No abstract

A forming process for heated sheet metal, such as hot-stamping, has limited use in deformable shapes. Higher temperature areas which have not yet come into contact with dies are more easily deformed; therefore, local deformation occurs at these areas which leads to breakage. To improve the formability of heated sheet metal, a deformation control technique utilizing the temperature dependence of flow stress is proposed. This technique can suppress local deformation by partial cooling around potential cracking areas to harden them before forming. In order to apply this technique to a variety of product shapes, a procedure to determine a suitable initial temperature distribution for deep drawing and biaxial stretching was developed with a coupled thermal structural simulation. In this procedure, finite elements exceeding forming limit strain are highlighted, and an initial temperature distribution is defined with areas of decreased temperature around the elements to increase their resistance to deformation. Subsequently, the partial cooling technique was applied to a deep drawing test with a heated steel sheet. The results of the experiment showed that the proposed technique improved 71% in the forming limit depth compared with results obtained using a uniform initial temperature distribution.

Springback in sheet metal forming is a deformation that occurs when a workpiece is released from dies after forming. Prediction of springback using the finite element method (FEM) has been used in the design of stamping dies. The present paper describes an effective approach by which to improve the accuracy of springback analysis, which can also suppress the increase in the calculation time. In general, FEM using the analysis condition to decrease discretization errors greatly increases the computation time. The proposed procedure is as follows: (1) Quick analysis using a large mesh size is performed. (2) Data mapping and morphology mapping for the forming results are then performed using the database based on a detailed partial analysis of the drawbead and the die shoulder. (3) Springback analysis is performed using the forming results modified by the data mapping and morphology mapping. A forming experiment was conducted to confirm that the proposed method using the database greatly improves the prediction accuracy of the shape after springback and also suppresses the increase in calculation time.

The numerical simulations of sheet metal forming are used practically in production preparation for automotive parts. Breakage and wrinkles are evaluated in most of the sheet metal forming by FEM analysis. The wrinkles are controlled by the restraining force with drawbeads. If the force is too large, a breakage occurs. Thus, the adjustment of the drawbeads are important in deforming the sheet metal without defects. The drawbead size is much smaller in comparison with the whole of automotive panels. If the whole panel is divided into finite elements with the mesh sizes corresponding to the shape of the drawbeads, the analysis takes several days. Therefore, the FE models of drawbead parts in the panels are normally omitted and drawbead force is given as the boundary condition, instead. A new calculation method of drawbead properties, in which a material and friction models conforming to the actual phenomenon were implemented, was proposed to improve the accuracy of sheet metal forming analysis. The difference between the predicted drawbead forces and the experimental values has been decreased to less than 10% at maximum. As a result of the application of these values to the calculation of forming an actual panel and a comparison with experimental results, the validity of the proposed method was verified.

In making the die of automotive outer panels, the most difficult process is fixing surface deflection. To fabricate high-quality outer panels without modifying dies, it is important to develop an evaluation method and a numerical analysis method for the surface deflection of outer panels. In this study, we developed a new evaluation method that uses the maximum value of curvature, calculated using reflecting curves in the surface. This made the examiner evaluation conform to digital evaluation. The method shows better agreement with the examiner evaluation than the conventional method. We proposed a new analysis method of predicting surface deflection correctly. By the proposed method, plastic deformation is calculated in consideration of stress in the thickness direction, and restriking conditions are examined. We applied our methods to the fabrication of automotive outer panels and verified that they were useful and practical.