The Mechanism of Formability Improvement in Multi-pass Incremental Flanging

As a flexible and low-cost forming process, incremental flanging has been widely used in the aerospace field. The multi-pass flanging strategy has been proven to be an effective means to further improve formability, but the mechanism of formability improvement is not clear. In this paper, a FE (Finite Element) model of the incremental flanging was established by introducing the FFL (Fracture Forming Limit), and the multi-pass incremental flanging with two different strategies was simulated. Based on forming defects, minimum thickness, and neck height of different strategies, it was found that the formability from low to high is single-pass flanging, multi-pass flanging with increasing flanging angle, and multi-pass flanging with increasing flanging diameter. Then, by analyzing the strain paths, it is found that the reason for the single-pass incremental flanging failure is that the strain value reaches FFL in the bidirectional tensile strain state, and the multi-pass incremental flanging forming with a gradually increasing flanging angle can reduce the strain value in the bidirectional tensile state, thus improving the formability. The multi-pass incremental flanging with increasing flanging diameter can avoid deformation in the bidirectional stretching state and further improve formability.