Experimental And Simulation Study On Ultrasonic Vibration Assisted Micro-upsetting Of Pure Copper

The development of plastic micro-forming is greatly limited due to poor filling performance of material,large interfacial friction,poor forming accuracy and non-uniformity during deformation.Ultrasonic vibration assisted micro-forming can improve the forming ability of materials,reduce the forming force and improve the friction between the mould and the materials,and has become a research hotspot in recent years.It has provided a new idea for solving the problems faced in micro-forming.The experimental and theoretical research on ultrasonic vibration assisted micro-forming has important theoretical significance and engineering application value for enriching and developing micro-plastic forming theory and process and promoting the application and development of ultrasonic vibration assisted micro-forming technology.This paper focused on ultrasonic vibration assisted micro-upsetting of copper,through experimental research,numerical simulation and molecular dynamics simulation,the effect of ultrasonic vibration on the forming load,material flow stress,microstructure,the interface friction and the dislocation evolution in material deformation are systematically studied.This paper also studied the forming mechanism of ultrasonic vibration in the micro-forming.The main work of this paper is as follow:The experiment of ultrasonic vibration assisted micro-upsetting of pure copper was carried out.The ultrasonic vibration at different amplitudes was applied to the micro-upsetting process of pure copper at a frequency of 28 kHz at room temperature.The effect of ultrasonic vibration on the material flow stress,forming quality and microstructure of workpiece was studied.The variation of flow stress during the ultrasonic vibration assisted micro-upsetting process under different sample sizes and grain sizes was studied,and the size effect was analyzed.Based on the comprehensive study of the existing material constitutive model and the dimensional parameter model,the calculation method of the proportion of surface layer and volume fraction of grain boundary of materials after optimization was given.The effects of grain orientation and grain boundary were further considered and the constitutive model of pure copper at the micro-scale was built based on the surface layer model.The established constitutive relationship was solved and verified using the experimental results,and had high accuracy.Based on the established constitutive model and Voronoi method,the FEA model of ultrasonic vibration assisted micro-upsetting of pure copper at the grain-scale was established.The effects of ultrasonic vibration parameters,process parameters and the billet parameters on the deformation behavior of materials during ultrasonic vibration assisted micro-upsetting process were studied.The forming load,flow stress,stress-strain distribution,material flow,interfacial friction and the degree of drum shape of the sample were analyzed.The mechanism of ultrasonic vibration on the surface effects and volume effects in the process of ultrasonic vibration assisted micro-forming was clarified.An ultrasonic vibration assisted compression model of[100]-oriented single crystal copper was established by molecular dynamics method.The relationship between the stress,the total dislocation length and the strain of the sample under conventional compression,and the changes of various types of atoms inside the crystal were analyzed.The motion state of atoms and dislocations under ultrasonic vibration and the change of bonding strength among atoms were studied.The deformation behavior and variation law of materials during ultrasonic vibration assisted micro-volume forming process were revealed,and the mechanism of ultrasonic vibration on material deformation behavior at the micro-scale were also revealed.