Tribological behaviors in microforming considering microscopically trapped lubricant at contact interface

In the study of microforming meeting the needs of miniaturization of parts to be formed, the size effects are important parameters to be considered seriously. The objective of the investigation is to establish an explicit friction model in micro/mesoscale to calculate the coefficient of friction (COF) considering size effects, which is very helpful in analysis of microforming processes. With the open-closed pocket assumption, a scaling factor was adopted to describe the size effects on tribological behaviors in microforming. Based on the general Wanheim/Bay friction law, a relationship between the real contact area and the forming load was obtained considering the microscopical contact interface and the pressure induced by the trapped lubricant liquid. An explicit equation was developed including fraction of real contact area, scaling factor, and properties of lubricant. The effects of scaling factor and lubricant properties were discussed by analyzing its effects on the fraction of real contact area and coefficient of friction. With the developed model, the coefficient of friction was calculated and introduced into the finite element simulation of micro-upsetting deformation using ABAQUS software. When the scaling factor is less than 9, the size effect of friction becomes the main reason which affects the shape parameter in micro-upsetting deformation. Comparisons show that simulation results are in good agreement with that of experiments, which means that the developed model is suitable for analyzing size effects of friction in microforming.