Consideration of Pile-Up Material on Identification of Friction Coefficient in Aluminum Forming Processes at High Temperature

The warm and hot upsetting sliding test (WHUST) is a direct measuring method of friction coefficient in metal forming processes. This specific test can conduct severe contact conditions referring to the tool-workpiece interface. In the WHUST, a contactor slides along a specimen with a constant penetration depth to generate a plastic strain in the vicinity of the contact surface. When performing the WHUST on aluminum alloys at high temperatures, pile-up material occurs in front of the contactor, causing a modification of the contact geometry. This study aims to indicate the impact of the pile-up material on identifying the friction coefficient. The analytical equation to determine the friction coefficient is presented, which considers the parameter of the pile-up material. In this study, the WHUST was scaled down from the existing device [1] and installed in the hot chamber to perform the tribological test at a precisely controlled temperature. The experimental data were used to identify the friction coefficient by different equations with and without considering the parameter of the pile-up material. Comparing the results show that the friction coefficient can be overestimated if the pile-up material is avoided to consider in the equation. Moreover, the evolutions of the friction coefficient and the material transfer at different sliding distances were experimentally investigated in this study by the WHUST on AA 6082-T6 aluminum alloy at 400 ℃.