An advanced approach in simulation of microforming processes

S. Geißdörfer, U. Engel, M. Geiger
Chair of Manufacturing Technology, University of Erlangen-Nuremberg, 91058 Erlangen, Egerlandstr. 11, Germany

Abstract

At microscale, the large ratio between mean grain size of the material and specimen dimension cause an increasing influence of single grain forming behaviour on the overall forming process. Thus the forming behaviour of these parts can no longer be regarded as to be homogeneous. This leads to a change in the material behaviour resulting in a large scatter of forming results, e.g. varying cup height in a cup backward extrusion process or varying spring-back angles in a micro bending process. Moreover, some correlation between the integral flow stress of the workpiece and the scatter of the process factors on the one hand and the mean grain size and its standard deviation on the other hand has been detected in experiments. Conventional FE-simulation which is by its nature size independent, is not able to consider these effects observed when scaling down processes, in particular represented by a reduction of the flow stress, an increasing scatter of the process factors and a local material flow being different to that obtained in the case of macro parts. Therefore, a new simulation model is being developed in order to take into account the identified effects and to determine the scatter of the process factors. The so-called mesoscopic model provides the discretisation of the simulated material into individual objects which represents the grain structure of the real material. To each object an individual flow curve is assigned, calculated on the basis of metal physics given by Hall-Petch and Ashby’s theory. The computational grain structure generation is based on the theory of a Monte Carlo Potts growth law.

The present paper deals with the theoretical background of the new mesoscopic model, its characteristics like synthetic grain structure generation and the calculation of micro material properties - based on conventional material properties. The verification of the simulation model is done by carrying out various experiments with different mean grain sizes and grain structures but the same geometrical dimensions of the workpiece.