Studies of the interactive effect of specimen and grain sizes on the plastic deformation behavior in microforming

Microforming process provides an efficient approach to fabricating microparts via microscaled plastic deformation. However, the material properties change and the size effect occur when the workpiece size is scaled down from macro- to microlevel, which makes the development of microforming system difficult. It is thus necessary to study the size effect phenomena and microscaled deformation behaviors. In this research, the upsetting of annealed pure copper cylinders with different sizes from macro- to microscale is conducted to investigate the interactive effect of specimen and grain sizes on material deformation behavior. It is found that flow stress decreases and surface roughening and inhomogeneous flow take place with the increase of grain size and the decrease of specimen size. Furthermore, the properties of grain interior and grain boundary change, and the properties of surface grains become significant in the overall deformation behavior. It leads to the deviation between the Hall–Petch relation and the experimental results. It is further revealed that the flow stress has a linear relationship with the ratio of specimen size to grain size (D/d) at a given strain, and the change rate of the flow stress with D/d could be independent of strain. By examining the changes of working hardening and slip distance, it is found that there could be a delay in the formation of dislocation cell, and the slip distance and the size of dislocation cell could increase with the decrease of D/d. The presented size effect phenomena and the discussed physics thus provide a basis for the further exploration of the microscale plastic deformation behavior.