摘要
采用物理模拟与数值模拟相结合的方法,针对高强度铝合金材料普遍存在的高温塑性成形性能差导致成形件服役性能下降的问题进行了深入的研究。本论文主要针对国产高强度7A04铝合金材料(Al-Zn-Mg-Cu合金)在热成形过程中的高温流动性能及成形构件疲劳性能两个重要环节开展相关研究：(1)建立了针对不同初始晶粒度的热流变本构方程并将其引入DEFORM-3D仿真,优化了实际成形过程中7A04铝合金零件的模锻工艺；(2)获得了经过多级时效处理后7A04铝合金材料的疲劳S-N曲线,将其引入ANSYS和FE-SAFE进行构件疲劳仿真,预测结果符合该构件的实际疲劳台架试验结果。本论文的研究成果对提升高强铝合金锻件生产效率和预测其构件疲劳服役性能具有重要指导意义,该成果已经成功应用于高速列车关键零部件国产化工程需求中。
采用物理模拟试验技术通过圆柱体单向热压缩实验获得7A04铝合金高温流变时的应力-应变曲线;获得了7A04铝合金在350℃-450。C温度区间、应变速率在0.01s-1～10s-1之间条件下稳态流变应力-应变的自然指数形式的本构方程,并将其引入DEFORM-3D对铝合金推杆模锻过程进行了精确模拟；初始晶粒度组织对流变应力本构关系方程具有显著影响：初始组织晶粒越细小,应力对应变速率的敏感性越强,稳态流变激活能越高；随着变形温度的升高与应变速率的降低7A04铝合金动态再结晶软化机制越来越明显,经过挤压变形的7A04铝合金更利于动态再结晶的进行：采用热加工图理论研究了7A04铝合金的热流变成形性能；建立了7A04铝合金的热流变功率耗散效率图和热流变失稳图,获得挤压态7A04铝合金最优热流变成形温度范围：360℃～430℃；基于7A04铝合金零件模锻工艺仿真,通过正交试验方法优化模锻成形温度参数为：坯料预热温度405℃,模具温度100℃。
采用轴向加载和四点弯曲疲劳试验法表征了7A04铝合金推杆材料的疲劳性能,建立了7A04铝合金在50%存活率条件下的Basquin方程,获得了推杆在承受脉动疲劳载荷(FMAX=45kN, FMAX=52kN)工况时的应力分布,采用主应力准则计算了构件的疲劳寿命分布,通过实际构件疲劳台架试验验证了理论计算模型的准确性。
本文主要获得了以下结论：(1)通过自定义本构关系方程采用DEFORM软件实现了7A04铝合金模锻过程的精确模拟,并结合7A04铝合金的热加工图,可以获得最优模锻成形工艺方案；(2)采用有限元仿真方法可以有效地预测构件的疲劳寿命及疲劳失效位置；(3)表面粗晶层组织显著削弱了7A04铝合金材料的疲劳强度,粗晶层组织在生产中应当予以排除；含表面半球形缺陷的推杆在缺陷处将产生严重应力集中,引发构件的疲劳破坏,生产过程中应采用严格的探伤工艺检测并排除缺陷。
本论文是在科技部、铁道部联合制定的“两部联合行动计划”框架下的高速动车组国产化背景下进行的,其产业化成果已在2010年初装车CRH5动车组安全运行至今,对高速动车组转向架关键零部件的国产化工作做出了重要贡献。
The present dissertation is study on the high strength aluminium alloy with the poor flowability in hot forging process which would weaken the service properity of the components. A series basic research on hot rheologic properity and fatigue properity for7A04aluminium alloy is carried out with the method of physical simulation and numerical simulation:(1) The constitutive equation of7A04aluminium alloy with different initial grain sizes was founded. The forming process of aluminium alloy part was optimized by DEFORM-3D simulation.(2) The fatigue strength of7A04aluminium alloy was predicted by ANSYS and FE-SAFE based on the measured S-N curve, the theoretical results correspond with the bench test. This work provided important guidance to raising productivity of7A04aluminum alloy forgings and predicting the component fatigue performance, this result has been successfully applied in the high-speed train key parts localization.
The thermal deformation behavior of7A04aluminium alloy during300℃-450℃was studied by isothermal compression of cylindrical specimens. The steady flow stress of7A04aluminium alloy during350℃-450℃can be represented by exponential correlation with a Zener-Hollomon parameter which discribles the flow stress change with strain rate and temperature.Constitutive equation of different initial grain size of7A04high strength aluminium alloy shows that the strain rate sensitivity factor and activity energy of the flow stress increase with the decrease of the grain sizes. The DRX is the dominant softening mechanism above350℃while DRV below350℃. DRX becomes more obviously with initial grain sizes decreasing. The processing temperature of7A04aluminium alloy was optimized during360℃to430℃based on the processing map method. The die forging process of a connecting rod in friction screw press was simulated with DEFORM-3D. The optimum preheat temperature are405℃for billet,100℃for container by orthogonal experimental design. The optimum processing pranneters are appilied in practical production.
S-N curve of7A04aluminium was obtained by high frequency axial loading fatigue test. The S-N curve was also describled by Basquin equation at50%survival rate. The effect of surface coarse grain on fatigue properity of7A04aluminium alloy has been studied by high frequency four-point bending fatigue test. The results show that the coarse grain reduced the fatigue crack initiation resistance and weaken the fatigue properties significantly. The accelerated life test was proposed to predict the fatigue fracture mechanism and the fatigue life. The FEM results show that the fracture would be occur at the surface hemisphere defect of the rod according to the actual defect shape under the accelerated life test loading spectrum(FMAX=27.2kN). The similar defect should be avoided in the production. The stress distribution of the connecting rod under the accelerated life testing loading was obtained by ANSYS. The stress and fatigue life distribution of the push rod was obtained and verified by bench test.
Under the background of "Ministry of Science and Technology and Ministry of Railways Joint Action Plan", the7A04aluminium alloy components completed the localization and run in safe service since2010. The work is significant for the localization of the key components of bogie.
引文
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