Determination of forming limit diagrams for thin foil materials based on scaled Nakajima test

摘要

For a better process understanding of micro deep drawing processes and reliable prediction of component failures in FE simulations accurate knowledge of actual material behaviour is necessary. However, it is not sufficient to describe material failure for a multi axial stress state in deep drawing using just one mechanical parameter from the elongation from tensile test. A forming limit diagram and a forming limit curve are more suitable for describing the limit of formability under deep drawing stress state conditions. Methods like hydraulic or pneumatic bulge tests are available to determine forming limit curves even for thin metal foils. Nevertheless, using these methods only positive minor strains can be achieved. Especially for a deep drawing process negative minor strains and the left side of a forming limit diagram are more important. Therefore, in this study, experiments based on scaled Nakajima tests are performed to determine complete forming limit diagrams for different foils with a thickness range from 20 μm to 25 μm. Scaling the dimensions of the test setup improves the handling of thin specimens. Results with a higher local resolution and smaller specimens' size is much closer to the actual size of a micro deep drawn component. Using this testing method forming limit diagrams for the materials Al99.5, E-Cu58, stainless austenitic nickel-chromium steel X5CrNi18-10 (1.4301/AISI 304), all produced by rolling, and an Al-Zr-foil, produced by a PVD sputtering process, were determined for the micro range.