Study On Prediction Of Distortion Due To Milling Process For Aerospace Monolithic Component Of Titanium Alloy

The distortion of monolithic component due to CNC machining is one of the most striking problems that aviation manufacturing has to face up to. Now titanium alloys are used widely in aerospace monolithic components owing to theirs excellent mechanical properties. Unfortunately, the machining distortion of titanium alloy monolithic component is more complex and troublesome because of its large machining loads during cutting process, small elastic modulus and complex distribution of initial stress. This state of affairs reduces the productivity and lowers the precision of the finished products. In view of the complex nature of distortion problem, a method combining finite element simulation with experiment and theoretical analysis is adopted in this dissertation. Then, the distortion prediction method and distortion law of monolithic component are researched deeply and widely.Firstly, the material model is studied, and the stress-strain relation is defined in cutting conditions. Based on Oxley cutting theory, the mathematic models of stress, strain, strain rate, temperature in shear zone and orthogonal cutting force are constructed. The ratio of length to depth in shear zone is taken as an iterative variable and the technology route is presented. Using the SHPB and orthogonal milling experiments data, the material constitutive model of titanium alloy is established, which can represent the big strain, high strain rate and high temperature characters under cutting conditions.To obtain the cutting force and temperature in milling process of titanium alloy, the finite element simulation and experiment are adopted. The helix double-blade finite element model of milling process of titanium alloy is developed for the first time. Several special finite element techniques, such as the chip separation criteria, friction model, heat conduction and adiabatic shear during cutting process have been implemented to improve the accuracy and efficiency of the finite element simulation. The law of cutting force and temperature in milling process are studied using this model, and the cutting force and temperature curves are established. At the same time, the cutting force and temperature experiments are designed and improved. Cutting force and temperature are measured and analyzed. Good agreement is found between the finite element results and the experiment values. That indicates that the cutting force and temperature curves are right.Based on the thermo-mechanical and elastic-plastic theories, an anneal analysis of titanium alloy is carried out by applying a sequentially coupled procedure, and the residual stress distribution of blank is gained. Then, the residual stress experiment is conducted by combining layer removal method with X-ray diffraction method. An agreement between experiment and simulating results shows the anneal simulation analysis is reasonable. According to the same dimension and position as designed part, the anneal blank of the aerospace monolithic components with initial residual stress are obtained.Material constitutive model, cutting force, cutting temperature and initial residual stress of blank are prepared by now. Based on an analysis of milling characters, an elastic-plastic finite element model is developed to simulate the milling process of titanium alloy monolithic component. Some key techniques, including material removal model, dynamic machining loads model, machining path and restriction transform, are explored and researched deeply. After that, the milling process of titanium alloy monolithic component is simulated using a restart analysis method, and the machining distortion is predicted successfully. At the same time, the machining distortion law of monolithic component caused by release of initial residual stress, cutting load and multi-factors coupling are researched respectively. That indicates the cutting load is the main element of causing machining distortion.Finally, the study conclusion in this dissertation is validated by comparing the distortion of monolithic components result from FEM and experiments.