Wrinkling in shrink flanging by single point incremental forming

Conventional flanging processes are commonly used in the automotive and aeronautical industry to provide stiffness or support for further assembly of thin-walled structures. Incremental flanging has emerged as a promising flexible and low-cost alternative to conventional processes for small batch production. This work presents a systematic experimental and numerical analysis of formability and failure within the forming limit diagram (FLD) of shrink flanges performed on AA2024-T3 sheet by single point incremental forming (SPIF). A series of tests varying the initial width, length and die radii of the flange are carried out under different process conditions, stablishing the practical process window for different flange radii, spindle speed and tool diameter. The results show two modes of failure for the larger flange radius, failure by wrinkling and failure by incipient wrinkling, and only failure by incipient wrinkling for the smaller die radius. The deformation mechanism of successful flanges and the wrinkling initiation are analysed obtaining strain points below the pure compression line (beta=- 2) in the FLD and pointing out that this limit does not provide a unique wrinkling limit curve (WLC) for this process. Finally, the stress evolutions of a set of flanges is analysed using an explicit finite element model. It is found that wrinkling in shrink flanging by SPIF occurs when the minor principal stress (compressive stress) reaches a certain critical level. Below this level, flanges are successfully formed. On that basis, a numerical methodology to predict the onset of wrinkling is proposed. This allows identifying the winkling limit as the minor principal stress at the edge of the flange that triggers the wrinkling process.