Comparison of tribological and wear performances of AlSi and Zn coatings in hot stamping of boron steel sheets

Since direct hot stamping has become the most relevant technology to manufacture 22MnB5 structural parts for automotive industry, the knowledge of the tribological conditions and the comprehension of the wear mechanisms are crucial for the process competitiveness. AlSi and Zn coatings, which are currently the most used to coat the steel sheets in order to avoid scale formation in the furnace, are supposed to act as lubricants during the forming stage. However, the thermal and mechanical conditions at the blank-dies interface may change significantly within the forming stage, depending on the geometrical features of the dies and the process parameters, so making critical the integrity of the coatings itself.The paper presents the results of experimental investigations on the tribological performances of AlSi and Zn coatings applied to 22MnB5, reproducing the different thermal conditions at the blank-die interfaces. Frictional and wear tests, reproducing the thermo-mechanical conditions typical of hot stamping, were carried out in the temperature range from 800. °C down to 600. °C. The results show lower friction coefficients for the Zn coating. For both the coatings, the dies wear appears as a combination of adhesion and abrasion, with the latter more relevant at the lower temperatures. Both the coatings show worse performances when the test temperatures are decreased.