【作者】 王辉；

【导师】 高霖；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2011， 博士

【摘要】 成形极限图(Forming limit diagram-FLD)是在金属板料塑性成形中判断板料成形成功与失败的一个重要判据。在利用有限元仿真方法进行冲压工艺优化、汽车钣金零件选材等方面有着重要的应用。板料成形极限图的准确获得与应用非常复杂,影响因素众多,针对FLD的研究仍然存在很多问题:随着新材料和工艺方法的不断出现,使得最初的一些试验与理论无法直接应用在新的情况下;FLD受加载路径,应变速率,加载温度等影响因素的研究仍然不够成熟;FLD在选材等方面的应用研究仍然较少。基于以上问题,本文以冲压领域常用的低碳钢板料和变形铝合金板料作为研究对象,围绕金属板料成形极限的获得方法、影响因素及其在板料成形有限元仿真和板料选材领域的应用等方面进行了理论与试验研究。论文共计8章,主要完成的内容和所取得的成果如下:(1)通过试验研究了FLD的试验方法与影响因素。通过对现有试验方法的分析,找出了影响试验结果的主要因素。通过试验研究了应变路径的获得方法、极限状态的判断方法、以及试样形状与板料各向异性对FLD试验结果的影响。为提高板料成形极限试验的准确性与实用性提供了重要的参考依据。(2)基于Hill-Swift失稳理论,M-K失稳理论,以及MMFC失稳理论的基本原理,利用各向异性屈服准则和不同的本构方程,推导了板料成形极限预测的数值迭代计算方法。通过试验对一种低碳钢和一种铝合金板料进行了不同加载路径下的失稳规律预测,与理论预测结果进行了对比。结果表明所建立的FLD预测程序能够较为准确的预测各种加载路径下板料的成形极限。(3)探讨了失稳现象在基于有限元仿真的板料成形模拟中的表现形式,研究了利用有限元仿真软件来计算FLD的原理与方法。通过分析板料塑形成形有限元仿真结果中板料失稳时的应力应变等的变现形式,提出了利用数值仿真方法计算板料失稳行为的原理与流程。研究了可以用来作为失稳行为的判断准则及其在有限元仿真软件中的实现方法。并对一种高强度钢板和铝合金板料进行了仿真与试验,研究了这种预测方法的实用性以及失稳判断方法、材料模型等对试验结果的影响。(4)通过试验研究了基于应力和应变的FLD路径相关性。分析了应力成形极限的理论与试验获得方法及其等效形式。通过对不同预加载后的板料进行单向拉伸试验,研究了不同加载路径下的应力成形极限的差别。并通过试验对反向推导得出的结论,对应力成形极限的路径相关性进行了验证。结果表明应力成形极限的路径相关性较应变成形极限小,其路径相关性是由板料的硬化行为决定的,等向强化条件下板料的应力成形极限与加载路径无关,而符合随动强化和混合强化模型的板料应力成形极限则与加载路径相关。(5)通过试验研究了两种铝合金板料AA5182-O、AA7075T6和一种镁合金板料AZ31B在温成形条件下的单向拉伸失稳行为和成形极限,找出了与材料在室温下失稳规律的区别。分析了成形温度和成形速度对成形极限的影响。利用材料的单向拉伸数据建立了材料温成形下的材料模型,并通过有限元仿真预测了这种材料模型下板料的FLD,与试验结果进行了对比分析。为了解板料温热状态下的失稳规律与成形极限提供了重要的科研依据。(6)开发了基于FLD的汽车板选材系统。基于汽车公司的成形工艺设计部门和材料采购选择部门的需要,根据充分利用材料性能,减少制造成本的基本原则,开发了基于板料成形极限的选材系统。利用数据库程序语言建立了材料数据库、零件数据库等,并编制了基于安全欲度分析的选材程序。该系统基于板料FLD和实际零件成形中的安全欲度,为每种零件选择合适的板料提供重要的参考依据。更多还原

【Abstract】 Forming limit diagram is a very important criterion in sheet metal forming for determination success or failure of a forming part. It is a good tool in FEM simulation and in material selection for the automotive parts. However due to the complexity of the forming process, many factors which effect the test and application has never been clearly understood.With some new material and forming technology are applied in industry, the traditional FLD study is not good enough. There are few study focus on material selection use FLD in China now. In this paper, experimental and theoretical methods for the development of FLD of the traditional softsteel and aluminum sheet were studied. Many factors that affect the FLD were investigated through experiment. The results and main contributions of the dissertation are as following:(1) Experimental investigation of FLD test method and factors which affect results. From the concept of forming limit diagram and its application. The process of conventional experiment method is analysised and factors in which affect the test results were find out. Realization of different strain path and estimation of forming limit condition are investigate and optimized. The effect of tools, specimens and strain measurement method was investigated through experiment. Application of the new developed automatic strain measurement in the experiment of forming limit diagram is introduced and analyzed.(2) Forming limit diagram prediction method based on the conventional plastic theory. A detail introduction of Swift diffuse necking theory, Hill localized necking theory and M-K theory is given. In addition, a numerical forming limit diagram prediction method based on these three theories is developed. A detail failure process of a soft steel and aluminum alloy is investigated through experiment. Results of theory prediction and experiment are compared with each other. Results show that the new developed FLD prediction method is convenient and can give a reasonable accurate prediction.(3) Numerical prediction method of the forming limit diagram based on Fem software. The forming limit diagram prediction method based on the FEM software is introduced. The failure criterion and theirs realization in the FEM software prediction are detailed. The effect of the material model and failure criterion to the results of the FEM predicted forming limit diagram are investigated through simulation experiment. A good model for the FEM forming limit diagram prediction method is developed. (4) Investigation on the path dependency of the stress and strain based forming limit diagram. The definition and develop method of the forming limit stress diagram are introduced. Path dependent of the stress and strain based forming limit diagram were investigated through experiment. An effective strain based path independent forming limit diagram is developed. A good guide for application of forming limit diagram in multistage forming process is given out.(5) Forming limit diagram for the forming process at elevated temperature. Failure criterion and forming limit of aluminum alloy AA5182、AA7075T6 and AZ31B at elevated temperature are investigated through experiment. Flow behavior of aluminum alloy at warm forming condition is investigated through tensile test and forming limit is got through experiment. Effect of the temperature and strain rate of the aluminum alloy are investigated through experiment. A fem based forming limit prediction method for aluminum at warm forming condition is developed.(6) Development of the material selection system for automotive sheet metal forming. A material selecting system based on forming limit diagram for the automotive company is developed. The system include a material database which can give a detail formability of different material, and a spare parts database which gives the information of different spare parts of automobile. This system can help the process and tools design department and the material supply department to select best material for certain part and reduce cost.更多还原