Friction effect of surface treated tools used for warm forming of Mg alloy sheets

Sheet forming of Mg alloy is usually performed at moderately high temperature since the formability of these alloys is very low at room temperature. However, it is difficult to use lubricant especially in a multi-stage sheet forming at moderately high temperatures. Therefore, tool coating is a better than lubricant application for Mg alloy sheet warm forming. In this study, some tool coating methods were investigated to reduce friction and prevent adhesion between the tool and sheet. Tools with three different non-metallic coating layers, namely, TiBCN, DLC, and TAC, were compared with a no-coated tool for friction effects by a pin-on-disc friction test. The friction coefficients for the different coating layers were measured at room temperature, 170 and 250°C. AISI H-13 and AZ31 were used as pin and disc materials, respectively. Two of the tool coatings, namely, TiBCN and DLC, were not good enough to prevent sticking between the tool and AZ31 sheet. The coulomb friction coefficient in the case of these coatings was 0.4 in ambient air. On the other hand, the friction coefficient of the TAC coating layer was lower(< 0.2~0.3) than the coefficients of these two coatings before the adhesion of the Mg alloy with the tool.