【作者】 黄诗尧；

【导师】 彭颖红； 李梅； 李大永；

【作者基本信息】 上海交通大学 ， 机械制造及其自动化， 2010， 博士

【摘要】 镁合金具有质轻、比强度、比刚度高以及容易回收等优点,在汽车、航空航天等行业具有广阔的应用前景。但由于镁合金是密排六方晶体结构,其室温成形性能很不理想,而在温热状态下具有良好的塑性成形性能。镁合金在塑性加工过程中,会由于滑移和孪晶使晶粒发生转动而形成强烈的织构。同时镁合金在热变形过程中容易发生动态再结晶,使晶粒细化。试验研究表明,镁合金成形过程中的晶粒尺寸与织构演化对其材料性能有着明显影响。因此,研究成形过程中微观组织的变化规律对预测镁合金产品性能具有重要的意义。集成计算材料工程集选材、设计、制造、优化于一体,是材料学科一个新的研究方向。它将材料初始的微观组织结构信息作为计算机仿真的输入,模拟宏观制造过程及相应的微观组织演变,最终获得不同度量尺度和加工过程里所有相关的材料信息,实现低成本下产品的最优化设计,并且能够缩短新材料的研发时间。本文根据集成计算材料工程的研究方法对镁合金挤压过程的微观组织演化进行研究,采用试验和模拟两种手段,系统学习了挤压对平均晶粒尺寸和织构分布的影响,为今后镁合金产品的性能预测奠定坚实的基础。晶体塑性力学在织构模拟中已经得到广泛的应用,本文对不同模型和模拟方法进行对比,确定最适合于AZ31镁合金的模拟方法。首先建立了滑移主导的率无关单晶体模型,通过模拟单晶体铝板的冲压验证了其正确性。基于该单晶体模型,分别根据泰勒模型和弹塑性自洽模型建立滑移主导的多晶体模型。进一步考虑孪晶在织构演化中的作用,建立了耦合滑移、孪晶的多晶体塑性力学模型。通过模拟AZ31镁合金挤压棒材在压缩过程中的织构分布,对不同的多晶体模型和模拟方法进行对比,该研究为后续的数值模拟提供理论基础。为了研究铸态AZ31镁合金高温变形时的微观组织演化,对铸态AZ31镁合金进行了不同变形条件(温度、应变速率和应变)下的恒温热压缩试验。在相同变形条件下,铸态棒材中间部位和边缘部位的应力应变曲线比较接近。利用金相试验观察压缩后样品的金相组织,研究了应变量对平均再结晶晶粒尺寸的影响;采用X射线衍射试验进行宏观织构的测定;采用电子背散射衍射试验研究了变形织构和再结晶织构分布。采用修正的Avrami方程拟合得到铸态AZ31镁合金的高温再结晶动力学方程,将该方程嵌入弹塑性自洽模型,提出了耦合动态再结晶动力学和晶体塑性力学的计算方法。通过拟合小应变下的应力应变曲线得到材料的初始临界剪切力和硬化参数。利用材料点模拟计算了热压缩过程的应力应变曲线、织构分布和平均再结晶晶粒尺寸的变化,通过和试验结果的对比验证了方法的有效性。对铸态AZ31镁合金进行了不同变形条件(挤压比、挤压温度和挤压速度)下的棒材挤压试验,并且根据汽车零部件(顶盖横梁)的实际尺寸,进行型材挤压试验。利用金相试验、X射线衍射试验和电子背散射衍射试验对挤压后样品的微观组织和织构分布进行观测。同时通过室温拉伸试验研究挤压对AZ31镁合金材料性能的影响。为了能够更好的描述挤压棒材的屈服强度,提出考虑晶粒取向影响的屈服强度描述方法,将Hall-Petch公式应用到每个晶粒上,采用电子背散射衍射的试验结果(每个晶粒的尺寸和取向)作为输入,拟合结果和试验结果较为一致。基于动态再结晶动力学和晶体塑性力学的耦合计算方法,考虑挤压过程中温度场变化的影响,对镁合金挤压过程进行宏微观集成计算,预测了挤压棒材和挤压汽车零部件的织构分布,模拟结果与试验结果较为相似。基于考虑晶粒取向影响的屈服强度描述方法,以挤压型材的模拟结果作为输入,得到挤压型材的屈服强度,和试验测量的挤压型材的屈服强度较为接近。因此,基于集成计算材料工程对镁合金挤压过程进行研究有助于揭示其微观组织的演化规律,能够为制定挤压工艺提供理论依据,促进镁合金挤压技术的推广和应用。更多还原

