Theoretical And Experimental Study On The Formability Of Mesoscale Sheet Metal Assisted By Ultrasonic Vibration

With the development of miniaturization,precision and intelligence of modern industrial products,as well as the market demand of micro medical devices,micro sensor,micro electro mechanical system(MENS)and wearable electronic products,the manufacturing technology of micro parts characterized by small shape and size or small operation size has developed rapidly.Therefore,mesoscale forming technology has become the focus of attention and research direction in industrial developed countries.However,when the size of the part is reduced to meso scale,the forming performance,deformation law and friction of the material show special changes,which makes the traditional macro forming theory could not be directly applied to the meso scale forming field.The introduction of ultrasonic vibration in mesoscale sheet metal forming have significant advantages in reducing scale effect,improving formability and improving surface quality.Although there are many advantages of ultrasonic vibration assisted meso scale forming,the change of forming force and flow stress caused by ultrasonic vibration named volume effect,the change of mechanical properties and friction state of contact surface named surface effect,and the influence of forming failure still needs to be further studied.Therefore,the research work in this dissertation focuses on the three problems of ultrasonic vibration assisted meso scale forming,namely,the correlation between volume effect,surface effect and forming failure.Based on the research conclusion of the theoretical models,the experimental device of ultrasonic vibration assisted micro-square cup drawing process was designed,and the new technology of ultrasonic vibration assisted micro-square cup forming was explored.The main research work of this paper will be carried out in the following aspects.(1)The volume effect mechanism of meso scale sheet metal forming assisted by ultrasonic vibration was studied.Firstly,the mechanism of volume effect in ultrasonic vibration assisted meso scale sheet metal forming was revealed from the perspective of thermal activation theory,dislocation density and motion,microstructure evolution and so on.Secondly,the plastic deformation behavior of sheet metal assisted by ultrasonic vibration is studied experimentally,and the stress-strain curves under different amplitude were analyzed.Finally,based on the thermal activation theory and dislocation kinematics model,a plastic constitutive model reflecting the volume effect was constructed by introducing ultrasonic energy field parameters.(2)The surface effect of ultrasonic vibration assisted meso scale sheet metal forming was studied.The mechanism of surface effect,which is the reduction of friction force in meso scale metal sheet forming based on ultrasonic vibration,was explained.The influence of ultrasonic vibration in different directions on contact friction was analyzed and discussed.The theoretical model was proposed to predict the effect of ultrasonic friction reduction based on adhesion plough friction theory,and a friction coefficient measuring device based on ultrasonic vibration was set up to carry out experimental research.(3)The forming limit experiments of sheet metal under different sizes,thicknesses and vibration frequencies were carried out.The correlation between ultrasonic vibration and forming limit was analyzed.The influence of different metal plastic instability theories on forming limit was compared.Based on the H-N theoretical model and considering the factors of ultrasonic vibration,a theoretical prediction model of sheet metal forming limit was constructed based on ultrasonic vibration.(4)An applicability about the forming process of micro-square cup parts was studied based on ultrasonic vibration.A device for micro-forming square cup based on ultrasonic vibration was developed.Combined with the previous research conclusions,a micro-square cup forming process control system based on c SPACE development platform was developed.The forming process of micro-square cup parts under different size specimen,different dies/punch and vibration parameters was studied experimentally.In this thesis,ultrasonic vibration was introduced into the forming process of meso size sheet metal,the mechanism of volume effect and surface effect caused by ultrasonic vibration was discussed,and the relationship between ultrasonic vibration and forming failure was studied.A prediction model for the limit drawing height of micro-square cup was constructed,which was combined with experiment and theoretical calculation.The prediction accuracy of ultrasonic vibration in micro-square cup forming process was verified and evaluated,which provides some reference for the research of new micro-forming process.