The size effect of flow stress is one of the main factors affecting the deformation behavior of materials in microforming. The traditional surface layer model can well explain the phenomenon that the material flow stress decreases with the declining size of the specimen at sub-millimeter and micron scales, while it also exhibits some deficiencies inconsistent with reality. In order to more accurately describe the deformation behavior of materials at the microscale, the modified surface layer model was proposed based on the experimental results and plastic forming theory, in which the region where the flow stress was lower than that of the internal polycrystalline rather than just a single layer of grains in the surface region was taken as the surface layer. The calculation method of the thickness of the surface layer and the stress in each region was also presented. According to the stress distribution in each region, a surface-layer-thickness model based on grain size was proposed. With the results of micro upsetting experiments, the surface-layer-thickness model of T2 copper was fitted. It was vali-dated that the modified-surface-layer model and surface-layer-thickness model can accurately predict the flow stress of the material at the microscale.
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