Research On Mechanism And Measurement Method Of Quenching-Prestretching Stress In Aluminum Alloy Thick Plate

Super-high-strength aluminum alloy thick plate is an important industrial material used in aeronautical industry and the main material for roughcast of thin-walled component in "large aircraft project". However, it has demanding requirement in homogeneity of material microstructure, material properties and the level of internal stresses to produce the aluminum alloy. Internal stress, also known as residual stress, which is the inevitable and self-balancing elastic force inside material during manufacturing, is determined by the inhomogeneity of plastic deformation. Unfortunately, the internal residual stress often cause quenching fracture of thick plate, material with internal defects, and distortion of pre-stretched thick plate, which is harm to the application of thick plate and will impede finished product rate and machining accuracy of subsequent machining. Therefore, the control and determination of internal stresses in thick plate has been a topic in aluminum alloy research, especially the formation mechanism and measurement methods need an in-depth study. Taking 7075 aluminum alloy hot-rolled thick plate as research object, this paper built thermo-mechanical quasi-coupled mathematical model and relevant finite element model centering on three aspects:formation mechanism and evolvement of residual stress, finite element simulation, and measurement methods of stresses. In addition, this paper revealed macro-micro formation mechanism of residual stress and its distribution and proposed accurate measurement methods for stress measurement.Based on elastic-plastic mechanics, thermodynamics and finite element method, the formation of residual stress in thick plate during quenching, the evolvement of stress field during stretching, and the mathematical description of stress-strain relationship in quenching and prestretching were analyzed. At the same time, the continuous variation of stress was discredited into a finite element mathematical model and the finite element models of quenching and pre-stretching were built by MSC.MARC. In the study, a discrete analytical method was used to calculate heat transfer coefficient, which provided convenience in discussing the relationship between quenching temperature and coefficient.Based on material science and theory of crystal deformation, formation mechanism and evolvement of residual stresses was studied. By studying aspects ranging from differences in macroscopic plastic deformation to anisotropic characteristic of local microstructure, this paper revealed that stress formation of thick plate results from the inhomogeneity of plastic deformation or local deformation. With abstract models of representative microstructure, this paper analyzed the inhomogeneity and variation of microstress in different local regions, and revealed the fluctuation mechanism of stress. Additionally, by finite element simulation and experiments, the general law of formation and evolvement of residual stress in thick plate during quenching and prestretching was studied and induced, including the formation cause of "M" curve of residual stress distribution through thickness and so on. Meanwhile, other problems which can influence distribution of stress field were studied, such as the failure of clamp and the initial defects of thick plate. Accordingly, relevant suggestion was made for these problems.For stress calculation, the integration method was introduced into layer removal experiments and relevant mathematical model in order to obtain the distribution of stress field. Meanwhile, by conducting related milling simulation and experiments, a modified method used for stress calculation in layer removal experiments was proposed to improve the accuracy of experimental result. Furthermore, the characteristic of layer removal method was summarized and the cause of error in the calculation was analyzed, keeping the total error of calculated stress less than 1OMPa. Besides, the influence exerted by sample size on measurement results of layer removal experiments was studied in order to improve the effectiveness of samples in layer-removal experiments. The experimental result shows that it is necessary to use sample with size twice the thickness of the thick plate or larger to measure its internal stresses. While common sampling method and experiment method have little influence on measurement results.In order to ensure the accuracy of results and the reliability of measurement methods, the simulation-experiment synthetical method is established to achieve multi-layer and complementary description of stress field, including measurement of surface stress by X-ray diffraction method, distribution of stress field by finite element simulation, and internal stress by layer removal method. Furthermore, the modification of heat transfer coefficient through measurement of surface stress by X-ray diffraction method is beneficial and will make finite element simulation model more coincide with real distribution of stress. Subsequently, based on milling simulation of samples, the root cause of accumulated error of deformation during conducting layer removal experiment was revealed and a modified function based on stress calculation in layer removal experiments was built.Based on analysis of stress measurement result from thick plates with different sizes and conditions, it was revealed that:(1) The distribution of quenching stress mainly depends on quenching intensity. Moreover, the distribution of plane stress through the thickness of thick plate is similar. (2) The analysis of stress field in stretched thick plate shows that the earliest internal plastic deformation is critical for the reduction of stress ef throughout the whole thick plate. Based on synthetical analysis, the reasonable stretch rate ranges from 1.8% to 2.5%. (3) Stress of prestretched plate is about±20MPa, which presents that prestretching process can even the residual stress of plate. (4) The integration model of layer removal method can accurately measure the stress field of thick plate regardless of the low stress level. So measurement methods can fully qualify for the stress determination of thick plate.The paper was supported by Key Project of Chinese National Programs for Fundamental Research and Development (2005CB623708).