Study On Process Optimization Of Sheet Metal Forming And Die Face Compensation Technique Based On Springback Control

Springback is a key defect existing in sheet metal forming. It causes deviation from the designed target shape and produces downstream quality problems and assembly difficulties. With the application of high strength steel and aluminum alloy steel in automobile body manufacturing, springback problem becomes more acute. Now springback is not only a problem that exists in industry but also a hot issue in academia.Generally, springback process involves material nonlinearity、contact condition nonlinearity and geometrical nonlinearity, and there are many factors that influence the accuracy of springback prediction. To control springback, traditionally, the on spot“trial and error”method based on experience would be applied to adjust the process parameters and the die-face. Thus, those measures as analyzing the main factors that influence the accuracy of springback prediction in numerical simulation、taking finite element simulation technique along with optimization technique to optimize process parameters aimed to control springback and developing new method to improve the efficiency of die-face compensation are effective to solve the springback problem.In this paper, present situation and research progress in those aspects as optimal design of sheet metal forming、springback control by adjusting process parameters and die-face compensation were introduced to give an overall understanding of springback control problem; Elastic-plastic theory in sheet metal forming and springback simulation, especially the important concept and develope procedure, was also introduced systematically which is the theoretical foundation for application and development.Accurate springback prediction is the prerequisite for springback control, thus the main factors that influence the accuracy of springback prediction in finite element simulation were analyzed. The rules for determining element size, which remains undecided and is one of the most important factor to influence the accuracy of springback prediction, was studied. Different phenomena were found which means that the thickness of the sheet metal should also be considered when determining the element size,it is then a wrong idea that the smaller the element size, the better the result. The effects of constraint point location on calculation efficiency and result accuracy for springback simulation were also analyzed. Results show that, on premise of convergence, constraint point position has little effect on result accuracy, but has considerable influence on calculation efficiency,The purpose of accurate springback prediction is to control springback. Aimed to control springback for high strength sheet, the optimization strategy and integration technology for multi-objcetive optimization model were studied. The selection criteria of objective function、the determination of constraint conditions and design variables in the optimization model were also analyzed. Then taking an automobile roof panel as an example, a multi-objective optimization model was established to control springback, the solution was compared with that of corresponding single-objective optimization models to validate the effectiveness and feasibility of the multi-objective optimization model.Process parameters optimization and die-face compensation are two main measures to control springback effectively. By taking an automobile roofbow panel as an example, and evaluating springback using an equivalent stress objective function (in which coordinates of integration point in the thickness direction were considered) and an equivalent plastic strain objective function, an integrated optimization model was established to optimize process parameters. The influence of optimize process parameters—variable blank holder force and drawbead restraining force on springback was also analyzed. By taking an automobile sidewall panel as an example, a multi-objective optimization model was established to optimize the parameters of the addendum; To improve the accuracy of the optimization result for the surrogate model, an approach was taken based on the even experiment design, optimization result of the surrogate model was also compared with that of the unimproved model and response surface model . It is not realistic to eliminate springback in practice, thus, die-face compensation hasbeen widely used to compensate springback. In this paper, the defect of current compensation directions was analyzed, a comprehensive compensation method which takes a direction factor into account was proposed, and taking a V-Shape bending panel and a complex blade as examples, the new method was compared with those traditional approaches which compensation in other directions, a U-Shape panel was also employed to verify the effectiveness of the new method experimentally. To determine springback compensation factor in displacement adjustment method, the defect of constant compensation factor was discussed firstly, then a variable compensation factor inductive method based on Lagrange interpolation algorithm was proposed. The new method was applied to a 2D U-shape panel and a 3D saddle panel and produced successful results, the 3D saddle panel was also employed to verify the effectiveness of the new method experimentally.