Experimental studies of the size effect affected microscale plastic deformation in micro upsetting process

Microforming is one of the promising approaches to fabricating microparts for its high productivity, low production cost and good mechanical properties. The material deformation behavior in microforming, however, is different from the one in macroforming. The macroforming knowledge is not applicable to the design and development of microparts. It is thus necessary to investigate the size effect on deformation behaviors and the physics behind in the microforming. In this research, the micro upsetting of cold-drawn and annealed pure copper with different billet sizes from macro- to micro-scale is conducted to investigate the size effect on material flow behavior, surface roughness evolution, flow stress and the hardening behavior. It is found that the compressive instability takes place for the cold-drawn specimens, resulting in the occurrence of double barreling. In addition, the inhomogeneous material flow occurs, surface roughness increases, flow stress decreases and the scattering range of the measured material properties increases with the decrease of workpiece size and the increase of grain size. To quantify the size effect, the ratio of the internal grain boundary surface area to the total grain boundary surface area of the workpiece is introduced. Based on the hardening behavior and the flow stress, the proposed modeling methodology for describing the size effect phenomena is verified. The reported experimental results and the modeling methodology thus provide an in-depth understanding of the size effect in microscale plastic deformation.