| Aluminum alloy thick plate is an important material in the fields of airplane, aerospace, shipbuilding and other industries. Structure made of thick plate is stronger and lighter, and it is then applied in the large military and civil aircraft of our country. However, there is high level residual stress, usually100-300MPa, caused by quenching in the plate. When stress-relief process is not designed well, the residual stresses left are sufficient to cause distortion and weaken the fatigue strength and the stress-corrosion-cracking resistance in aircraft components machined from this material. Meanwhile, lower residual stresses are required for the thick plate used in the large aircraft. Therefore, it is important to study the mechanism and establish the analytical model for pre-stretch in order to improve the process and lower the residual stress in the plate.The thesis focuses on the mechanism and modeling of stress-evolution of the residual stress during quench-stretch process by stress experiment, strain-based modelling of elasto-plasticity, analysis on in-plane deformation of thin plate and multi-layer method modeling of residual stress in thick plate:1. Residual stresses in quenched and pre-stretched thick plates of2A12and7075aluminum alloy were measured by layer removal method. The thick plates were quenched in spray and immersion to get different quenching intensities. The results show that plate was cooled down in shorter term under the immersion-quenching than that of the spray-quenching and residual stress caused under the former circumstance was higher than that of the latter. Spraying process caused high level residual stress close to the surface and nearly zero stress around the midplane, while immersion bringing inverted U-shape of stress in the plate. Besides, pre-stretch process significantly reduced the residual stresses in quenched plates.2. The evolution of mechanical properties of7000series aluminum alloy during quench-stretch process and values of properties at key stages were determined according to the test condition. It was found that the yield strength is lower than residual stress of as-quenched plate, indicating that residual stress is increasing during aging process. The through-thickness inhomogeneity was confirmed in the quenched-stretched7075plate by uniaxial tension test and hardness test. Material close to the surface has the higher strength than that near the midplane in the thick plate.3. Based on general form of strain-space incremental plasticity relation, a new model for linear hardening materials was proposed. The model, suitable for isotropic hardening materials like aerospace aluminum alloy, was carried out by determining yield criteria, flow rule, hardening function and plastic modulus in strain space.4. A calculation model for strain in the thickness-direction of isotropic linear hardening plate was proposed, based on the theory of stress and strain space elastic-plastic constitutive model and analysis of stretching process of aerospace aluminum alloy thick plate. It was found that when ratio of stress fractions is constant during in-plane loading, ratios of strain components under various loading conditions are linearly related and these points of ratios form an η-η line. Under these simple loadings, strains in thickness direction can be easily calculated by the η-η line equation without integral and differential work. When the plate is under more complicated loading conditions, the thickness can be computed by the proposed optimization and piecewise calculation model. The model was then verified against the results of proved theories and FE simulation in MSC.Marc.5. A mathematical model is developed to calculate the residual stress in stretched aluminum alloy thick plate. It is based on multi-layer method, balance of stresses, two-dimensional plasticity and a conception of free size. The model was verified against the experiments performed on7075aluminum plate. It was found that the computed residual stress basically express the stress level of the stretched plate under the given stretching ratio and that through-thickness inhomogeneity plays an important role in residual stress.6. The mechanism of stress-relief during stretch was studied. The stress was reduced by narrowing of differences of free sizes at various depths of the plate. Study showed that residual stress in the transverse direction is higher than that of the rolling direction when the tension ratio is relatively small and the latter is larger than the former when tension ratio reaches a certain level. Non-uniform plastic deformation caused by stretching was also studied. This work was funded by the State Key Fundamental Research Program of China (Grant No.2005CB623708and No.2010CB731703). |
PDF Full Text Request