Characterization of the interfacial friction properties in the hot stamping process

In this study, an experimental machine for conducting flat strip drawing tests at high temperature was developed to investigate the interfacial tribological properties and evaluate the coefficient of friction in hot stamping. The strength and rigidity required for the testing machine were examined by finite element analysis and confirmed by actual drawing tests. The consistent and reproducible data further validated the reliable performance of the developed friction test machine. The high temperature tribological characteristics at the interface between the sheet specimen and clamping dies were also investigated. The thermal contact condition between the die and the blank was found to be significantly affected by the geometry of the clamping die, and an optimal clamping die design was then proposed accordingly for generating a nearly uniform contact condition in the experiment. With the developed testing machine, high temperature friction tests were conducted on three kinds of boron steel to evaluate the coefficients of friction under various temperatures and contact pressures. The experimental results revealed that the coefficient of friction at elevated temperatures was more sensitive to temperature than to contact pressure. A lower coefficient of friction was also noted for coated boron steel than for the bare equivalent.