【作者】 李振红；

【导师】 张质良；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2009， 博士

【摘要】 伴随着经济发展和制造全球化的热潮,市场竞争日益加剧。采用节约高效的先进制造技术已成为制造业重要的发展趋势。精密塑性成形技术是一种以高效、低成本获取高质量的净形或近净形零件的先进制造技术,而亚热控制精密成形则是在温成形和热成形温度范围之间成形的一种新的精密成形工艺。该工艺具有变形抗力较低、简化工序、减少设备吨位等优点,并可通过控制锻后冷却及利用余热进行热处理获得力学性能优良、尺寸精度高的优质锻件,达到节约能耗的目的。因而这种方法具有广泛的应用前景,但目前尚未有系统的研究报导。为深入认识亚热成形特性,本文以汽车精锻件常用合金结构钢40Cr和DIN优质碳素钢Cf53为研究对象,以汽车外星轮为载体,采用物理模拟和数值模拟相结合的方法对亚热控制精密成形特性及变形过程中的模具磨损进行了系统的理论和实验研究。本文基于热模拟试验,对40Cr钢和Cf53钢的亚热流变行为进行了试验研究及分析,结果表明随变形工艺参数的变化,流变应力曲线表现为不同的型式。在变形温度较高(1000℃和950℃)、变形速率较低(0.1s-1和0.5s-1)的时候,流变应力曲线出现明显的峰值,属于动态再结晶型。峰值应力时的应变(峰值应变)随变形温度的降低和变形速率的增大而升高。变形温度、变形速率及变形量对流变应力均有影响,其中变形速率和变形温度是最敏感的影响因素,是工艺控制中的主控因素。通过40Cr圆环亚热压缩试验,探讨了亚热变形工艺参数对摩擦因子的影响,研究结果为实际成形及数值模拟分析中摩擦因子的选取提供依据。基于热模拟试验的分析结果,在对现有流变应力模型进行评价的基础上,建立了40Cr钢和Cf53钢的亚热变形稳态应力模型,采用双曲正弦表达式计算40Cr钢和Cf53钢的亚热变形稳态激活能。40Cr钢亚热变形激活能为:270.321 KJ·mol-1,Cf53钢亚热变形激活能为:290.782 KJ·mol-1。并计算得出不同变形条件下的lnZ值,对应相应的表格即可快速算出特定变形条件下的峰值流变应力,进而可方便地为模具设计、设备选择等方面提供重要理论依据。基于Zener-Hollomon参数的研究,将变形激活能及其他参数与应变相关联,建立了一个基于动态变形激活能的亚热流变应力模型,计算了40Cr钢和Cf53钢在连续变形过程中的变形激活能及其变化情况,得到了变形激活能与变形量之间的关系。模型计算结果表明,所有工艺下的预测值均与试验值相吻合,为将本文构造的流变应力模型应用于数值模拟分析中提供了依据。采用编写用户子程序的方法将本文建立的流变应力模型耦合到MSC.Marc/Mentat有限元软件系统中,建立了亚热精密成形工艺模拟系统。利用该系统对40Cr钢和Cf53钢的亚热挤压成形过程进行了数值模拟,分析了变形温度、凸模速度、变形程度和摩擦因子对亚热挤压成形过程中的应力场、应变场分布和金属流动规律的影响,为确定合理的亚热成形工艺参数提供了科学依据。在分析现有磨损模型基础上提出适用于有限元模拟的模具磨损模型,建立了模具磨损模拟系统,首次采用数值模拟方法获得了各工艺参数对模具最大磨损深度及磨损分布的影响规律。研究表明摩擦因子和凸模速度对模具的磨损深度和磨损分布影响很大。将模具磨损模型预测结果与实际生产实例所测模具磨损轮廓对比,发现二者趋势相吻合,证明了模型的正确性。研究结果为预测模具寿命、预测产品尺寸及模具设计的补偿研究提供理论基础,具有重要的实际意义。通过物理模拟进行了不同变形条件下亚热正挤压和反挤压成形,比较研究了数值模拟结果和实验结果。研究结果表明,模拟与实验所得变形载荷大小及趋势基本一致,证实了本文的流变应力模型及数值模拟系统在40Cr钢和Cf53钢亚热变形过程的应用中具有较高的准确性和可靠性。应用基于本文建立的流变应力模型、模具磨损模型的亚热控制精锻模拟系统,以Cf53钢的汽车外星轮为例,对其亚热成形过程进行工艺分析,确定了工艺方案,成功实现了Cf53钢的亚热控制精锻成形,并投入了批量生产。结果表明采用本论文研究的亚热控制精锻工艺,降低了材料变形抗力,取消了单独正火处理的工序,采用成形后的余热控制冷却,从而获得优良的内部组织与较高的力学性能,具有简化工艺,节约能源,使产品兼具优良的内外质量的特点。更多还原

