Research On Milling Stability In Ultrasonic Assisted Milling Of Titanium Alloy Thin-walled Parts

With the rapid development of aerospace industry,the utilization rate of titanium alloy thin-walled parts has increased rapidly,and the structure is more complex,the thickness of the wall becomes thinner.During machining process of titanium alloy thin-walled parts,the difficult machining characteristics of titanium alloy and the low stiffness of thin-walled parts tend to causes chattering phenomenon,which has negative effects on machining quality,precision and efficiency.Thus the development and application of titanium alloy thin-walled parts in aerospace field is seriously limited.Therefore,it is of great significance to improve the stability of the thin-walled parts of titanium alloy during the machining operation.In this thesis,the stability of ultrasonic assisted milling of thin-walled titanium alloy parts is theoretically and experimentally studied.The trajectory of the tool nose in longitudinal,torsional and longitudinal-torsional combined ultrasonic vibration assisted milling(side milling)were studied to explore the contact state between the cutting edge and the chip.The method of solving the contact point and the separation point between the cutting edge and the chip during the single ultrasonic vibration machining is established by comparing the cutting edge movement angle and the cutting edge helix angle,and then the net cutting time and duty ratio of the cutting edge in single ultrasonic vibration are solved,finally the influence of the process parameters on the net cutting time and the duty ratio were studied.Combined with the analysis of the movement of the cutting edge in ultrasonic vibratory milling,the cutting force model of ultrasonic vibration assisted milling was established by introducing the function of the contact state between cutting edge and chip based on the normal milling cutting force model.The cutting force coefficient between the four-edge and two-edge end mill and the titanium alloy was solved by experiments.The feasibility of the model was verified by comparing the experimental value and the predicted value of the cutting force.Based on the full-discretization method,the stability prediction model of longitudinal,torsional and longitudinal-torsional combined ultrasonic vibration assisted milling system(side milling)were studied,and the influence of each technological parameter on stability of ultrasonic vibration milling system was achieved.By comparison,it is found that the stability lobe map of the ultrasonic vibration assisted milling system is higher than that of the ordinary milling system,and the stability of UVA system is better.On the basis of theoretical analysis,titanium alloy thin-walled parts with special structure were designed,and the modal properties of thin-walled parts were measured by modal tests.The variable-depth cutting tests on thin-walled parts were carried out to verify the feasibility of this method for the stability prediction of ultrasonic vibration assisted milling systems.