Study On Ultrasonic Mechanisms And Its Influence On Materials Deformation Behaviors In Ultrasonic Vibration Assisted Micro-forming

With the trend of product miniaturization,micro manufacturing technology is gaining more attention.Meantime,the field of micro-manufacturing has gained more and more attention.Micro-forming technology is one of the most promising methods in the field of micro-manufacturing.But due to the existence of the "size effect",plastic forming theory based on macroscopic continuum mechanics and conventional forming processes and equipments can not fulfill the needs of micro-forming,there exists some problems in application of micro-forming such as changes of deformation behavior and serious local defects.In micro-forming,ultrasonic vibration can reduce forming load,reduce interfacial friction,improve material forming performance and enhance the part quality,which can efficiently address the problems faced by traditional micro-forming.Ultrasonic-assisted micro-forming has recently become a hot spot for research.However,the existing theoretical system of micro-plastic forming is still immature,and the mechanism of ultrasonic vibration on material forming is still unclear.After the introduction of ultrasonic vibration into micro-forming,the material forming performance and deformation pattern in ultrasonic vibration-assisted micro-forming become more complicated.In particular,the influence of ultrasonic vibration parameters on material flow and deformation law,the coupling effect of ultrasonic vibration and size effect,the constitutive relationships and theoretical models of materials cannot accurately reflect the mechanism of ultrasonic effect,etc.,become bottlenecks that restrict the ultrasonic-assisted micro-forming technology.Accordingly,an ultrasonic vibration-assisted micro-forming experimental system was designed and fabricated,and the influence of ultrasonic volume and surface effects on metal deformation behavior in micro-forming was systematically investigated in typical plastic forming experiments such as micro-upsetting,micro-extrusion and micro-coining of T2 pure copper.After that,coupling effect of size effect,volume effect and surface effect was analyzed,and the mechanism of ultrasonic volume and surface effects was explored from macroscopic and microscopic perspectives using finite element method（FEM）and molecular dynamics（MD）method.The main works are as follows:To address the problems in the conventional "surface layer model",such as the thickness of the surface layer is too small,and the surface layer stress and internal stress are discontinuous,which are not in line with reality,the surface layer model is modified based on plastic forming theory and the transition layer model,and the calculation method of surface layer thickness and stress in each layer is given.Then the surface layer thickness model based on grain size is proposed.According to the results of microcompression experiment conducted by previous authors and this paper,the proposed modified surface layer model and surface layer thickness model can accurately predict the deformation behavior of the material at microscale.In order to study the influence of ultrasonic vibration on the metal deformation behavior in micro-forming systematically,an ultrasonic-assisted micro-forming device was designed and produced based on acoustic theory and modal analysis method,and the micro-upsetting and micro-double cup extrusion experiments on T2 pure copper were conducted.The results indicate that there is a clear coupling between ultrasonic effects and size effect.The smaller the billte size,the more significant the ultrasonic effects.Based on the material hardening model and the experiment results in microupsetting,the constitutive model of T2 pure copper material under the coupling effect of size effect and volume effect was constructed.It was also found that the conclusion that the interfacial friction increased by ultrasonic vibration in double-cup extrusion was an "illusion" obtained by using the conventional method to calibrate the friction factor even though the friction conditions of the upper and lower punches were different due to ultrasonic vibration.To address the problem that the current finite element simulation cannot accurately reflect the macroscopic mechanism of ultrasounic vibration,a new method is proposed to represent the ultrasonic volume and surface effects respectively by using the material flow stress drop and friction coefficient reduction.The ultrasonic vibration-assisted upsetting and U-groove extrusion are simulated using this method,and the ultrasonic volume and surface effects are quantified and analyzed.It is proved that the method can reflect the material forming behavior under the action of ultrasonic vibration,and there are significant differences in the ultrasonic volume effect,surface effect and their effects on the forming process in the upsetting and U-groove extrusion processes.In response to the lack of theoretical support for the microscopic mechanism of ultrasonic vibration on material forming,this paper adopts a molecular dynamics approach to simulate and analyze the ultrasonic vibration-assisted single-crystal copper micro-tension,micro-compression and relative sliding processes.It is shown that ultrasonic vibration enhances the vibration energy of internal atoms,which promotes the dynamic evolution of dislocation generation,motion and annihilation,which is the main mechanism of ultrasonic bulk effect.Ultrasonic vibration provides additional energy for surface atoms to break the energy mat barrier more easily and reduce the stick-slip friction behavior,which is the main mechanism of ultrasonic surface effect.The coining of micro-cylindrical process can be improved significantly by the application of ultrasonic vibration,with a maximum increase of 500%in microcylindrical height.The ultrasonic action time and the maximum forming load are the main factors affecting the micro-cylindrical height.The micro-cylindrical height increases with ultrasonic action time,but when the ultrasonic action time exceeded 10 s,the material reached the forming limit under ultrasonic action,and the extension of ultrasonic action time will gain no more.In addition,ultrasonic frequencies closer to the natural frequency of the punch and larger ultrasonic amplitudes also significantly enhance the effect of ultrasonic vibration.