Investigation of factors affecting the formability of metallic sheets in dieless incremental hole-flanging

Incremental sheet forming (ISF) is recognized to have the capability to enhance the formability of metallic sheets in comparison to press working (PW), but there is insufficient knowledge regarding the physical mechanisms underlying this phenomenon. This paper examines the forming limits of Al 6061 and AISI 304 sheets in dieless incremental hole-flanging (IHF), which is an important supplement to conventional ISF. The effect of discontinuous deformation and strain-induced martensitic transformation in 304 steel on deformation is especially considered. It was found that the limiting forming ratios (LFRs) of all the blanks in IHF were improved compared to those in PW, and the LFRs of the 6061 blanks were improved more. In the uniaxial tensile tests under repeated loading-unloading, the elongation of the 6061 specimens slightly improved, while that of 304 steel obviously decreased with the application of cyclic loading. The results indicate that cyclic straining can contribute to the ductility of 6061 blanks in IHF. For the 304 blanks, the strain-induced martensitic transformation was found in all the processes, and the martensite content increases with the deformation amount and the frequency of cyclic loading. The TRIP (transformation-induced plasticity) effect of 304 sheets does not dominate in the tensile tests and IHF. Moreover, a special failure mode of the 304 blanks, the petaloid neck, which is prone to appear before fracturing in IHF, was addressed.