An Insight in Friction and Wear Mechanisms during Hot Stamping

Hot stamping is often used in the automotive industry to combine formability and strength. However, during forming process at high temperatures, friction and tool wear are determining factors that can affect the efficiency of the whole process. The goal of this paper is to investigate the effects of temperature on the local coefficient of friction and tool wear and to provide an insight in the phenomena which take place at the tool-sheet metal interface during hot stamping processes. For this purpose, hot friction draw tests between uncoated tool steel and Al-Si coated press hardening steel were carried out at several temperatures between 500-700°C. Consecutive tests were performed to mimic industrial hot stamping process and to investigate the effect of tool wear on the friction phenomenon. Finally, tool-sheet metal tribological behavior and the interaction between the friction and tool wear mechanisms were analyzed using different imaging and chemical characterization techniques. The results show that several stages can be distinguished at the interface between tool and sheet metal coating during hot stamping: flattening due to initial normal contact, ploughing of tool asperities through coating, secondary ploughing in the coating by adhered material on the tooling, and abrasive wear in the tool by embedded particles in the sheet metal coating. Furthermore, tool wear shows some major differences in the temperature range of 500-700°C. At high temperature a larger abrasive area and more severe compaction galling occurs that can be explained by material properties of Al-Si coating at elevated temperatures. The results of this study can be used for more efficient process design and a more realistic modelling of the hot stamping process.