The effects of temperature on friction and wear mechanisms during direct press hardening of Al-Si coated ultra-high strength steel

Direct press hardening is a non-isothermal sheet metal forming method which combines forming and heat treatment in a single process. However, due to the high temperatures during the forming phase, tool wear is severe and friction is high. In this paper, hot strip draw tests are utilised to assess the influence of the forming temperature on the coefficient of friction (COF) and active wear mechanisms during sliding of Al-Si coated press hardening steel (PHS) strip in contact with uncoated tools under typical hot forming process conditions. The COF is found to be temperature dependent during initial sliding against a virgin tool surface. Whereas, for 10 consecutive strip draws, COF is only temperature dependent for the first samples over the temperature range from 400 °C to 750 °C. This would be due to the tribolayers which form in the tool-sheet contact during the test series. Conversely, the wear mechanisms active in this temperature range are temperature dependent: at higher temperatures (> 600 °C) an area of severe abrasive wear is found that precedes a thick layer of compaction galling while at lower temperatures, (< 600 °C) adhesive wear is dominant. Furthermore, the results show that particles leading to compaction galling are predominantly generated from the Al-Si coating and their size depends on temperature and are related to the fracture of the Al-Si coating.