Research On Numerical Simulation Analysis Of Pure Titanium Plastic Flow Stress On Meso Scale With Ultrasonic Vibration

The softening effect of ultrasonic vibration was found in the middle of 20 th century.With the rapid development of ultrasonic vibration technology,ultrasonic vibration has been widely used in many kinds of plastic deformation processes.Ultrasonic vibration can effectively make plastic deformation process easier,especially in micro plastic deformation.In micro plastic deformation,due to the micro size of specimen,the size effect makes the plastic deformation process more difficult.However,the introduction of ultrasonic vibration can help improve the surface quality of parts.But the theories which can explain the softening effect are still in discussion,and research about micro mechanism of softening effect is still few.This article mainly discussed about how the ultrasonic vibration will affect the dislocation dynamics.On the meso scale,this article set the two dimensional discrete dislocation dynamics model of pure titanium.According to the characteristics of pure titanium in plastic deformation,the model simplified the dislocation dynamics of pure titanium.And the model also introduced the ultrasonic vibration and periodical boundary condition.Using C++,this article created a program according to the model set.And the article succeeded in obtaining the stress strain curves and dislocation density versus strain curves of pure titanium in different deformation situations.In order to verify the results of simulation,this article used the ultrasonic vibration assisted micro upsetting experiment system from the research group and did experiments in different deformation situations.At the same time,nanometer indentation experiments were done to calculate the dislocation density and compare the experiment results with ones from simulation.At first,the results from experiments and simulations in different deformation situations were compared.And it was proved that the model of dislocation dynamics was reasonable for pure titanium.On the basis of this comparison,the dislocation density during the plastic deformation was investigated.It was found that the appearance of ultrasonic vibration helped decrease the dislocation density compared to the situation without ultrasonic vibration.At the same time,it was found that with higher ultrasonic vibration energy,the dislocation density became lower.These results can explain why the ultrasonic vibration results to softening effect.The appearance of ultrasonic vibration helped dislocation move faster and further.And it was more likely that dislocation can meet the dislocation with the opposite sign which lowered the dislocation density and the flow stress of pure titanium.The effect of frequency of ultrasonic vibration on the dislocation dynamics of pure titanium was also investigated.And it was found that under different ultrasonic vibration power,the dislocation density did not change with the frequency in the scale of 1kHz~160kHz.Simulation of macroscopic model was also done.After comparing the simulation results with experiment results,it was found that there was a huge difference between experiment results and simulation results because dislocation dynamic was not considered into the model.