【作者】 廖凯；

【导师】 吴运新；

【作者基本信息】 中南大学 ， 机械设计及理论， 2010， 博士

【摘要】 超高强铝合金厚板是重要的航空工业材料,也是国家“大飞机”项目薄壁构件板坯的主要用材,因而对制备过程中材料组织均匀性、机械性能和内应力水平的控制都有着极高的要求。内应力又称残余应力,是生产制备过程中不可避免的平衡于材料内部的弹性力,是塑性变形不均匀的必然结果。工程中,残余应力对厚板的使用与加工有很大负面影响,例如板内应力致使板材淬火断裂、材料内部缺陷、预拉伸厚板加工扭曲变形等,既影响了成材率,也造成后续构件的加工精度难以达到要求。因此,开展对厚板应力形成机理与测试方法的研究具有十分重要的意义。本文以高强铝合金7075轧制厚板为研究对象,围绕厚板淬火-预拉伸残余应力的形成、分布与演变规律,从理论分析到实验研究,建立了残余应力形成过程热-力准耦合数学模型和有限元仿真模型,揭示了残余应力场形成的宏微观机理和分布规律,并研究了准确求取厚板残余应力的测试方法。运用弹塑性力学、热力学和有限元理论方法,分析了厚板淬火热传导过程中残余应力的形成,以及预拉伸塑性变形对材料应力场的演变,并对两者物理过程中应力-应变关系进行了较为完整的数学描述。同时,将应力连续变化过程离散为有限元数学模型,通过非线性有限元软件MSC.Marc建立了厚板淬火-预拉伸应力场有限元仿真模型。研究过程中,提出用离散解析方法求解换热系数,解决了模型热边界条件的求取问题,以便分析淬火温度与换热强度之间的关系。运用材料学和晶体变形理论,展开了对厚板淬火-预拉伸残余应力形成机理和演变过程的研究,从宏观塑性变形的差异,到局部微观组织表现的各向异性特征,揭示了厚板应力形成的本质在于塑性变形或局部变形的不均匀和不一致性所致。文中运用典型微结构抽象模型分析方法,分别对不同微区结构中晶粒相互作用形式,进行了微观应力不均匀和变化波动演变机理的分析,较好地揭示了微区应力变化波动现象。通过有限元仿真和实验计算,研究归纳了厚板残余应力形成与演变的一般规律,包括预拉伸板应力“M”双峰型分布形成原因、预拉伸板锯切区范围讨论等。同时,探讨了拉伸状态非对称因素对预拉伸厚板应力场分布的影响,如拉伸夹钳失效问题、厚板裂纹缺陷问题等,提出了可供参考意见和建议。为了获得厚板真实应力场,将积分算法引入到层削实验方法中,构建内应力计算数学模型,求取厚板深度应力场。同时,借助相应的铣削仿真与实验研究,提出层削实验应力计算的修正方法,提高了测试方法的正确性。进一步,总结了积分型层削法特点,分析层削实验计算的误差来源,求得其应力计算总不确定度小于10MPa,证明积分型层削法具有较高的计算精度。同时,为揭示厚板取样对测试的影响机理,研究了厚板取样过程中存在的应力分布非均匀特征,实验结果表明：取样平面尺寸大于2～3倍板厚对保证测试稳定性非常关键,而常规取样和实验加工方法对测试结果的影响很小。为保证测试结果的正确性和测试方法的可靠性,建立了仿真-实验综合方法,多层次互补地实现对厚板应力场的准确描述,该方法包括X-ray表面应力测定、应力场有限元仿真、力学实验测试方法。X-ray衍射测试用于标定厚板表面应力,修正换热边界条件,使有限元仿真模型计算结果能较好反映实际应力分布。同时,为揭示层削实验中变形累积增大的误差根源,构造层削实验应力计算修正函数,为完备实验测试方法的正确性研究提供依据。借助实验测试方法,完成对不同规格、不同状态下厚板残余应力的测试分析,结果表明：(1)淬火应力分布主要取决于淬火强度,沿深度两平面应力的分布状况基本相似。(2)预拉伸应力评估的结果显示,内部最先塑性变形是整个厚板应力消减的关键点,综合考虑消减效果和拉伸效率,1.8%～2.5%拉伸量是一个合适的拉伸范围。(3)预拉伸板应力水平基本处于±20MPa左右,说明预拉伸工艺对厚板残余应力分布有一定均化作用。(4)积分型层削计算方法能准确测得厚板应力场分布,尤其对预拉伸板这类低应力测试对象,仍能较客观地反映厚板应力分布,说明上述测试方法的研究成果能够用来解决厚板应力场测试问题。本文得到了国家重点基础研究发展计划项目“高性能铝材与铝资源高效利用的基础研究”,其中第八子课题“大规格铝材非均匀多相组织和内应力场的产生与演变(2005CB623708)”的资助。更多还原

【Abstract】 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).更多还原