Some Remarks on Formability and Microstructural Features of Incrementally Formed Sheets as a Function of Geometrical Parameters

Several investigations have been carried out in recent years to study the fundamental aspects of incremental sheet forming operations, with particular attention to determination of material formability and evaluation of the formed component geometry. Some of these investigations based on both experimental and numerical studies tried to give a comprehensive explanation of deformation mechanisms that arise during incremental sheet forming, and which possibly affect the material formability. However, none of the proposed theories are today fully accepted by the scientific community. The objective of the paper is to study the effect that geometrical parameters have on the component formability and on its microstructural characteristics. Axi-symmetric parts characterized by a varying slope with depth and by different initial slopes were incrementally formed until the first crack appeared. The formed components were measured with a coordinate measuring machine and their geometry and thickness profiles evaluated. It is shown that the final thickness of the component is strongly dependent on geometrical parameters: in particular, the presence of one or two minima in the thickness is outlined, as a consequence of the different predominant deforming mechanisms that lead to sheet failure. In order to study the relevant deformation mechanisms, microstructural characterization was conducted on the failure zone of the formed parts, by means of optical microscopy to evaluate the part microstructure as compared to the non-deformed blank. The observed microstructural features were then compared to the equivalent ones of tensile sheet specimens deformed until fracture, in order to evidence similarities and differences, and utilized to support or deny theories about incremental forming deformation mechanisms available in literature.