Fully Coupled Thermomechanical Numerical Simulation Of Ultrasonic Vibration Friction Stir Welding

Ultrasonic vibration friction stir welding(UVFSW)is developed on the basis of conventional friction stir welding(FSW).Its basic working principle is an additional axial ultrasonic vibration is added to the stirring tool,using ultrasonic vibration's volume effect and surface effect to affect the material's microstructure,improve the material properties of the workpiece to be welded and increase welding quality.The ultrasonic vibration also reduces the load on the stirring tool and increases the stirring tool service life.In this paper,7075 aluminum alloy is taken as the research object.Based on the basic theory of solid mechanics,the fully coupled thermomechanical numerical model of welding process is established by ABAQUS finite element software.The reasonable assumptions and simplifications of the welding process are made and a feasible simulation procedure is put forward according to the current research status.In the model,the ALE algorithm is used to separate the material points from the grid nodes to avoid the problem of grid distortion in the process of plastic deformation.At the same time,Johnson-Cook elastic-plastic constitutive model is used in combination with the properties of strain hardening and temperature softening.Considering the friction heat generated between the stirrer head and the workpiece to be welded and the plastic deformation and heat generation of the weld metal,the temperature of the metal material is increased and enters the thermoplastic state,which limits the unlimited growth of the friction force.Therefore,using the modified Coulomb friction law describes the contact condition.Then the temperature distribution of friction stir welding was analyzed,and the calculation results were compared with the experimental results of relevant literature to verify the rationality of the model.Considering the influence of ultrasonic vibration on the properties of the workpiece and the contact state,a fully coupled thermomechanical numerical of ultrasonic vibration friction stir welding model was established.On the one hand,the influence of ultrasonic vibration on the plastic deformation of aluminum alloy was analyzed based on dislocation theory,and Johnson-Cook model was modified.The flow stress calculated by the model is compared with the related test results to verify the reliability of the model.On the other hand,based on the ultrasonic vibration surface effect,the mechanism of ultrasonic vibration reduction friction is qualitatively analyzed.According to the calculation results,the effects of axial ultrasonic vibration on equivalent plastic deformation,temperature distribution and load on the stirring tool were analyzed.Finally,the calculation results under different vibration parameters are compared,and the influence of vibration parameters such as vibration frequency on the ultrasonic vibration welding process is summarized.In addition,the acceleration algorithm of mass scaling is used in the calculation to improve the operation efficiency and evaluate the stability of the calculation.