Wrinkling modeling and controlling in hydroforming of conical thin shells

Conventional fabrication of conical thin shells inevitably suffers from welding-induced profile deformation. Accordingly, this investigation proposes using the hydroforming process to manufacture these components. However, the small end of the bulging zone of the conical tube blank is prone to wrinkling under the compressive axial stress induced by internal pressure during hydroforming. Accordingly, a critical wrinkling pressure model was first established using the energy method, and the wrinkling mechanism was revealed through parametric analysis. Furthermore, the effects of lubrication, geometry, and loading mode on the deformation behavior of conical shells during hydroforming were revealed. Results indicate that the region near the small end of the bulging zone is susceptible to wrinkling under the loading mode where the axial force of the large punch, F, equals the counterforce exerted on the large punch by the internal pressure, F_p, (F=F_p). Moreover, the critical wrinkling pressure, p_cr, predicted by theoretical analysis and simulation agrees well with experimental results. A variable loading path (VLP) was adopted to regulate the end forces of the bulging zone by setting F>F_p in the early stage and F=F_p in the late stage of hydroforming, thereby effectively suppressing wrinkling. Reducing the Coulomb friction coefficient, μ, and the height-to-diameter ratio of the constraint zone at the large end, λ₁, facilitates material supply to the bulging zone and thus avoids excessive thickness reduction, but also results in the failure to completely eliminate the concave defect caused by wrinkling. Finally, a smooth conical shell with a large-end diameter of 1.1 m and a height of 0.8 m was hydroformed using a 1Cr18Ni9Ti stainless steel tube blank, thereby verifying the feasibility of hydroforming for large size and thin-walled conical shells with curved generatrices.