Integrated Free-bulge Forming Method for Thin-walled Metallic Spherical Cap Structures

The end surfaces of large storage tanks used in various industries are often composed of thin-walled metallic spherical cap structures. The ability to process these components at a low cost and with high manufacturing precision is an important research challenge. In this study, a new integrated free-bulge forming method is proposed to fabricate thin-walled metallic spherical cap structures. This method involves fixing the perimeter of a circular forming sheet, applying internal water pressure, and uniformly bulging the central portion of the sheet to achieve a spherical cap structure. To analyze the forming performance of the proposed method, formulas for calculating the plastic strain and average thickness during the process of forming the spherical cap from the circular sheet are derived, enabling a clear understanding of the workable range of the free-bulge forming method. Additionally, by deriving a prediction formula for the internal water pressure required for the free-bulge of the spherical cap structure, the key process design factors are identified. For verification, a free-bulge forming device is developed, and thin-walled metallic spherical cap structures are processed. The results confirm that the spherical cap shape is sufficiently precise and can be stably produced using the free-bulge forming method. Furthermore, a specialized device for measuring the shape accuracy of the spherical cap formed using the proposed free-bulge method is developed, and the surface shape of the spherical cap structure is measured. The results show that the formed spherical cap shape has a maximum deviation of 2.3% from the theoretical shape, demonstrating adequate precision for practical applications. To further verify the processing performance of the free-bulge forming method, the thickness distribution of the processed thin-walled metallic spherical cap is measured along its diameter. The results show that, compared to the original thickness of 1.0 mm, the minimum thickness of 0.858 mm occurs at the center of the spherical cap, representing a thickness reduction rate of -13.2%. It is confirmed that the free-bulge method can be stably applied to typical thin-walled press materials.