Simulation of friction characteristics in sheet metal stamping forming

Abstract: In the sheet metal forming process, the friction force between the sheet and the die is extremely complex, and its rule is not understood well enough. Based on friction theory, the friction force is divided into 4 components: the shear stress on the adhesion area, the shear stress on the boundary lubrication area, the ploughing force caused by protuberance on the die, and the shear stress on the area with much lubrication. Automobile panel products with high quality shall have no quality defects of stamping forming, such as fracture, wrinkle, spring back and surface defect, and the material flow of product shall be uniform and the thickness changes also shall be uniform during the forming process. Therefore, this article proposes that stamping forming quality index which includes defect quality index and thickness variation uniformity quality index. As different types of automobile panel contain different materials and have different quality problems, the defect quality index of different panels will also be different. The automobile's back plate reinforcement in this paper is complex internal covering parts, its main forming defect is fracture and wrinkle. So a novel criterion evaluating quality of sheet metal forming is developed. When the minimum thinning rate satisfies design requirements, the variance of each element thinning rate calculated in finite element simulation can be taken as evaluation index, and this index is inversely proportional to forming quality, in other words, the smaller evaluation index is, the more even sheet metal becomes, then the better forming we can obtain. In addition, the concept of equivalent friction coefficient is proposed, and the relationship between 4 components and friction force in FE simulation is built up; and then the accuracy of friction force in simulation is improved. Finally, numerical simulation and analysis about influence of friction and lubrication condition change on friction characteristics and forming quality is conducted.The result shows that the variance of each element thinning rate is direct proportional to friction coefficient, and the uniformity of sheet thickness becomes bad obviously when friction coefficient is great than 0.175. The total friction force is increasing persistently with the stamping development, and has the same variety trend and distinct terminal values in different lubrication conditions. It is proved that sheet forming quality can be enhanced by improving lubrication condition, and the components of friction force and their proportions can be known in the certain lubrication condition. After analyzing the curve of friction force with respect to time, the conclusions can be drawn as follows: 1)The change law of friction force at different nodes is different, and the thinning rate is directly proportional to the mean of friction force in thinning sheet area；2) In thickening area, the relationship between friction force and thickening rate is confused, and the variety of friction force is more irregular with comparison to that in thinning area.The research provides a reference for reducing friction force, avoiding scratching of the surface of parts, extending the service life of die and controlling the sheet material flow, and has theoretical and practical sense for the improvement of sheet metal stamping quality.