【Abstract】 Accompany with the upsurge of the economy development and the manufacturing globalization, and to make the product have more market competition abilities, the advanced manufacturing technique having low cost and high quality has becoming the manufacturing industry development trend. Precision plasticity forming is the process which can get net shape or near net shape part with a low cost. The semi-hot precision forming process is one of the precision plasticity forming whose forming temperature range is between warm forming and hot forming. The semi-hot precision forming has some advantages: saving energy consumption, lower the forming resistance, reducing the equipments input and acquiring good property products. Although this advanced process has been gotten widely using, its mechanism has not been system researched. In order to investigate the forming property, takes 40Cr and Cf53 steel as the subjects, systematic experimental and theoretical studies on the forming behavior and microstructure evolvement of the semi-hot precision forming was carried out, then conducts numerical simulation and experimental verification of the process , and has also applied for the invention patent.Using the ring compressing test, process parameters on the impact of friction factor has been studied. Results show high temperature and low velocity decrease friction factor.Plastic deformation resistance is an important factor which can affect process developing, die life and equipment selection etc. Based on the thermal-simulation test, the flow behavior of 40Cr and Cf53 steel was analyzed. The results show that flow stress curve represent different types with the deformation process parameters changing. Strain rate and temperature is the most sensitive factors.Based on the thermal simulation results and the evaluation of the existing flow stress models, the forming activation energies of 40Cr and Cf53 has been calculated using a hyperbolic sine expression. They are 270.321 KJ·mol-1 and 290.782 KJ·mol-1 respectively.Based on the Zener-Hollomon parameters, semi-hot activation energy has been associated with the strain. A new creep model has been constituted. The calculated results using this new model showed that all the values are in line with the experiment, so this flow stress model can be applied to the follow-up numerical simulation analysis.The simulation of semi-hot extrusion of 40Cr and Cf53 steel was conducted out by the thermo- mechanical coupled finite element method and the new creep model was used in the simulation to calculate the flow stress. The simulation predicts the distribution of strain, stress and temperature in extrusion test. The deformation temperature, the forming velocity, deformation extent and friction factor on the semi-hot extrusion process has been analyzed. The results may provide the scientific guidance to determine reasonable forming process parameters.On the basis of the analysis of existing wear models, a new wear model has been applied to the finite element simulation of die. Comparing the actual contours with the simulated wear contour, we find that the two contours are match which prove that this model can be used to the research of mould. Then forming parameters on the wear of die has been studied using this model. The results show that forming velocity and friction factor is the most sensitive factors, which should be controlled in the process design.Physical simulation has been conducted to verify the accuracy of the flow stress model and the die wear model. The results show that the flow stress model and numerical simulation system in the semi-hot forming process of 40Cr and Cf53 steel have a high stability and reliability.Semi-hot controlling precise forging simulation system was adopted to analyze the semi-hot forming property of an automobile outer race, and the process scheme has been determined. The semi-hot controlling precise forming of Cf53 steel was realized for the first time, and metallographic analysis for the outer race forging was preceded. The results show that the semi-hot controlling precise forging process in this study can decrease the material forming resistance, and the single normalizing treatment procedure can be omitted. The cooling can be controlled by the residual thermal. Then the finer internal structure and the higher overall mechanical property are obtained. This method has good characteristics of simplifying working process, conserving energy and making final products have better internal and surface quality.更多还原