Micro forming processes are very well suited for manufacturing of small metal parts in large quantities and micro deep drawing provides a great application potential for the manufacturing of parts with complex shapes. But size effects like changed tribology and material properties usually result in smaller process windows for micro forming operations. Process caused wear as well as large inaccuracy in manufacturing of micro forming tools is responsible for geometrical deviation of the tools from nominal size. Both influences can have essential impact on the process window size and process stability. A better understanding of the influence of tool geometry on process stability can help to improve and optimize process control in micro forming. In addition, a quantitative judgment of the impact of wear and manufacturing inaccuracy will be possible. Therefore, in this study, the impact of different tool geometries on the punch force in micro deep drawing was investigated. Significantly varied tool geometries were punch diameter, drawing gap, punch and drawing die radius and shape of the die edge. FEM simulations as well as experiments were used to determine tool geometry influence on the punch force of a micro deep drawing process. Hereby, it was possible to classify each geometry variation regarding its impact on the punch force and therefore on one important parameter of the process stability. Results show that the greatest impact on the punch force was caused by modifications of the punch diameter and variation of the drawing gap. Changes in punch or drawing die radii proved to be of minor importance.
For a better process understanding of micro deep drawing processes and reliable prediction of component failure in FE simulations, it requires the most accurate knowledge of actual material behaviour. However, it is not sufficient to describe material failure for a multi axial stress state in deep drawing using a mechanical parameter as the elongation from tensile test. A forming limit diagram and a forming limit curve are more suited to describe 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 foil materials. 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 Nakazima tests were performed to determine complete forming limit diagrams for different foil materials with a thickness range of 20 µm to 25 µm. Scaling the test setup improves the handling of thin specimens. Results with a higher local resolution and the 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.
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