Interactive effects of height-to-diameter ratio and strain on tribological behavior in micro compression of pure nickel cylinder

In micro scale metal forming processes, the plastic deformation behaviors are complicated and difficult to predict because of size effects. In this study, micro cylinder compression tests have been carried out to determine the plastic deformation behaviors in micro bulk metal forming. The effects of specimen height-to-diameter ratio and strain on the plastic deformation behaviors have been investigated extensively by compressing pure nickel micro cylinders. It is found that both the flow stress and friction coefficient decrease with the increase of the specimen height-to-diameter ratio. Friction coefficient decreases with the increase of compression ratio for specimens with the same height or decreases with the increase of the specimen height-to-diameter ratio under the same compression ratio. A relationship between normal pressure of lubricant friction, strain and height-to-diameter ratio (H/D) is established. A modified friction model is proposed based on the assumption of open-close lubricant pockets by introducing the specimen geometrical parameters. The calculated friction coefficients by the developed model and their tendencies are in good accordance with the experimental ones. It is also found that the ratio of the real contact areas (RCAs) decreases with increase of the specimen height-to-diameter ratio under the same compression ratio which can explain the decrease of the equivalent friction stress between the tool and the specimen. The new developed friction model enables more flexible modeling of friction behaviors in microforming processes.