Hydroforming sheet metal with intermittent changes in the draw-in condition of the flange

The hydroforming process is performed with and without the presence of the inward flow of the flange. In single-stage hydro forming, improvement in sheet formability is observed when draw-in of the flange is present. The formability of the blank material is further examined when the blank is subjected to two double-stage hydroforming processes of different deformation paths. In Path A, the first stage, which is the prestraining stage, involves hydroforming the blank with draw-in permitted to a polar height before the onset of instability, followed by the second stage in which draw-in is prevented. Path B involves hydroforming with draw-in prevented as the first deformation stage, followed by hydroforming with draw-in permitted as the second. The good correlation between the theoretical and experimental results suggests that double-stage hydroforming can be modelled as a single-stage process having intermittent changes in the draw-in condition of the flange. The findings herein would also provide better understanding of how the formability of the blank material is affected by the level of prestrain in each deformation path.