Research On Evolution And Evaluation Model Of Residual Stress In Aluminum Alloy Thick Plate

Aluminum alloy thick plates with high strength and toughness, excellent corrosion resistance and low residual stress are of great importance as structural materials in the industry of aerospace. These aluminum alloys gain their favorable integrate performance through heat treatment involving a severe quenching operation, which inevitably introduces a very high level of residual stress. The existence of residual stress in thick plates has direct influences on after machining, which results in unexpected distortion frequently. Therefore, it is of great importance to investigate residual stress and its evaluation and prediction method in aluminum alloy plates.The present dissertation is funded by the National Basic Research Program of China (2005CB623708) and (2010CB623708), Hunan Provincial Innovation Foundation for Postgraduate.A series of basic research on several key procedures for processing7075aluminum alloy thick plates is carried out with the combination of numerical simulation and experiment method, which could provide a theoretical guide for the optimization of the processing technic parameters and residual stress adjusting and controlling in large-scale high performance aluminum alloy plates. The primary investigation contents of this paper include three main parts:First, the prediction model for residual stress in aluminum alloy thick plates was built. Calculation expressions of transient temperature field and transient stress filed during quenching and transient stress filed during pre-stretching were deduced. Nonlinear analysis model of the residual stress field of the plate was established by taking the three nonlinear factors into consideration:the nonlinearity of material thermal parameters, the nonlinearity of heat transfer boundary condition on the surface of quenching plate and the nonlinearity of constitutive relation of aluminum alloy. On this basis, the MSC.Patran/Marc parametric model was built. Using the model, evolving rules of inner stress in aluminum alloy plates was analyzed, then distribution rules of residual stress in aluminum plates and the influence of pre-stretching on residual stress were summarized, a quantified effect of stretching ratio on plate residual stress was constructed. Then plates warping induced by non-uniform quenching was studied, moreover, the effect of stretching on plates shape was investigated. These works can lay a theoretical foundation for latter experiment research and provide evaluation technique of residual stress for planning the preparation of plate.Second, the application of crack compliance method for inner residual stress measurement in aluminum alloy plates was studied. The calculation uncertainty of crack compliance method was assessed. Then, the release rule of residual stress after sampling was also studied, the influence of specimen dimension on the measurement results was analysed, and the reasonable specimen dimension was summarized. The correction scheme was proposed by taking the experiment error into consideration. Using FEM, the calculation ability of crack compliance method and layer removal method for inner residual stress were validated. Combined with FEM and experiment method, the calculation results of crack compliance method and layer removal method were compared and analyzed, which conformed the validity of the methods for inner residual stress measurement.Third, an evaluation model of residual stress field in aluminum alloy plates was constructed. Based on FEM results, a shape function of residual stress distribution in aluminum plates was proposed using mathematic method, and the regression model of stress values of key points of stress distribution within aluminum plates was obtained using nonlinear regression technology. Based on experiments, a shape function of residual stress distribution from experimental results was proposed, and the accuracy of the regression model was improved by revising the model using experiment results.