Abstract:A new polycrystal model was presented from the viewpoint of polycrystal structure of the billets considering free surface effects. In the model, the billet was divided into three portions, such as free surface portion, transition portion and internal portion. The grains in free surface portion were considered the single grains, and the anisotropy of the grains was taken into account by introducing grain orientation to explain the inhomogeneous deformation. In the transition portion, the effects of the neighbouring grains were adopted in the model. The grains in the internal portion were considered the polycrystalline material. With the developed model, the upsetting deformation process was simulated by the MSC Superform software. The scatter of the flow stress and inhomogeneous deformation was observed by analysis of the model. The comparisons show that the computational results are good agreed with the experimental results. This means that the presented model is effective.

Key words: micro forming; size effect; polycrystal model; inhomogeneous deformation