Size dependence of flow stress and plastic behaviour in microforming of polycrystalline metallic materials

Microforming which exploits the advantages of metal-forming processes has been considered to be a very promising manufacturing technology for microparts. However, the fundamental understandings on microforming have not been well established yet since the extensive findings and know-how of conventional forming cannot be simply transferred to the microscale. This article attempts to offer an effective theoretical and experimental modelling to describe the so-called 'size effect' on the behaviour of metals in microforming. A theoretical model was developed based on the dislocation theory to quantify the grain size and feature size effects on the flow behaviour of polycrystalline materials. Tensile tests of copper sheets of various specimen thicknesses and grain sizes were carried out, and thus the effectiveness of the theoretical model can be confirmed by comparing the predicted flow stress, dislocation density, and Hall-Petch constant with experimental measurements. The general trend of decreasing flow stress with increasing grain size and decreasing specimen thickness was observed. The ratio of feature size to grain size was identified as an important parameter to characterize the flow stress level, grain boundary strengthening, and failure characteristics in microscale deformation processes.