【作者】 胡丽娟；

【导师】 彭颖红； 李大永； 张少睿；

【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、比刚度高以及良好的散热性和减振性等特点,在汽车、航空航天以及3C行业具有广阔的应用前景。但由于镁合金的密排六方晶体(HCP)结构,决定其在室温下塑性很不理想,而在温热状态下具有良好塑性变形能力。镁合金在温热变形过程中会发生晶界滑移及动态再结晶,能细化晶粒、增强组织均匀性,有效地改善了合金的微观组织,提高材料的力学性能。因此如何控制热变形条件,使微观组织演化向着所希望的方向发展就具有重要的理论和实际意义。对镁合金合金变形过程中晶界滑移及动态再结晶的研究手段主要有理论研究和试验研究两种。由于试验条件与研究手段的限制,要对合金变形过程直接进行动态观察相当困难,不能揭示热塑性成形过程复杂的微观物理现象。因此本文在分析变形条件与动态再结晶相互关系的基础上,建立考虑动态再结晶影响的宏细观本构模型。通过实验与模拟技术相结合的方法,研究变形条件对晶界效应及动态再结晶演化的定量影响规律。模拟镁合金板材成形过程中微观组织演化趋势,获得成形过程中各物理场的分布规律,进而预测镁合金板材的成形性能,为成形工艺的优化提供了依据。为此,本文着重进行了以下几个方面的研究:首先采用等温拉伸试验和金相观察方法,研究了AZ31镁合金在温热条件下的动态再结晶行为。研究变形条件对组织演变以及动态再结晶体积分数的影响。该项研究为后续本构方程以及动态再结晶组织演化数值模拟提供了研究基础。在分析变形条件对流变应力影响的基础上,分析应力指数、变形激活能、峰值应变、硬化率曲线、临界应变以及变形条件对动态再结晶体积分数的影响,并建立考虑动态再结晶影响的宏细观本构模型。该本构模型可以描述镁合金动态再结晶对宏观力学特性的影响规律,为镁合金板材成形提供理论依据,发展了薄板温热成形理论。利用热粘塑性本构模型,对AZ31镁合金薄板在不同温度下的单向拉伸进行了有限元数值仿真。采用代表性体积元的方法建立起考虑晶界影响的有限元模型,研究AZ31镁合金薄板晶界效应与温热变形条件变化的关系。该研究明确了变形条件对晶界演化的影响,得到变形过程中晶粒的应力应变分布,这为研究动态再结晶过程中由位错密度增加引起的再结晶形核分布提供了依据。将元胞自动机方法与再结晶理论相结合,建立镁合金动态再结晶组织演化模型。并通过数值模拟的方法研究了变形条件对镁合金热拉伸过程中动态再结晶动力学的影响。研究结果表明计算所得的AZ31镁合金热拉伸过程的应力应变曲线、组织转变及拉伸终了组织中晶粒尺寸分布等与实验结果一致。AZ31镁合金在热拉伸过程中发生了明显的动态再结晶,先形核长大的再结晶晶粒其晶界在变形条件满足时则成为再次再结晶的形核位置,使晶粒尺寸更加细化和均匀化。动态再结晶动力学分析结果表明,与典型再结晶动力学曲线相比,温度低、应变速率大时AZ31镁合金薄板热拉伸过程未发生完全再结晶,再结晶动力学曲线呈不完全“S”形。最后基于上述研究基础,将基于元胞自动机方法得到的材料本构模型引入冲压成形数值仿真中,研究结果表明镁合金温热变形条件下,应力和应变呈均匀分布,晶粒尺寸在凹模圆角处较小,在侧壁拉伸区较大。并通过试验的方法对仿真结果进行了验证,结果表明数值模拟与试验研究相一致。通过对镁合金温热变形微观组织演化的理论和数值模拟研究,一方面能够揭示镁合金温热变形微观组织演化规律,另一方面能增加研究手段的多样性,有助于丰富镁合金板材成形理论。对镁合金动态再结晶组织演化的研究能够实现对镁合金板材成形性能的定量预测,可以为制定镁合金的塑性加工工艺提供必要的理论依据,进而促进镁合金板材成形技术的推广和应用。更多还原

