Study On The Thermo-mechanical Effect And Material Flowing In Ultrasonic Assisted Friction Stir Welding Of Aluminum Alloy

Ultrasonic assisted friction stir welding gradually highlights the advantages in the welding of aluminum structure due to its high welding quality and small welding load.The research on the behaviors of temperature,stress,strain and material flow in the welding process is of great significance for in-depth understanding of the welding mechanism and effective control of welding defects.In this study,the influence of ultrasonic vibration on material properties and welding process were considered,and the numerical model of temperature field and material flowing in ultrasonic assisted friction stir welding was established.Combined with the welding test,the influence of welding parameters on the temperature field and plastic deformation behavior in the weld region was analyzed.The main research work and results of this study are as follows:(1)Based on the discrete element method and particle contact model,combined with the material characteristics of aluminum alloy,a visco-elastic-plastic particle contact model suitable for 6061 aluminum alloy was established through the API interface of EDEM2020 and the C++ language.The correctness of the contact model was verified by the mechanical test.Based on the contact model,the vibration-assisted tensile model of 6061 aluminum alloy was established.The influence of ultrasonic vibration on the tensile process of aluminum alloy was analyzed from the microscopic scale by using the discrete element method.The results show that the ultrasonic vibration can aggravate the movement of particles and promote the failure of the contact between particles under lower load.Macroscopically,The yield strength of 6061 aluminum alloy decreases(2)Considering the influence of ultrasonic vibration and welding temperature on the workpiece material,the high temperature mechanical test and ultrasonic vibration assisted tensile simulation of 6061 aluminum alloy were carried out.The mechanical properties of materials under different temperatures and ultrasonic parameters were obtained.The parameters in the J-C constitutive model were fitted according to the results,and the constitutive model of 6061 aluminum alloy suitable for numerical simulation of ultrasonic assisted friction stir welding was established.(3)Considering the influence of ultrasonic vibration on the contact force and friction coefficient between the stir tool and the workpiece,the thermo-mechanical coupling model of ultrasonic assisted friction stir welding is established based on Coulomb ’s contact law,and the temperature field distribution and peak temperature change curve during welding are obtained.The accuracy of the model was verified by welding test,and the influence of welding parameters on welding temperature field is studied by control variable method.The results show that the temperature field on the workpiece surface is asymmetrically distributed as a whole,and the high temperature region is distributed below the shoulder feeding side.With the increase of ultrasonic amplitude and frequency,the area of high temperature region and the peak temperature decreases.(4)The thermal-mechanical coupling model of aluminum alloy ultrasonic assisted friction stir welding was established based on Coupled Eulerian-Lagrangian method.The material flowing behavior in the welding process was simulated,and the distribution of equivalent plastic strain and temperature field in the weld area was obtained.The influence of welding parameters on the distribution of equivalent plastic strain in the welding area was studied by control variable method,and the accuracy of the model was verified by welding process test.The results show that with the increase of ultrasonic amplitude and stir tool speed,the maximum equivalent plastic strain at the weld and the nugget zone increases.With the increase of the feed rate of the stir tool,the maximum equivalent plastic strain in the welding and the nugget area decreases.During the welding process,the material in the welding first moves to the bottom of the workpiece,and then flows to the upper surface of the workpiece with the rotation of the stir tool and accumulates behind the stir tool to form the welding.