AZ61镁合金热变形行为研究及微观组织演化模拟

发布时间：2019-06-21 20:36

【摘要】：随着轻量化时代的到来,镁合金越来越受到青睐,在航空航天、汽车、3C中都有较广的应用前景。由于镁合金密排六方的晶体结构,塑性变形能力较差,因此当前产品中压铸件比较常见。但压铸件有时无法满足高性能零件的要求,塑性变形可获得晶粒细小、综合性能较好的组织,力学性能要优于压铸件,因此镁合金的塑性变形对镁合金的发展和应用具有重要意义。金属的宏观力学性能由微观组织决定,通过研究热变形过程中微观组织演变及动态再结晶行为,从而建立适当的模型对热变形过程中组织进行预报,是目前较常用的研究方法。元胞自动机作为交叉学科的出现,引入了曲率驱动机制、热力学驱动机制和能量耗散机制,能够更真实地反映晶界迁移的物理过程,在现在模拟方法中是较受欢迎的一种。传统的热变形研究方法是通过应力应变曲线建立动态再结晶动力学模型和本构模型,这些是研究动态再结晶行为的基础,可为模拟模型提供基础性的材料参数。本文通过建立包含应变参量的流变应力应变模型,动态再结晶动力学模型,研究了挤压态和铸态的AZ61镁合金的热变形行为,通过引入晶粒拓扑变形机制,变化的激活能等,建立了动态再结晶CA模型,对挤压态AZ61镁合金进行了微观组织演化模拟。确定合适的热处理方法对AZ61镁合金进行了优化热处理,获得均匀组织后,进行热挤压,获得挤压态镁合金。通过Gleeble-1500模拟试验机,对铸态和挤压态的AZ61镁合金进行热压缩实验,获得不同变形条件下的应力应变曲线。建立高温应力应变模型,研究了应变对本构参数α、Q、n、lnA的影响。研究发现不同状态下的AZ61镁合金具有相同的热变形行为规律,挤压态镁合金的动态软化现象较明显。通过动态再结晶动力学的研究发现,挤压态动态再结晶的发生要易于铸态,挤压态的晶粒尺寸较小,更易发再结晶,铸态镁合金在变形初期会通过产生孪晶来协调变形。基于元胞自动机原理,结合曲率驱动机制、能量耗散机制建立了初始晶粒长大模型。结合动态再结晶理论建立了元胞自动机动态再结晶模型,引入了晶粒拓扑变形技术,考虑变形过程中晶粒形状的变化,通过引入本构方程,考虑了激活能随应变的变化。利用MATLAB完成了模型程序的编程,针对不同变形条件下的热变形进行CA模拟。研究结果表明:初始晶粒生成模型的模拟结果能够准确地反映晶粒长大过程的特点。动态再结晶CA模型可准确地再现变形参数对动态再结晶的影响规律。模拟获得的微观组织形貌与实验获得的金相组织也较相似,获取的应力应变曲线、动态再结晶动力学曲线也能够较准确地反映曲线的特点,获得的峰值应力、稳态应力、平均晶粒尺寸等都与实验值吻合良好,因此所建立的元胞自动机模型可用于该材料动态再结晶过程的模拟。引入变化的激活能,通过影响动态再结晶形核率,进而影响再结晶行为,结果表明,引入变化的激活能后更贴合实际。
[Abstract]:With the advent of the age of light weight, the magnesium alloy is more and more popular, and has a wide application prospect in the aerospace, automobile and 3C. Due to the hexagonal crystal structure of the magnesium alloy, the plastic deformation ability is poor, so the pressure casting in the current product is more common. But the plastic deformation of the magnesium alloy is of great significance to the development and application of the magnesium alloy. The macroscopic mechanical properties of the metal are determined by the microstructure, and the microstructure evolution and the dynamic recrystallization behavior of the metal are studied by studying the microstructure evolution and the dynamic recrystallization behavior in the thermal deformation process, so that an appropriate model is established to forecast the tissue in the thermal deformation process, and the method is the most commonly used research method. Cellular Automata, as a cross-discipline, introduces the mechanism of curvature driving, the mechanism of thermodynamic driving and the mechanism of energy dissipation, and can more truly reflect the physical process of grain boundary migration, and it is a more popular one in the current simulation method. The traditional method of thermal deformation is to set up a dynamic recrystallization dynamic model and a constitutive model through the stress-strain curve, which is the basis of studying the dynamic recrystallization behavior, and can provide the basic material parameters for the simulation model. The thermal deformation behavior of the extruded and as-cast AZ61 magnesium alloy is studied by establishing a rheological stress-strain model and a dynamic recrystallization dynamic model which contain the strain parameters. The dynamic recrystallization CA model is established by introducing the grain topological deformation mechanism and the activation energy of the change. The microstructure evolution of the extruded AZ61 magnesium alloy was simulated. And a suitable heat treatment method is used for optimizing the heat treatment on the AZ61 magnesium alloy, and after the uniform tissue is obtained, the hot extrusion is carried out to obtain the extruded magnesium alloy. The stress-strain curves under different deformation conditions were obtained by the Gleeble-1500 simulation test machine, and the AZ61 magnesium alloy in the as-cast and extruded state was subjected to thermal compression experiments. In this paper, a high-temperature stress-strain model is established, and the effects of strain on the parameters, Q, n, and lnA, are studied. It is found that AZ61 magnesium alloy in different states has the same thermal deformation behavior, and the dynamic softening of the extruded magnesium alloy is more obvious. It is found that the dynamic recrystallization of the extrusion state is easy as cast, the grain size of the extruded state is small, and the recrystallization can be more easily, and the as-cast magnesium alloy can coordinate the deformation at the beginning of the deformation by generating twinning. Based on the cellular automaton principle, the initial grain growth model is established by combining the curvature driving mechanism and the energy dissipation mechanism. In this paper, the dynamic recrystallization model of the cellular automata is established in combination with the dynamic recrystallization theory, and the grain topological deformation technology is introduced, and the change of the grain shape in the deformation process is considered, and the change of the activation energy with the strain is taken into account by the introduction of the constitutive equation. The programming of the model program is accomplished by using MATLAB, and the CA simulation is carried out for the thermal deformation under different deformation conditions. The results show that the simulation results of the initial grain generation model can accurately reflect the characteristics of the grain growth process. The dynamic recrystallization CA model can accurately reproduce the influence of the deformation parameters on the dynamic recrystallization. The microstructure of the obtained microstructure and the metallographic structure obtained by the experiment are also similar, the obtained stress-strain curve and the dynamic recrystallization dynamic curve can accurately reflect the characteristics of the curve, the obtained peak stress and the steady-state stress, The average grain size and the like are in good agreement with the experimental values, so the established cellular automaton model can be used to simulate the dynamic recrystallization process of the material. The activation energy of the change is introduced, and the recrystallization behavior is influenced by the influence of the dynamic recrystallization nucleation rate, and the results show that the activation energy of the introduced change is more practical after the activation energy is introduced.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.22

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