metal foils

A. Diehl, D. Staud, U. Engel
Chair of Manufacturing Technology, University of Erlangen-Nuremberg Egerlandstr. 11, D-91058 Erlangen / Germany

Abstract

Ongoing miniaturisation leads to increasing complexity of micro parts linked with continuously decreasing development time. Hence, the demand for reliable material data and means to collect these data in a most efficient way is rising. Since the mechanical properties and thus material forming behaviour are dependent on the stress and strain conditions, the test methods have to be as close as possible to real conditions. Further, due to the so called size effects, data gathered from conventional length scale experiments cannot be used for the description of material used for parts with feature sizes in the micrometer range. In the present paper, the hydraulic bulge test as a means for the mechanical characterisation of thin metal sheets with thicknesses in the range of 25 μm to 500 μm is discussed and compared to data obtained by conventional tensile testing. Challenges due to the small sheet thickness are emphasized and the effect of strain rate on the flow curve is shown. The influence of geometric dimensions on the evaluation of the experiments is investigated by downscaling of the hydraulic bulge test. The material flow curves, as well as the forming limits are discussed in dependence of the sheet thickness.

A. Diehl, U. Engel, M. Geiger
Chair of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstr. 11, D-91058 Erlangen / Germany

Abstract

Metal foils attract a large field of applications, e.g. in micro-technology they are being used for sensors, actors, micro-electro-mechanical systems and in medical devices. Conventional sheet metal forming processes are in principle applicable for metal foil forming. Reducing the sheet thickness to the order of micrometers, however, causes various scaling effects. Therefore, the know-how of conventional sheet metal forming cannot be transferred directly to metal foil forming. A known phenomenon during foil forming is the reduction of strength of the material with decreasing thickness due to the increasing share of surface grains with fewer constraints to plastic flow on the overall volume. The opposed phenomenon is the increase of material strength regarding foils with mean grain sizes in the range of the foil thickness or even higher.

In the present paper basic research via scaled free bending tests is performed to investigate size effects in order to provide basic knowledge for the design of the process and of the components, respectively. An important factor in production accuracy of bending processes is the spring-back. In the current research spring-back of aluminium foils (Al 99.5) in dependence of the foil thickness is investigated with foil thicknesses ranging from 25 to 200 microns. Variation of the mean grain size/foil thickness ratio is achieved by different heat treatments. The experimental results are being compared with FE-simulations.