THE SIGNIFICANCE OF FRICTION IN THE SINGLE POINT INCREMENTAL FORMING PROCESS

The Single Point Incremental Forming Process (SPIF) makes use of a single, small forming tool to incrementally deform a metal sheet into a complex shape. Without a need for dedicated tooling, this process meets the demand for small batch production and Rapid Prototyping of industrial shell-like structures. The sheet is formed due to a locally applied stress field on its surface, consisting of a normal and a shear component. The shear component is strongly influenced by friction with the dragging forming tool. In common practice, SPIF is performed under well lubricated conditions and with tool rotation, both designed to minimize friction. Through-Thickness Shear (TTS), which is part of the deformation mechanism in SPIF, may be intuitively linked to the friction applied during the process. In this paper, the role of friction in SPIF is experimentally investigated through the forming of an aluminium alloy sheet (AA3103-O) into a large wall angle cone with and without imposed tool rotation. It is found that only the force component along the instantaneous tool movement direction depends on the tool rotation, i.e. on the friction conditions. Finite Element (FE) simulations are also presented, which use (i) continuum elements, so that TTS can be taken into account, (ii) a sub-millimetre FE mesh, so that the small contact zone may be adequately modelled, and (iii) a mixed isotropic-kinematic hardening law that captures the anisotropic hardening until large strains. Under these three conditions, a quite accurate prediction of the forming force is obtained. Furthermore, it is shown how the validation of the FE constitutive law may be decoupled from the validation of the FE contact model. Finally, it is seen that overall TTS predictions are affected by tool rotation, which shows that the deformation mechanism in SPIF may be controlled to a certain extent by the imposed tool rotation.