Experimental And Simulation Study Of Mesoscale Titanium Plastic Flow Under High Frequency Vibration

With the development of new technology,the need of miniaturized products is increasing largely.The technology of micro-products is increasingly concerned by researchers and technicians.Ultrasonic vibration-assisted micro-forming is one of the hot spots.Ultrasonic vibration-assisted micro-forming is a complex forming process that ultrasonic vibration with specific frequency is set on the mold or blank in a particular direction on the basis of traditional micro-forming to form the required products.Compared with conventional micro-forming,ultrasonic vibration-assisted micro-forming has the advantages of reducing the forming force,improving the quality of the contact surface of products and so on.However,most studies focus on the factors such as amplitude and size.The effects of frequency,time duration and amplitude on plastic flow characteristics of pure titanium are investigated by ultrasonic vibration-assisted micro-upsetting experiments and numerical simulation.A set of high frequency vibration system is designed.The effects of frequency,amplitude and time duration on flow stress of ultrasonic vibration-assisted micro-upsetting are studied by two sets of vibration system.Based on the experiment data,the simulation of ultrasonic vibration-assisted micro-upsetting is built and verified.The influence of frequency and amplitude on flow stress is analyzed depending on experimental data and simulation model.Combined with the two-dimensional discrete dislocation kinetic model,the experiment results are interpreted on the macroscopic and microscopic perspective.Finally,the mechanisms of frequency and amplitude on soften effect of pure titanium are given.It is found that frequency and time duration have little impact on flow stress and decline of flow stress increases with the increase of amplitude.According to the experimental data,the micro-upsetting process of pure titanium is simulated by finite element model.The result shows that the plastic deformation process can be described objectively by the finite element simulation.The ultrasonic vibration-assisted micro-upsetting experiments are simulated by applying the displacement-time resonant curve and the average decrease of the simulated flow stress is less than that of true stress decrease in the experiment.The stress superposition leads to a decrease in the deforming force,but is not the main mechanism of its decline.The ultrasonic vibration assisted micro-upsetting with different amplitudes and frequencies is simulated and the results show that the frequency has an effect on flow stress mainly as decrease of flow stress increases with the increase of frequency,but the increase is not insignificant.The effect of frequency on the dislocation density is analyzed from a microscopic point of view using the two-dimensional discrete dislocation kinetic model.The result shows that the frequency on the range has the same effect on the dislocation density.By simulating the micro-upsetting of ultrasonic vibration with different amplitudes,it is found that the decrease of flow stress caused by the macroscopic stress superposition has a linear relationship with amplitude.