【Abstract】 Magnesium alloy is promising metal in automotive and aerospace applications because of its low density, high specific strength and stiffness and recyclability. Due to the close-packed hexagonal structure, the formability of magnesium alloy is poor at room temperature, while it is much better at warm condition. Grain reorientation caused by slip and twinning can induce the formation of texture during deformation. Besides, dynamic recrystallization is easily activated during warm forming process of magnesium alloy. The experimental research shows that grain size and texture evolution during deformation of magnesium alloy has significant influence on its material properties. Hence, it is necessary to gain a better understanding of microstructure evolution during forming process from the point of view of predicting material properties.Integrated computational materials engineering (ICME) is a new field of study that is evolving within the global materials profession. It enables concurrent analysis of manufacturing, design, and materials within a holistic system. The forming process and microstructure evolution during forming process are simulated with the input of the initial microstructure distribution. ICME entails integration of information across length scales and forming processes for all relevant materials phenomena. In fully mature form, integrated computational materials engineering offers a solution to the industrial need to quickly develop durable components at the lowest cost. It also has important potential for accelerating the development of new materials. In this paper, microstructure evolution during extrusion process is studied with ICME methodology. A systematic research is conducted to investigate the influence of extrusion process on average grain size and texture distribution. This work lays a solid foundation for predicting material properties.Crystal plasticity has been widely applied to predict texture evolution recently. To find out the one which fits for magnesium alloy AZ31, comparison between different models and simulation methods has been carried out. A rate dependent single crystal model is built for slip dominated metals. The model is validated by modeling earing behavior during stamping of aluminum sheet. Based on the single crystal model, Taylor model and elasto-plastic self consistent (EPSC) model are built respectively, the effect of twinning is also incorporated into the polycrystalline plasticity model. Comparison between material point simulation and crystal plasticity based finite element method (CPFEM) is conducted by simulating texture evolution during compression of AZ31 alloy. The research lays the foundation for numerical simulation of magnesium alloy.To gain a better understanding of microstructure evolution during hot deformation of casting magnesium alloy AZ31, the isothermal compressions have been carried out at different deformation conditions (different temperatures, strain rates and strain). The sampling locations on the casting billet do not have obvious effect on the stress-strain curves. The microstructure distribution is measured by optical microscopy, and the influence of strain on the average recrystallized grain size is studied. The evolution of macro-texture distribution is measured by X-ray diffraction, and electron backscattered diffraction is employed to studied the deformed texture and recrystallized texture.The dynamic recrystallization kinetics is formulated as a function of strain, temperature and strain rate based on the modified Avrami equation. The dynamic recrystallization kinetics is then implemented into crystal plasticity model. The initial critical resolved shear stress and hardening parameters are obtained by fitting the stress-strain curve. The coupled model is employed to predict stress-strain curve, texture and averaged recrystallized grain size evolution during compression, the model is validated by comparing with experimental results.Direct extrusions of casting AZ31 alloy are carried out at different conditions (different extrusion ratios, extrusion temperatures and velocities), component extrusion test is conducted based on the dimension of an automotive part (roof rail). The microstructure and texture distribution of extruded samples are measured by optical microscopy, X-ray diffraction and electron backscattered diffraction respectively. Tensile tests are conducted at room temperature to reveal the influence of extrusion on the material properties. The experimental yield strength can not be solely described by average grain size. The grain size and orientation in the extruded rods are characterized by electron backscattered diffraction, and Hall-Petch equation is applied to each individual grain with the input from electron backscattered diffraction results (individual grain size and orientation). The yield strengths of tensile sample (polycrystalline aggregate) and individual grain are related by Taylor assumption. The predicted yield strength shows the same trend as experiment results.The integrated simulation of extrusion process is carried out based on the coupled method of dynamic recrystallization kinetics and crystal plasticity model, while the effect of temperature variation during extrusion process is also considered. Texture evolution during direct extrusion and component extrusion are simulated, the predicted results are in qualitative agreement with experimental results. The yield strength of extruded component is calculated with the predicted texture and grain size, the predicted yield strength is similar with the experimental value. Hence, investigating extrusion process of magnesium alloy with ICME methodology is possible to reveal its microstructure evolution. It can also provide theory basis of devising extrusion process design, and promote the extrusion technique of magnesium alloy.更多还原