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The marginal-restraint mandrel-free spinning method can solve the problem of instability when spin-forming thin-walled dome blanks. However, when shear-expansion compound forming is adopted for a large-diameter dome, the sinking phenomenon that occurs during the expansion spinning stage makes it impossible to accurately form the dome. Through finite element simulation, it is found that the smaller the roller fillet radius is, the smaller the feed ratio, and the larger the angle is between passes, the more obvious the sinking phenomenon and the worse the forming accuracy. It is further revealed that the mechanism of the sinking phenomenon is shear deformation in addition to expansion deformation in the expansion spinning stage. To solve the sinking problem, this paper proposes a marginal-restraint mandrel-free spinning method with a large bottom reserve. Research shows that a larger roller fillet radius and more passes can improve the contour accuracy of the dome by using the cylindrical roller structure. Finally, it is verified that the new spinning method can achieve accurate forming of large thin-walled aluminum alloy domes with a diameter of 2250 mm and a forming angle of 60°. [ABSTRACT FROM AUTHOR]

The special-shaped tank dome of a launch vehicle is a large, thin-walled, curved structure that is difficult to form using the conventional center-restraint spinning method. This study proposes a two-step marginal-restraint mandrel-free spinning method for forming large domes. The finite element analysis results indicate that a larger roller fillet radius and larger feed ratios lead to a larger upper convex angle and the minimum thickness value for the bottom contour. This study explored the impact of shape parameter variations on the upper convexity and transition rounding angle on forming accuracy. The results show that the convexity of the bottom of the special-shaped domes increases with a larger roller fillet radius and larger feed ratios while the overall height decreases. The forming accuracy is adversely affected by larger transition rounding angles and smaller upper convexities. For the accurate forming of domes, the mutual coupling influence during two-step forming should be considered, and a suitable process and suitable trajectory compensation parameters must be carefully selected. Finally, the study verified that a two-step marginal-restraint mandrel-free spinning method with a 10 mm roller fillet radius, a 2 mm/r feed ratio, and the corresponding trajectory compensation can achieve the precise forming of 2250 mm thin-walled special-shaped domes. [ABSTRACT FROM AUTHOR]

The special-shaped tank dome of a launch vehicle is a large, thin-walled, curved structure that is difficult to form using the conventional center-restraint spinning method. This study proposes a two-step marginal-restraint mandrel-free spinning method for forming large domes. The finite element analysis results indicate that a larger roller fillet radius and larger feed ratios lead to a larger upper convex angle and the minimum thickness value for the bottom contour. This study explored the impact of shape parameter variations on the upper convexity and transition rounding angle on forming accuracy. The results show that the convexity of the bottom of the special-shaped domes increases with a larger roller fillet radius and larger feed ratios while the overall height decreases. The forming accuracy is adversely affected by larger transition rounding angles and smaller upper convexities. For the accurate forming of domes, the mutual coupling influence during two-step forming should be considered, and a suitable process and suitable trajectory compensation parameters must be carefully selected. Finally, the study verified that a two-step marginal-restraint mandrel-free spinning method with a 10 mm roller fillet radius, a 2 mm/r feed ratio, and the corresponding trajectory compensation can achieve the precise forming of 2250 mm thin-walled special-shaped domes.