Spring-back behaviour of thin metal foils in free bending processes

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.