Determination of the tribological size effects in microforming through the scaled upsetting tests of disks

Microforming is a very attractive and suitable manufacturing technology for the production of microscale parts since it enables to utilize a wide range of materials with high production rates and minimized material loss. Previous researches on microforming have shown that the traditional friction model can lead to significantly erroneous results. Thus, for successful development and applications of microforming process, a critical consideration of the tribological size effect is required. This paper investigates the tribological size effect in microforming of pure copper by scaled upsetting experiments of disks. In order to explain the mechanism of the friction size effect, a simple theoretical analysis was performed based on the upper-bound theorem and the lubricant pocket model. The sensitivity of the friction to the deformation characteristics of the material was analyzed using finite element analysis (FEA) under a wide range of friction conditions. In addition, upsetting tests of disks enabled to verify the effectiveness of the theoretical and FEA models and explained the change of the frictional behavior with the reduction of the part size qualitatively and quantitatively.