【作者】 张学宾；

【导师】 李志刚； 柳玉起；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2008， 博士

【摘要】 随着产品质量要求的不断提高和开发周期的不断缩短,仅仅依靠经验和理论解析方法已经不能满足金属环件冷辗扩工艺应用和研究开发的需求,迫切需要仿真方法。但是,由于金属环件冷辗扩是材料非线性、几何非线性和接触非线性的复杂过程,其成形过程仿真非常困难。对冷辗扩过程进行仿真研究,并开发专用的软件系统,是冷辗扩新产品开发、模具优化和新型设备研究的迫切需要。本文在国家自然科学基金重点项目“金属环件冷辗扩塑性成形机理及工艺设备研究”(No.50335060)的资助下,重点研究了环件冷辗扩成形仿真算法关键技术,开发了金属环件冷辗扩成形仿真系统。本文研究了环件冷辗扩过程弹塑性动力显式有限元方法,研究了冷辗扩的运动微分方程、几何方程、Mises各向同性强化的后继屈服函数、应力和位移边界条件等,建立了动力显式有限元方程及中心差分求解算法。研究了保证计算稳定性的临界时间步长和环件单元最小名义长度计算方法,给出了冷辗扩中对临界时间步长的缩放比例。冷辗扩接触面积小,易形成单点载荷,沙漏难以控制。针对这些难点,提出了稳定矩阵沙漏控制方法,这种方法不需要用户人为地设定沙漏控制系数。沙漏力由沙漏控制程序根据单元的应变情况计算。可以避免因经验不足或者系统复杂选择不合适的沙漏控制参数而造成辗扩成形仿真失败。由于系统自动地确定沙漏力和大幅度地减小沙漏能,保证了仿真过程的能量平衡,提高了计算的精确度。基于环件冷辗扩的接触特点,本文采用全局搜索和局部搜索的两步法,有效减小了接触判断时间。接触判断时间在总的计算时间里只占10%以下。接触力的计算采用罚函数法。为了提高保证仿真过程的精度,解决了冷辗扩过程环件单元的重划分和新旧单元之间场量的继承问题,保证了重划分前后场量的一致性。本文提出了冷辗扩动力显式求解器循环式并行化的思路,研究的共享存储式并行求解器能够获得1.9的加速比,并行化的求解器准确度高,串行和并行仿真结果仅相差0.1%。在理论研究的基础上,首次开发了金属环件冷辗扩成形过程专用仿真系统。此系统能够仿真分析金属在环件辗扩中的流动规律。仿真系统包括前处理、求解器和后处理三个子系统。前处理子系统提供了辗扩系统几何模型创建和离散、导入冷辗扩CAD/CAE模型、辗扩工艺参数设定、单元重划分、沙漏控制参数设定等功能。后处理程序可采用云图、等值线和视频输出等多种方式显示辗扩过程中各种场量的变化。采用开发的冷辗扩成形仿真系统对环件冷辗扩进行了仿真,通过仿真结果与物理实验的结果的比较,验证了仿真计算结果的正确性。同时,与国际著名的通用仿真软件ABAQUS的仿真结果进行了比较,验证了仿真系统的可靠性。更多还原

【Abstract】 As the requirement demand of product quality become higher and higher, and the product development cycle is constantly shortened, the experience based and pure analytical method can not meet the demands of application and study of cold ring rolling process. Therefore,the study on numerical simulation method is an urgent task. Because cold ring rolling process is a complicated and integrated process with the nonlinearity of material, geometry and boundary,its process simulation is very difficult. The study on the numerical simulation method of the process and developing a special software system have become an urgent need of the development of new products, new equipments and die structure optimization.This study is supported by the National Natural Science Foundation Key Project, metal cold ring rolling forming mechanism and process equipment research (No. 50335060). The forming simulation algorithm and key technologies are studied. A ring cold rolling process simulation system has beed developed.The elasto-plastic dynamic explicit finite element analysis method for ring rolling process simulation is studied. Based on the kinetic differential equation of cold ring rolling process, geometrical equation, Mises isotropic hardened subsequent yield function, and stress and displacement boundary condition, the dynamic explicit finite element equation and its central difference solver algorithm are established. The element minimum nominal length is calculated. To ensure the analysis stability, the critical time-step and the scale ratio of the time-step is provided.Since the contact area is small in the cold rolling process, a single point contact is easily to appear, and the hourglass is difficult to control. To control the hourglass, a new control method is proposed. User defined control factor is not required any more in this method. Hourglass resistance force is calculated in accordance with the strain of the element. Failed simulation resulted from the lack of experience or complexity of the system is avoided. The hourglass energy is decreased by the automatically determined hourglass force so that the energy balance of the system and accuracy of the calculations are improved.According to the cold ring rolling contact characteristics, a two-step method including global search and local search is adopted. The contact judgment time is reduced to the level less than 10% of the total computing time. The punishment method is used to calculate the contact force.In order to improve simulation accuracy, remesh technology of the element are studied in the study on ring rolling process simulation. The field variables are transferred to the new elements from the old ones after remesh to ensure the accordance of the field variables in the remesh.A dynamic explicit parallel algorithm is proposed. The structure of the dynamic explicit algorithm is studied. With the paralleled solver based on shared memory, computation acceleration ratio can reach more than 1.9. The solver has high accuracy after paralleled. The simulation results difference between the parallel and sequential solver is only 0.1%.Based on the theoretical research, a special simulation system has been developed for cold ring rolling process for the first time. The system can simulate metal flow in the ring rolling process. The simulation system is consists of pre-processing module, solver module and post-processing module. In the pre-processing module, the geometric model of ring rolling system can be created and discretized. The ring rolling model can also be imported from CAD/CAE data format. The forming parameters, remesh parameters, hourglass control method can be defined in the module. In the post-processing module, the results data is read in, and the variation of the field parameters, such as stress, strain and displacement, is displayed in the ways of cloud, contour or video.The cold ring rolling process is simulated with the newly developed simulation system. Compared with the experimental results, the simulation results are proven to be accurate. The simulation results of the new ring rolling process simulation system is compared with the simulation results of the famous software ABAQUS to verify the reliability of the simulation system.更多还原