The miniaturization of products requires the mass production of microparts. The microforming can well meet this requirement. Due to the emergence of decreasing flow stress scale effect in the micro scale, the traditional forming process and theory may fail. Based on the crystal plasticity theory, upsetting tests of micro copper cylinders with different dimensions and grain sizes were simulated, and the decreasing flow stress scale effect was studied and discussed. Results show that with the decrease of billet dimensions, the flow stress is gradually decreased, and the decreasing flow stress scale effect is emerged; with the increase of grain size, the decreasing flow stress scale effect is more remarkable. It can also be seen that the decreasing flow stress scale effect can be well simulated with the crystal plasticity theory, and the necessary relevant information is provided for deeper understanding on this scale effect, as well as the design of processes and die structures in the microforming.

Based on the composite model of polycrystalline plastic deformation, a new composite model was built, with which the relation of flow stress to grain size, dimension and shape of the billet can be well explained. Through the comparison of this model with experimental data and the surface layer model, its rationality was testified. Based on this model, with the definition of scale effect factor, the decrease of flow stress in micro scale was discussed and predicted, and conclusions were drawn as below: with the downscaling of billet dimension, the decrease of flow stress is gradually increased; with the increase of grain size, the decrease of flow stress is also gradually increased; the shape of billet has a significant influence on the decreasing flow stress scale effect.