A 3D defect-driven analytical framework linking geometric evolution to failure limits in ring hydroforming

Achieving precise geometric control in the forming of complex ultrathin-walled components remains a persistent challenge due to the intricate coordination of multi-process parameters. Critically, the underlying mapping mechanism between geometric evolution and failure initiation has not been fully elucidated. This study proposed a novel analytical framework that quantitatively reveals the nonlinear coupling effects of process parameters on geometric evolution and defect transitions in ring hydroforming. Combining finite element simulations and experiments, the deformation behavior at each stage of the ring hydroforming process was analyzed. Obvious wall thinning was observed during the pre-bulging and feeding-bulging stages. The pressure and displacement exhibited a coupling effect on the geometric shape, the inappropriate loading paths of them may lead to geometric defects such as insufficient height or material accumulation. Based on the mechanical analysis through static equilibrium relationships, the geometric contour evolution characterization models that mathematically characterize the cross-sectional shape transition from circular to elliptical to U-shaped profiles were established, which were utilized to quantify the interrelations among process parameters and cross-sectional shape changes. Furthermore, by integrating critical constraints including minimum plastic deformation pressure, maximum burst pressure, geometric folding and excessive thinning, a 3D “displacement-bulging height-pressure” parameter-defect mapping space was constructed to visually analyze intrinsic relationship. Displacement and pressure exert a non-linear coupled influence on geometric evolution and defect formation. Specifically, an increase in pre-bulging pressure diminishes the driving efficacy of axial displacement on bulging height, explicitly resulting in a contraction of the parameter-defect mapping space. As the cross-sectional profile evolves, the dominant failure mechanism shifts from excessive thinning to either bursting or geometric folding, with the transition point determined by component geometry and loading history. The M-shaped ring, triple-peak ring and multi-wave corrugated flattened tube were successfully manufactured under loading paths designed within the contracted mapping space. Comparative analysis of experimental, simulation and geometric model results regarding cross-sectional profile evolution and wall thinning distributions further validates the mechanistic insights and demonstrates the framework's potential for guiding high-precision forming of complex ultrathin-walled components.