Influence of welding distortion of preform shell on shape variation during hydro-forming process of ellipsoidal shell

An ellipsoidal shell is first welding assembled by 12 pieces of lateral petal and 2 pieces of polar plate, and then hydro-formed into the needed dimension. In order to analyze the effect of welding distortion of preform shell on dimension accuracy of ellipsoidal shell, welding assembly and subsequent hydro-forming of ellipsoidal shell with the axis length ratio 2.2 were conducted by experimental research along with simulation. Welding distortion was experimentally evaluated for every sequential number of weld seams, and the displacement of each weld seam was recorded by 3D-video measurement. It is shown that the radial displacements on the welding seams are inward the radial direction, and the magnitudes of deformation increase with the progress of welding assembly. Considering the welding distortion of preform shell, simulation was conducted on hydro-forming process; latitudinal stress distribution and thickness distribution were given. An experiment on hydro-forming process was carried out, and the axis length variation, volume variation, and residual stress on the weld seams were stated. It is shown that the latitudinal stress, thickness on lateral petals, and long axis variation are asymmetrical at the early stage of deformation, but the wrinkling does not occur. With the increase of internal pressure, sectional profiles at latitudinal direction will trend to circle and a sound symmetric ellipsoidal shell has been obtained. The magnitude of residual stress after hydro-forming is very small and sometimes is compressive, which can significantly improve the ability of resist stress corrosion for pressure vessel. It is proved that the welding distortion can be eliminated by severe plastic deformation and it has less influence on the forming accuracy. However, it may be considered when the plastic deformation is slight such as hydro-forming of sphere.