Experimental and numerical study of frictional size effects in micro-metal forming

Due to the size effect phenomenon, the conventional friction models commonly used in metal forming are not accurate for use in micro-metal forming. In this study, the frictional size effect, as one of the most important phenomena in micro-metal forming, has been investigated. Different frictional models developed based on open and closed lubricant pockets theory have been investigated in both dry and lubricated frictional conditions. Those models use a scale parameter to quantify friction on the micro-scale and a real contact area to calculate the friction force at contacting surface. The models were implemented into ABAQUS finite element package via the VFRIC_COEF subroutine interface. The ring compression test with specimens of different sizes was used to determine the parameters of the models. By reducing the dimensions of the specimens in the ring compression test, no size effect was observed in dry friction conditions. However, in the lubricated frictional conditions, it was observed that the coefficient of friction increased significantly with reducing the specimen size. As the dimensions of the specimen decrease and the scale parameter approaches 1, the gap between the coefficient of friction curves increases significantly, and the coefficient of friction converges to those obtained in dry friction conditions.