Study On Evolution And Prediction Of Quenching And Pre-Stretching Residual Stress Of 7055 Aluminum Alloy Plate

7xxx series aluminum alloys(Al-Zn-Mg-Cu series)are one of the most important structural materials in aerospace because of their high specific strength,specific stiffness and toughness,and excellent comprehensive performance.With the development and application of the large monolithic structures,the demand for high performance 7xxx aluminum alloy plate becomes more and more urgent.The evolution,distribution and control of residual stress in aluminum alloy plate have always been one of the difficult problems in this field.In the present dissertation,ultrahigh-strength 7055 aluminum alloy large size plate was taken as the research object.The evolution,distribution and influence factor of residual stress in aluminum alloy plate were studied using Finite Element Method(FEM)simulation and experimental measurement.The comprehensive characterization method of residual stress was studied.The prediction method for quenching residual stress distribution was established and the effect of non-uniform quenching on residual stress was searched,which provides a reliable reference for the control of quenching and pre-stretching residual stress of 7055 aluminum alloy large size plate in factory.The FEM simulation model of quenching and pre-stretching residual stress in aluminum alloy plate was constructed by the ANSYS.The required thermo-physical parameters of the material were determined by experiments.The boundary conditions of heat transfer in quenching process were calculated by the inverse method and iteration method.The evolution and distribution of thermal-stress during quenching process were studied by the established FEM model.The results showed that the quenching residual stress was compressive on the surface and tensile in the center,and the location of transformation point was at D/4.There was an uniform plane stress state in the plate,and the stress gradient only existed in the thickness The influence of pre-stretching on the residual stress distribution were studied.The reduction ratio of stress increased rapidly at the beginning and then slowly as the pre-stretching ratio increased.Residual stresses in the 7055 aluminum alloy plate were studied by both experimental measurement and FEM simulation.The measurement results of surface stresses with different widths showed that both the hole-drilling and X-ray diffraction testing had good stability and accuracy.The X-ray diffraction and neutron diffraction results were consistent with the FEM simulation results,so the FEM model had a satisfied accuracy for the prediction of residual stress distribution.The FEM simulation and hole-drilling results of residual stresses with different quenching medium parameter were consistent with the crack compliance method results,so the accuracy of the crack compliance method measurement was vefied.Therefore,the optimization principle of surface stress measurement method was proposed and the comprehensive characterization method of residual stress of 7055 aluminum alloy plate was established based on the hole-drilling method,X-ray diffraction method and crack compliance method.Effect of quenching medium parameter on the heat transfer coefficient was studied.The heat transfer coefficients decreased as the water temperature or PAG concentration increased while increased as the spray water flow increased.The order of heat transfer capability of different quenching medium was:10%NaCI+KNO3 solution,20? water,13.76L/min spray,15%PAG solution.Effect of quenching medium parameter on the quenching residual stress of 7055 aluminum alloy plate was studied.The quenching residual stress decreased as the water temperature or PAG concentration increased.The order of quenching residual stress of different quenching medium was:10%NaCl+KNO3 solution,20? water,5%PAG solution.Influence of dimension on the quenching residual stress of 7055 aluminum alloy plate was studied systematically.The influence on the stress level showed that the quenching residual stress level with various length or width fluctuated obviously when the length or width was relatively small.The stress level tended to be stable when both the length-thickness ratio and the width-thickness ratio were more than 6.The influence on the stress distribution showed that the stress level was basically unchanged and the ratio of the fluctuating area decreased gradually as the length increased.The stress level and ratio of the fluctuating area decreased gradually as the width increased.However,the stress level and ratio of the fluctuating area increased gradually as the thickness increased.A method to predict the residual stress distribution in the length direction and width direction was proposed by the study of influence of the length and width on the quenching residual stress.The quenching residual stress distribution of x×y×1 model can be obtained by the prediction method and the quenching residual stress distribution of 5×5×1 model,where x>y>5.The study of experimental measurement and FEM simulation of the surface quenching residual stress distribution with different widths showed that the prediction method was credible.The prediction model of through-thickness quenching residual stress distribution was established by the research of stress distribution function in thickness direction and the prediction model of center residual stress.Then,a comprehensive method to predict the residual stress distribution of 7055 aluminum alloy large size plate was proposed by the combination with the prediction method for residual stress distribution in the length direction and width direction.Through the proposed method,the quenching residual stress distribution of 7055 aluminum alloy large size plate can be predicted by the quenching residual stress distribution of small size plate with the surface residual stress of large size plate.Effect of non-uniform quenching on residual stress of 7055 aluminum alloy large size plate was studied.When the top and bottom surfaces of large size plate were under asymmetric quenching,the through-thickness asymmetrical quenching residual stress distribution was between the two distributions corresponding to the heat transfer coefficients of the top and bottom surface respectively.The surface and center stress levels were equal to the average value of stress levels corresponding to the two surface heat transfer coefficients.When the surface was under non-uniform quenching,the surface stress distribution was the same as the heat transfer coefficient distribution and the stress level was equal to the stress level corresponding to the average heat transfer coefficient at each location.However,the center quenching residual stress distribution was approximately uniform and the stress level was equal to the average value of stress levels corresponding to the maximum and minimum heat transfer coefficients.The distribution of the surface residual stress of the large size plate was similar to that of the multi-section linear non-uniform quenching.Effect of non-uniform quenching on pre-stretching residual stress distribution was studied.The result showed that pre-stretching only had regulatory effect on the inhomogeneity of internal stress distribution.