Deformation Mechanism And Prediction Of Aluminum Alloy Monolithic Component In The Milling

With the increasing development of the aircraft functions, the properties requirements for the aircraft structure are more critical. The reliability of machining deformation of large aerospace monolithic components is highly concerned. The problem of how to enhance the reliability of products has become the bottleneck to achieve the battle effectiveness of weapon equipments and the market competence of civil large aircraft under development.The factors of affecting the deformation include the material properties, workpiece structural characteristics, blank initial residual stress, cutting stress, temperature etc. Due to rather few comprehensive studies about machining deformation mechanism in the domestic and overseas report, and there no effective deformation prediction system, it is greatly limited the application in the actual production and development. With the aid of the national natural fund project "Study on correcting theory and method for aviation monolithic component based on residual stress adjustment applied on the local" (51275277) and a military project "Aluminum alloy aviation monolithic component deformation prediction and compensation system development on CATIA" (Department of science and technology11-018), we have done the research of the aerospace monolithic components machining deformation mechanism.7050-T7451 aluminum alloy is the research material. This dissertation is about to resolve "why deformation" and "what is the primary and secondary factors influencing the deformation". In the end, deformation prediction system is established for engineering application.Based on the combination of theoretical analysis, finite element modeling and experimental research, we research the relation between stiffness characteristics, residual stress redistribution and workpiece deformation. Considering the single factor and multi-factor coupling action, such as stiffness, initial residual stress, machining residual stress, temperature, we established the aerospace monolithic components deformation forecast. It provided theoretical basis for the machining deformation forcast, control and process parameters optimization.First of all, accurate measurements of the coefficient of thermal expansion of 7050-T7451 aluminum alloy are conducted. The measurement is carried out with the help of thermo mechanical analysis(TMA). Numerical fitting method is adopted to obtain the CTE variation rules, which is nonlinear, with temperatures increasing. Under Windows’s Access environment, we establish the coefficient of thermal expansion database. The inherent nature of CTE variation was revealed based on 7050-T7451 composition and metallography. In the end, workpiece deformation prediction analysis was carried out using the average CTE and accurate CTE models respectively. The results show that the maximum relative error got by the accurate CTE dropped from 17.4% to 9.5% comparing with the average CTE.We analyzed the deformation characteristics of the workpiece in the machining process and the corresponding residual stress state, and revealed the evolution regularity of residual stress; based on the equivalent stiffness calculation method of the bending strain energy, we got the stiffness change rule of the aerospace monolithic components in the NC machining process; in the end, using the three frames aerocraft beam as an example, the mapping relation model of stiffness change, residual stress evolution and deformation was established.A high-speed milling experiment by means of orthogonal method with four factors was conducted for aluminum alloy 7050-T7451. The residual stresses on the surface and subsurface of the work piece were measured using X-ray diffraction technique and electro-polishing technology. An exponent’s mathematical model for milling residual stress of 7050-T7451 aluminum alloy was established related to machining parameters and tool geometry. The effect of cutting speed, feed rate, width and depth of cut on residual stress was investigated using the ANOVA techniques. The effects of material initial residual stress and machining induced residual stress on the deformation of aluminium alloy plate are studied. The theoretical model of the plate is analyzed firstly, and the experiments of the milling deformation under different initial residual stress conditions are performed. The results show that the machining induced residual stress is the primary factor of distortion. The coupling action of compressive initial residual stress and machining induced residual stress increase the plate deformation, and the coupling action of tensile initial residual stress and machining induced residual stress decrease the plate deformation. To study the effect of blank initial residual stress on 3-frame monolithic beam deformation, chemical milling was used to remove the machining induced residual stress on the machined surface of the components. The research results show that the deformation caused by machining residual stress accounted for about 7.8% of the total deformation of the component, and the deformation caused by the blank initial residual stress accounted for 92.2% of the total deformation of the component.The finite element simulation system for predicting machining deformation is developed by customizing applications of secondary development of ABAQUS, in which aerospace monolithic components are modeled considering the relationship between the machining stress and process parameters, structure parameters of tools. Using a civil aircraft long beam as an example, we analyze the machining deformation characteristics under multi-factor coupling action. The stiffness evolution of complex beam in machining is studied. The results show that the bending and twisting is the mainly deformation.