【正文】 2020 Elsevier . All rights reserved. Related articles Tangential bending and stretching of thin ma... Materials amp。, S. K (b) forming limit diagram (FLD) of LZ91 sheet. View thumbnail images 3. The finite element model The tooling geometries were constructed by a CAD software, PRO/E, and were converted into the finite element mesh, as shown in Fig. 3(a), using the software DELTAMESH. The tooling was treated as rigid bodies, and the fournode shell element was adopted to construct the mesh for blank. The material properties and forming limit diagrams obtained from the experiments were used in the finite element simulations. The other simulation parameters used in the initial run were: punch velocity of 5 mm/s, blankholder force of 3 kN, and Coulomb friction coefficient of . The finite element software PAM_STAMP was employed to perform the analysis, and the simulations were performed on a desktop PC. 10 Fig. 3. The finite element simulations. (a) Finite element mesh and (b) fracture at the corners. View thumbnail images A finite element model was first constructed to examine the oneoperation forming process of the hinge. Due to symmetry, only one half of the top cover case was simulated, as shown in Fig. 3(a). The simulation result, as shown in Fig. 3(b), indicates that fracture occurs at the corners of flanges, and the minimum thickness is less than mm. It implies that the fracture problem is very serious and may not be solved just by enlarging the corner radii at the flanges. The finite element simulations were performed to study the parameters that affect the occurrence of fracture. Several approaches were proposed to avoid the fracture as well. 4. Multioperation stamping process design In order to avoid the occurrence of fracture, a multioperation stamping process is required. In the current industrial practice, it usually takes at least ten operational procedures to form the top cover case using the magnesium alloy sheet. In the present study, attempts were made to reduce the number of operational procedures. Several approaches were proposed to avoid the fracture, and the fouroperation stamping process had demonstrated itself as a feasible solution to the fracture problem. To limit the length of this paper, only the twooperation and the fouroperation stamping processes were depicted in the following. . Twooperation stamping process The first operation in the twooperation stamping process was sidewall forming as shown in Fig. 4(a), and the second one was the forming of flange of hinge presented in Fig. 4(b), the height of the flange of hinge being 5 mm. Fig. 4(c) shows the thickness distribution obtained from the finite element simulation. The minimum thickness of the deformed sheet was mm and the strains were all above the forming limit diagram. It means the fracture defect could be avoided. In addition, the height of the flange conformed to the target goal to be achieved. However, this process produced a critical defect of 11 wrinkling, as shown in Fig. 4(d), on the flange of hinge, which induces a problem in the subsequent trimming operation. Hence, even though the twooperation stamping process solved the fracture problem at the corner of the bottom and the flange of hinge, a better forming process is still expected to solve the wrinkling of flange of hinge. Fig. 4. Twooperation stamping process. (a) Formation of sidewalls, (b) formation of hinges, (c) thickness distribution and (d) wrinkle. View thumbnail images . Fouroperation stamping process The fouroperation forming process proposed in the present study starts with the forming of three sidewalls and the flange of the hinge with a generous corner radius, as shown in Fig. 5(a). Since the sidewall close to the flange was open and the corner radius was larger than the desired ones, the flange was successfully formed without fracture. S