Temperature and strain dependent fracture of AlSi coating in hot stamping of press hardening steel

In hot stamping severe tool wear occurs. The main tool wear mechanism is the so-called compaction galling, in which fractured particles from the coating of the part (blank) being build-up and become compacted upon the tools. Since fractured particles play an important role in the wear mechanism, fracture of the AlSi coating is investigated. For this purpose, simplified B-pillar parts are pressed at several temperatures to investigate the effect of temperature, strain and tool contact on the fracture behavior of the AlSi coating. Three positions which exhibit different strain modes are analysed by means of optical topography and optical microscopy measurements. The results indicate a strong correlation between the AlSi coating fracture, strain and temperature. For instance, at relatively low forming temperatures (i.e. 650 oC), mode I fracture as well as spallation of the coating and at higher temperatures (750 and 800 oC) mainly mode I coating fractures are observed. Interestingly, this type of mode I coating fractures are found to widen, eventually giving rise to mode II coating fractures along the coating-substrate interface. The onset strain of coating fracture is low and approximately at strains of 0.017, coating cracks are observed at all start temperatures. At contact-dominated regions, additional coating cracks are observed mainly due to a drop in blank temperature and added shear stresses due to the relative sliding between the blank and tools.