Estimation of grain size and grain orientation influence in microforming processes by Taylor factor considerations

In the last decade many common forming processes were scaled down to microdimensions, to supply the increasing market of microparts, e.g. for electronic or medical devices. Knowledge of microforming processes is mainly based on in house experience and trial and error procedures. In most cases a decrease in reproducibility is observed with an increasing degree of miniaturization. Among other scaling effects especially three main effects are reported: (I) A decrease of geometrical accuracy. (II) A decrease of flow stress. (III) An increase of scatter for flow stress and part geometry. These phenomena are often related to the dominant influence of single grains inside the volume or the cross-section, because the forming process is geometrically scaled down while the microstructure is not. Accordingly, geometrically scaled down deep drawing experiments from 8 mm down to 1 mm punch diameter have been carried out, varying additionally the grain size to foil thickness ratio. Remarkable little variation was observed for the punch forces while increasing roughness affected significantly the cup geometry. This paper attempts to explain these findings using a basic model of a unit volume cube with different numbers of grains inside. The introduced model considers the Taylor factors to estimate the conditions under which a microforming process is likely to be influenced by single grain orientations and to explain the difference between flow stress determination in microcompression or microtensile tests versus punch force in microdeep drawing.