Residual Stresses And Deformations Of Aerospace Aluminum Alloy In Machining

Monolithic components are being widely used for aerospace applications because of their combination of lower height, higher assembly quality and higher structural efficiency. However, these components are large in size, complex in structure, and are often thin-walled. In the milling process of monolithic components, more than 90% of the materials would be removed which result in their low rigidity. Therefore, the large machining deformations of the monolithic components are often observed. Such machining deformations can be attributed to the following factors: (1) the release and redistribution of the original residual stresses, (2) machining-induced residual stresses, (3) the action of dynamic cutting loads and clamping forces. Considering above influencing factors, a method combined experiment with theoretical model and computer simulation was proposed in this dissertation to study machining deformation mechanism of monolithic components, which could provide a theoretical guide for predicting and controlling the machining deformations of monolithic components.The rolling and quenching processes result in high residual stresses during the production of 7050-T7451 aluminum alloy pre-stretched plate. These residual stresses are relieved by applying a uniform plastic strain in the rolling direction. In order to measure a full through-thickness original residual stress profile of aluminum alloy thick plate, a method named as the crack compliance method was presented. The compliance functions were calculated by finite element method. An optimal expansion order was obtained based on minimizing the total stress uncertainty which was evaluated by considering the two main sources in calculation of stress uncertainty: the random errors in strain data and model error. Then the residual stresses depth profiles in pre-stretched aluminum alloy plate 7050-T7451 were determined. The results revealed that Legendre polynomials that the fit order is 9 can evaluate accurately through-thickness residual stresses of aluminum thick plate. It is also shown that the residual stresses distribution in pre-stretched aluminum alloy plate can be revealed by "M type curves", that is, the exteriors are residual compressive stress and the interiors are residual tensile stress. These works can provide a basis for analyzing and predicting the effect of the blank's original residual stresses on machining deformations of monolithic components.Based on finite element software DEF0RM-2D and DEF0RM-3D, a two-dimensional orthogonal cutting model and a three-dimensional oblique cutting model for aerospace aluminum alloy were built, respectively. The material's flow stress behavior was described with Johnson-Cook constitutive equation. The separation of the chips with the workpiece was realized by the combination of adaptive remeshing technique and separation criterion. The material's failure was defined by adopting Cockcroft & Latham fracture criterion. The tool-chip friction model was the combination of a Coulomb friction model and shear (sticking) friction model. To validate the finite element model, orthogonal cutting tests were conducted. The simulation models can predict non-uniform stresses, strains distribution in the chips and the workpiece. The effects of tool geometrical parameters such as flank wear, cutting edge inclination and corner radius on cutting forces and cutting temperature fields were analyzed by three-dimensional oblique finite element model.The superficial residual stresses in milling aerospace aluminum alloy were measured by Doelle-Hauk method. The measurement results show that the principal plane is approximately parallel with the machined surface of the workpiece, which reveals that these stresses are under two-dimensional plane stress state. In the experiments, the machining-induced residual stresses in depth were measured by using X-ray diffraction technique in combination with electro-polishing technique. Particular attention was paid to the influence of cutting parameters, such as the spindle speed, feed rate and flank wear on residual stresses in milling aluminum alloy. The results revealed that the flank wear would appear to influence on machining-induced stresses most strongly, and spindle speed appear to be of minor influence on machining-induced stresses, while the feed rate could have a few less effect on machining-induced stresses. In order to correlate the residual stresses with the thermal and mechanical phenomena developed during milling, the orthogonal components of the cutting forces were measured using a Kistler 9257A type three-component piezoelectric dynamometer. The temperature fields of the machined workpiece surface were obtained with the combination of infrared thermal imaging system and finite element method. The formation of the residual stresses can be explained by thermo-mechanical coupling effects.The machining deformations caused by the release and redistribution of original residual stresses were studied by the theoretical analysis and finite element method. The research results show that the release and redistribution of original residual stresses result in bulking distortion of the plate part. To satisfy force and moment equilibrium in the finite element model, nine basis functions were chosen to represent the machining-induced stresses profile through the depth in the thin machined specimens. The machining deformations caused by machining-induced stresses were analyzed by finite element method. The simulation results show that machining-induced residual tensile and compressive stresses induce also bulking distortion of the frame component, and machining-induced residual shear stresses cause pure twisting distortion of the frame component. A finite model was built to study the effects of bulkhead processing sequences and processing routes on machining deformation of multi-frame monolithic components. An optimal processing scheme was then proposed based on minimizing the machining deformationThe machining deformations prediction model was developed considering multi-factors coupling effects including original residual stresses, clamping load, milling mechanical loads, milling thermal loads and machining-induced residual stresses. The machining deformation of a frame monolithic component was predicted by this model. To validate the prediction model, a true frame component was machined and its deformation was measured on a Coordinate Measuring Machine. The deformations by prediction model show a good agreement with the experiment results. The machining deformations prediction model can provide an effective way to study further control strategies of the machining deformations for monolithic component.This project is supported by National Natural Science Foundation of China under Grant No.50435020.