Fold defects mechanism of aluminum alloy thin-walled stiffened cylinders during flow forming

Flow forming is an advanced technology for manufacturing integrally thin-walled cylinders with orthometric stiffeners (longitudinal stiffeners (LSs) and transverse stiffeners (TSs)), and fold is a typical defect in this process. To understand the forming mechanism of fold defects, a flow forming finite element (FE) model of a representative cylinder with orthometric stiffeners was established, based on physical experiments. Meanwhile, the influence of geometry and process parameters on fold defects was investigated. In order to eliminate fold defects, the influence of sensitive parameter coupling on fold defects was investigated. It was found that the gap between the stiffener and groove wall in the flow forming process is the primary factor for the formation of fold defects, which is larger at the cross-region than that at TS and LS, and forms a surface pit during the subsequent forming process. Finally, the surface pit develops into the fold defect under the compression of material circumferential flow. Parametric analysis shows that the roller attack angle, groove corner radius and rotation trajectory of roller are sensitive to the development of fold defects. By coupling any two sensitive parameters, fold defects can be further weakened. By utilizing all three of them simultaneously, it is possible to completely eliminate fold defects. The results provide methods and guidance for improving the quality of aluminum alloy cylinders with orthometric stiffeners during flow forming.