可控温度场作用下的控制相变闭塞挤压变形机制及成形极限研究

The economic pressures from international competition, as well as from the increasing application of government controls on air, water, noise and safety, is having new requirements on the development of precision forging technology. According to the status-of-art in precision forging technology and the high demanding from real industry, a new proposal of controlled transformation extrusion process with closed-die (CTCD extrusion process) is put forward. Firstly, the deformation mechanism of CTCD extrusion process under ideal temperature distribution will be revealed, the microstructure evolution of specimens after extrusion with a non-uniform temperature field will be carefully observed, and the effect of preheating temperature of workpiece, deformation degree of extrusion, temperature gradient after extrusion, and parameters related to die cooling system will be investigated in details. Secondly, a hardness prediction model considering grain size effect, dislocation strengthening and deformation strengthening will be established to predict the hardness distribution of the extruded part. Thirdly, the quantitative calculation method of the closing force for the closed die during forming or cooling process will be obtained to support the structural design of the closing element. Fourthly, the forming limit of the CTCD extrusion process under multi-constraint conditions such as the temperature gradient, microstructure evolution, hardness after extrusion and the magnitude of closing force will be explored. Finally, an optimized stable process window of the CTCD extrusion process would be created and applied to the practical forming, to realize the efficient, environment-friendly and short precision manufacturing of typical automotive forgings. Then, the global competitiveness of auto parts manufacturing industry could be enhanced, and this could provide better support to the national major development strategy in “Energy saving and new energy vehicles”.

日益激烈的全球竞争与日益强烈的环保诉求，对精锻技术发展提出新要求。基于精锻技术的发展现状与工程实际的问题需求，提出控制相变闭塞挤压精密成形的新构想。拟通过深入系统研究，揭示可控温度场作用下的控制相变闭塞挤压变形机制，探讨挤压大变形及非均匀温度场条件下的金属微观组织演化机理，分析坯料加热温度、挤压变形程度、挤压后温度梯度分布及模内冷却相关参数对挤压件微观组织及其演化的影响规律；建立考虑晶粒尺寸效应、位错强化及形变强化的硬度预测模型，预测挤压件的硬度及其分布；形成不同工位的闭塞力定量计算方法，为合模机构的结构设计提供科学依据；探索温度梯度分布、微观组织演化、挤压后硬度及闭塞力大小等多约束条件下的成形极限；构建控制相变闭塞挤压成形工艺的稳定窗口并应用于实际，实现典型汽车锻件的高效、环保、短流程精密制造，提升我国汽车零部件制造业的全球竞争力，更好地支持“节能与新能源汽车”的国家重大发展战略。

基于精锻技术发展现状与工程实际的问题需求，提出控制相变闭塞挤压精密成形的新构想。本项目围绕该工艺构想，重点开展了挤压温度场理论计算、材料变形流动行为和微观组织演化机理、考虑综合效应的挤压件硬度预测模型、闭塞挤压力计算，以及具体汽车典型零件的工艺案例等具体研究工作。提出一种基于圆环压缩的接触传热系数测定方法，利用接触传热系数标定曲线实现了挤压坯料与模具间接触传热系数的定量测试。根据热传导原理，确定了组合凹模在挤压过程中的稳态温度场理论计算公式，同时搭建两套不同管道结构的模内控制冷却平台，为控制相变技术创造了必要的工艺条件。利用热模拟压缩实验，确定44MnSiVS6、TL1438和42CrMoS4等多种钢材的热塑性变形流动行为，构建相应的材料流动应力模型，为闭塞挤压大变形模拟提供可输入的材料模型。深入分析了20MnMoB和45钢两种材料在不同调质热处理制度下微观组织演化规律，为控制相变提供参考依据。提出一种材料硬度-应变原位测量方法，并在硬度数据测试的基础上，依据材料混合强化机制，构建了引入位错密度和等效应变的硬度预测模型。针对闭塞挤压成形过程的特点，探讨锻件复杂程度、材料流动应力与模具结构等对闭塞力的影响规律，确定了闭塞力经验计算公式。提出一种新的锻件形状复杂系数计算方法，与现有方法相比新定义的复杂系数可以更好地反映锻件复杂程度，便于闭塞力的准确计算。以汽车发动机用喷油器体为例，开展闭塞挤压成形工艺试验研究，通过经验公式计算了喷油器体挤压成形的闭塞力，并对其材料模内控制冷却后的挤压件进行硬度测试，初步测试结果可以满足该类零件的技术要求。此外，对汽车发电机用爪极的闭塞成形工艺可行性进行了初步探索；提出一种测试稳定性良好的爪极磁性能无损检测方法，为后续面向服役的爪极闭塞成形工艺优化奠定了基础。