Shape improvement of a dieless hydro-bulged sphere made of hexagonal and pentagonal shaped panels

• Experiments and simulation concerning dieless hydro-bulging shell were performed.
• Corner cutting or bending hexagonal or pentagonal panels lead to higher precision.
• After bulging the centers of the panels were thinner than the edges.
• Depression area appears on the wielding seam due to the buckling instability.
• The residual stress is on the wielding seams and ‘corner-cutting’ regions.

Dieless hydro-bulging of a spherical shell comprising welded hexagonal and pentagonal shaped panels, in the manner of a ‘soccer ball’ construction using ‘corner-cutting’ technique to improve shape accuracy, is described in this paper. The ‘corner-cutting treatment’ of the panels is used to decrease the roundness error of a bulged spherical ‘soccer ball’ shell with a diameter of 1300 mm. The strains on the surface of shell were measured versus the internal pressure and the equation for calculating the maximum internal pressure is given. The nonlinear commercial simulation software ADINA was used to simulate the forming process. During simulation, the thickness of welding seam was considered for the first time compared to previous research of shell hydro-bulging, because the thickness of welding seam is larger than that of sheet. In order to investigate the variation of the stress state during hydro-bulging, the trajectory of the principal stresses of typical points on the soccer shell was plotted on the yield locus by numerical simulation method. Simulation results suggest that the springback deformation and the residual stress after unloading of the internal pressure are small. The experimental and simulation results show that the plastic deformation of the hexagonal blanks is larger and occurs earlier than that of pentagonal blank in the hydro-bulging process. Moreover, the roundness error of the shell after ‘corner-cutting treatment’ is significantly decreased.