of lightweight and high-strength alloy materials,aluminium-lithium alloy has a wide range of application prospects in aerospace and rail transportation fields.However,aluminium-lithium alloy is prone to porosity,thermal cracking and burning of alloying elements during fusion welding,which seriously weakens the quality of the joint.Friction stir welding(FSW)is a conventional method of welding high-strength aluminiumlithium alloy,although it can overcome the shortcomings of the fusion welding method,but there are still problems such as low welding efficiency and narrow process window.Numerous experimental process studies have shown that the addition of ultrasonic vibration to FSW can effectively improve the shortcomings of FSW.There have been many experimental studies of ultrasonic assisted friction stir welding for aluminium-lithium alloy.However,the mechanism of the effect of additional ultrasound on the microstructure evolution of Al-Li alloy FSW joints is still unclear.In addition,it is difficult to directly observe the microstructural evolution of the joint by means of experiments.Therefore,the establishment of a microstructure evolution model for ultrasonic vibration enhanced friction stir welding(UVeFSW)of Al-Li alloy can help to analyze the microstructure evolution behavior of Al-Li alloy during the FSW process,and help to quantitatively analyze the influence of applied ultrasonic vibration on the microstructure evolution of the weld.This has important theoretical significance and practical engineering value for the rational use of ultrasonic energy to promote high-quality and efficient welding of Al-Li alloy.Firstly,based on the ultrasonic preheating and acoustic plastic softening effects,this paper established a numerical model of 2195 aluminium-lithium alloy during UVeFSW and obtained information on the temperature,strain and strain rate of the material in the typical region of the joint during welding.By comparing the simulation results of FSW and UVeFSW,it was found that ultrasonic vibration has a certain preheating effect on the material in front of the tool.The introduction of ultrasonic energy could increase the strain of the material in the influence zone of the tool pin to a certain extent,but the effect of applying ultrasound on the strain rate history of the weld material was not significant.Subsequently,the dislocation density was corrected based on acoustic hardening effects and a Monte Carlo model of the microstructural evolution of the Al-Li alloy UVeFSW joint was established.Compared to existing models,the following improvements have been made in this work.On the one hand,the influence of grain size as well as morphology on the distribution of deformation storage energy was taken into account in the model.On the other hand,in order to better match the actual process of microstructure evolution in welded joints of aluminiumlithium alloy,precipitation phases and thermal gradients were introduced in the model.Afterwards,the macroscopic model of welding thermal coupling and the microstructure evolution model of the joint were combined to numerically simulate the microstructure evolution of materials at different positions of the joint during the welding process.The Monte Carlo simulation results were validated using the EBSD characterization results of the joint microstructure.The model results were in good agreement with the experimental results,and the established model can better describe the evolution law of microstructure during the welding process.Finally,the microstructure evolution of Al-Li alloy FSW joints and UVeFSW joints was investigated by analysing the simulation results.The effect of ultrasound on the microstructure evolution and grain size change of Al-Li alloy FSW joints was also quantified.It was found that acoustic hardening triggers a large proliferation of dislocations,which increases the deformation storage energy of the material and increases the nucleation rate of dynamic recrystallisation,making the dynamic recrystallisation process in the weld nugget zone of the joint more complete.As a result,the width of the weld nugget zone increases and the grains in the weld nugget zone are refined.Under the welding process parameters of 1000 rpm-120 mm/min,the peak dislocation density in the weld nugget zone of UVeFSW joints increased from 2.49×1013 m-2 in conventional FSW joints to 3.49×1013 m-2,and the grain size in the weld nugget zone was refined by approximately 10.5%compared to FSW joints.However,additional ultrasound does not change the process of microstructural evolution of the FSW joint,complete dynamic recrystallisation occurs in the weld nugget zone,and when the dynamic recrystallisation is completed,the recrystallisation grain growth process is mainly influenced by the thermal cycle of welding.The grain size in the weld nugget zone is dominated by the welding heat input,and the average size of the equiaxed grains in the weld nugget zone is slightly larger at higher rotational speeds than at lower rotational speeds.