Experimental studies of plastic deformation behaviors in microheading process

The demand on microparts is increasing significantly due to the global trend of product miniaturization. Microforming provides a promising approach to fabricating microparts with good mechanical properties at low cost and high productivity. However, when the part dimension is scaled down to micro-level, the size effect occurs such that the deformation behaviors are different from those in macroscale. The investigations on size effect and the physics behind the phenomena observed are thus needed. In this research, the size effect on material deformation behavior is investigated based on the two heading cases, viz., central heading and end heading of the rods of the cold-drawn and annealed pure copper. It is found that the grain flow of the cold-drawn material is different from that of the annealed material and the coarse-grained material undergoes nonuniform deformation when the part size is in microscale. These phenomena can be explained by the strong texture in the cold-drawn material and the strain incompatibility among different grains in the coarse-grained material. In addition, the tooling surface texture generated in grinding process makes the friction different in different flow directions, resulting in the variation of formed profiles in both macro- and microparts. The surface roughening effect on the formed part becomes significant in microheading process. It has a close relationship with the grain size and crystallographic texture of the forming material. Furthermore, the punch-die and workpiece-tooling interfaces have a certain effect on deformation pressure. This leads to the scattering effect and the difference of the deformation pressure-stroke curves between the macroheading and microheading. The reported experimental results and the articulated findings provide a basis for understanding of material size effect in microheading process.