شبیه سازی عنصر محدود سفت و سخت پلاستیکی برای طراحی فرایند تشکیل پروانه توپی

A torque converter impeller hub is usually made through sequential cold forming processes: forward extrusion, upsetting, piercing and finishing. The finishing process is a closed-die forging where the load increases abruptly due to flash formation and the defect of the under filling in the finished product is happened occasionally. In this study, rigid–plastic finite element simulation was applied to analyze the deformation characteristic of the whole impeller hub forming processes and to optimize the process. As a result, two kinds of improvement for the impeller hub forming process satisfying the limit of the machine’s load capacity and the geometrical quality are suggested and the results are verified by experiment.

In the cold forming process, the material in a die flows continuously into the complex geometrical shape of the die under the progressive forming procedures. When the die design and the preform design of material are not optimized, internal damages of material like as void and micro-crack occur. According to the accuracy of initial volume of material shape, difference between the designed and the final shapes are sometimes happened [1]. A poorly estimated initial volume of material may cause the geometrical forming defects such as the underfilling or the overlapping in the forging process. Therefore, the optimization of the forming process is required to obtain the proper product without any damage. Recently, the numerical simulation techniques using the rigid–plastic finite element method (FEM) have been successfully applied to investigate the forming characteristics of various forming processes such as the stress–strain state of the material and clarify the effects of various forging parameters on formability [2], [3], [4], [5] and [6]. The forming process of torque converter impeller hub used in an automobile’s transmission consists of the sequential cold forming processes: forward extrusion, upsetting, piercing and finishing operations. The final finishing operation governing the final shape of impeller hub is a closed-die forging process. In this forming operation, the forging load increases abruptly due to flash formation and geometrical forming defects of the underfilling occasionally occurs due to the limit of the forging machine’s load capacity. Moreover, excessive initial volume of material may cause the failure of forming die and forming machine. In this research, cold-forming processes of a torque converter impeller hub are analyzed by the rigid–plastic FEM, and process improvements to obtain the proper product without any underfilling defects and failure of the forming machine are suggested.

The forming process of an impeller hub used in torque converter was analyzed using the rigid–plastic FEM. Forming defects deduced from excessive large volume of the initial material and improper die design in the conventional forming process could be predicted. Therefore, two kinds of improvement are suggested to tackle these problems. The conclusions with the finite element analysis can be summarized as follows: (1) Conventional forming process. Due to excessive initial volume of material and poor die design, the underfilling problem occurred in the finishing operation. Also large flash formation in the finishing operation yields overloading problem for the forging machine. (2) Improvement I. The volume of initial material is reduced about 7% compared with the conventional forming process. Because of small flash formation, the forming load was reduced to 5.4 MN which is a 14% lower value compared with the conventional forming process. The forming process could be controlled under the effective load capacity of the press, 5.26–5.49 MN. However, the underfilling problem in the finishing operation could not solve in the improvement I. (3) Improvement II. The die of the upsetting and the finishing operation was mainly modified to reduce the forming load within the range of the effective load capacity of the press without any underfilling defect. The die shape of the finishing operation was changed entirely to the backward extrusion. Compared with the conventional forming process, the forming loads in forward extrusion, upsetting and finishing operations were reduced about 18, 9 and 17%, respectively. As a result, a press with a maximum load capacity of 6.37 MN could be used without any problem. Moreover, the volume of the initial material was reduced about 17% compared with the conventional forming process and the underfilling problem in the finishing operation was solved.