Prediction And Experimental Study Of Surface Residual Stress In Complex Surface Milling

Controllable pitch propeller is a typical complex surface part,which is an important part of the all-direction propeller.The surface performance of the propeller after processing directly affects the propulsion efficiency and service performance of the all-direction propeller.In recent years,the research and development of propellers with high service performance has put forward higher requirements for their machined surface integrity.As one of the important indexes of surface integrity,the residual stress distribution of blade directly affects the working stress,and has an important impact on fatigue life.In industry,torus-end cutter is widely used to machining large complex surface parts.According to the formation mechanism of residual stress in cutting,the sources of residual stress are mechanical stress under cutting force and thermal stress under cutting temperature and temperature gradient.As the last process,finishing cutting state will affect the surface quality of parts,so it is necessary to study the cutting force and cutting heat state under this process,and then calculate the residual stress under this state.So far,the achievements in the modeling of residual stresses in three-dimensional milling,especially for complex surface processing,are quite limited.In the thesis,the residual stress prediction model for complex surface milling is studied.At present,mechanical model and a large number of experimental data are mainly used to predict the milling force of complex surface machining.Experimental methods are mainly used to predict the milling temperature and residual stress of complex surface machining.These methods do not analyze the formation mechanism.In order to solve this problem,an analytical modeling method of milling force,milling temperature and milling residual stress are proposed to analyze the mechanism of residual stress under the effect of mechanical-thermal effect in milling process.The process of residual stress modeling can be divided into three stages: milling force modeling,milling temperature modeling,stress loading and unloading modeling.An analytical prediction method of cutting force in surface machining of torus-end cutter is proposed.Based on the principle of metal cutting,the cutting force of oblique cutting is modeled analytically.The geometric model of torus-end cutter is established,and the coordinates of cutter location and cutter vector at any time in five-axis machining are calculated.Based on Z-map method,the cutter-worker contact zone is calculated and the instantaneous chip thickness is calculated.The cutting force of micro element is calculated,and the milling force is obtained by space integration.At the same time,the micro-element shear force required for study of temperature field and the mechanical stress load in the shear zone required for the study of residual stress are obtained.The characteristics of Ni Al Bronze material and recommended cutting parameters are summarized,and the cutting force is verified by experiments.An analytical prediction method of the end temperature of the workpiece machined by a torus-end cutter is proposed.Using the idea of space and time history discretization of surface heat source,the cutting micro-element heat source can be regarded as surface heat source at each moment,which can solve the problem that the intensity of heat source changes continuously along the tool rotation angle and time history.According to the coordinate transformation relation in cutting force modeling,the expression of any point on the workpiece in the shear element coordinate system is obtained,and the contribution of heat source element to the temperature rise of the point is calculated.According to the space integral and the time integral,the temperature rise history of this point with time is obtained.The distribution of temperature and temperature gradient for residual stress calculation is obtained.The validity of the cutting temperature model is verified by milling temperature experiment.An analytical prediction method of residual stress in surface milling of torus-end cutter is proposed.Firstly,the mechanical stress caused by cutting force is calculated.According to the idea of discretization of cutting force modeling and temperature field modeling,cutting edge is discretized in space and cutting time is discretized in time.The contribution of load distribution in shear coordinate system to the stress at a point on the workpiece can be obtained at each moment.The problem of changing chip thickness and orientation in shear coordinate system can be solved.In this way,the orthogonal cutting stress model can be applied to three-dimensional oblique cutting.Then,in view of the problem of simplified orthogonal cutting temperature model used in the previous literature on residual stress prediction,according to the temperature field calculated by the proposed milling temperature model,the thermal stress generated under temperature and temperature gradient is further calculated.Finally,according to the rolling contact theory,the residual stress on the workpiece surface is obtained by loading-unloading-releasing analysis.The residual stress of nickel-aluminium bronze material was measured by X-ray method,and the correctness of the proposed model was verified.The validity of the model is further verified by comparing with the existing literatures data and the results of cutting simulation by AdvantEdge FEM software.Based on the proposed prediction model algorithm and MATLAB GUI,a simulation software for milling residual stress is developed.The milling residual stress of controllable pitch propeller is simulated.Taking a certain type of proportional controllable pitch propeller as an example,the simulation analysis and experiment of milling residual stress are carried out to verify the validity of the proposed prediction model.The theory adopted provides a theoretical basis for the subsequent optimization of machining parameters and helps to improve the processing quality.