Tribology and hot forming performance of self-lubricious NC/NiBN and NC/WC:C hybrid composite coatings for hot forming die

Adhesion between blank and die is the main reason for seizure and short tooling life during metal forming processes at elevated temperatures. This study applied novel complex-structured composite coatings, NC/NiBN and NC/WC:C, to the hot forming die flange and radius to reduce the use of lubricant. The high-temperature adhesion resistance and hot forming properties of the hybrid coated dies were tested and correlated with the microstructure and mechanical properties of the contacted blank/tool surfaces. The results show that the coated dies experienced little adhesion at elevated temperatures, consequently, ultra-low coefficients of friction of 0.11 against steel and 0.10 against Al at 350 °C were obtained. The minimum weight of lubricant per unit area required for a successful deep drawing of AA6082 was quantified by hot forming tests at various temperatures, and it was reduced by 83% at 400 °C. Complete lubricant-free deep drawing was achieved with limited blankholding force and forming temperature. Based on this experimental data and theoretical analysis of two disparate stress states, a model comparing the frictional state of material surfaces using the geometric features on formed parts is proposed.