【Abstract】 Magnesium alloys are the lightest structural material therefore are very suitable for application in the transportation industry, aerospace and 3C industry. However, magnesium alloy has traditionally been considered one of the less shapable metals at room temperature due to its close-packed hexagonal structure, while its formability is excellent at warm condition. The previous investigations have shown that magnesium alloy might undergo the grain boundary (GB) sliding and dynamic recrystallization (DRX) pheomena during hot working processes. The grain refinement during DRX is insignificant at high temperatures due to the rapid grain growth. So controlling warm deforming condition is important to make the microstructure evolution of magnesium alloy to the expecting status.There are two main research methods to develop the sliding GB and DRX of magnesium alloy during warm deformation. However, it is different to observe GB sliding and DRX at warm deforming conditions of magnesium alloy due to the limitation of test conditions and research way. Therefore, based on the relationship between the deforming conditions and dynamic recrystallization, the macro-micro constitutive model including the effect of DRX has been developed. According to the experimental and numerical method, the quantitative influence of deforming conditions on GB effect and DRX evolution has been studied. The microstructure evolution has been simulated during forming process of magnesium alloy sheet. And the distribution of physical field has been obtained. According to the researches, the formability of magnesium alloy sheet can be predicted and the forming technology can be optimized.Through isothermal tensile test and metallographical observation, the DRX of AZ31 magnesium alloy sheet has been studied. The effect of deforming conditions has been explored, and the influence of deforming conditions on the volume fraction of DRX has been analyzed. This research can be used as the foundation to construct the constitutive model and simulate the evolution of DRX.Based on the analysis of effect of deforming conditions on the flow stress, stress exponential, deforming activation energy, peak strain, hardening ratio curves, critical strain and influence of deforming conditions on the volume fraction of DRX have been studied. According to these researches, a macro-micro constitutive model containing the effect of DRX has been put forward. This constitutive model can describe the effect of DRX on the macro-mechanical properties of magnesium alloy sheet, and it can be used as a theoretical method for formation of magnesium alloy sheet and improve the warm forming theory of thin sheet.The uniaxial tension of AZ31 magnesium alloy sheet at different temperatures is simulated using the thermoviscoplastic constitutive model. A representative volume element taking from the tension model is constructed to simulate the relationship between GB and deforming conditions. The results show the influence of deforming conditions on the GB and distribution of the stress and strain in grain and GB.Combining the Cellular Automata (CA) method with DRX theory, the DRX evolution model of magnesium alloy has been developed. The effect of deforming condition on the DRX kinetics during warm tension of magnesium alloy has been analyzed. The results show that the stress-strain curves, the structural transformation and the grain size distribution of ultimate tension agree with the test results. The phenomenon of DRX is obvious during warm tension of AZ31 magnesium alloy. When the deforming condition fits the critical condition of DRX, the grain boundary of DRX grains first nucleating and growing will become the position of next nucleation. This will make the grain fined and homogeneous. The results of analysis of DRX kinetics show that the DRX of warm tension of AZ31magnesium alloy does not complete at lower temperature and high strain rate. Compared with the classical recrystallization kinetics curve, the DRX kinetics curve is incomplete shape of S.Based on the above research, the constitutive model according to the CA method is introduced into the numerical simulation of stamping. The results show that the distribution of stress and strain of cup is homogeneous. Grain size at region of die profile is smaller than it at wall of cup. And the grain size of numerical simulation agrees well with the experimental results.The research of theoretical and numerical simulation of microstructure evolution of magnesium alloy at warm deforming conditon can make microstructure evolution rule obvious and improve the forming theory of magnesium alloy sheet. The forming properties of magnesium alloy sheet can be quantitatively predicted according to the study of DRX evolution. The research of DRX evolution can supply necessary theory of plastic processing technic and promote the forming technique of magnesium alloy sheet.更多还原