Formability of AlSi and Zn coating during hot stamping

Hot stamping is widely used in the automotive industry due to the combination of high strength (±1500 MPa), low springback and good formability. The sheet is austenitized in a furnace transported to a press and formed at high temperatures (600-800°C). Forming at lower temperatures could be interesting with respect to multi-step hot stamping. Coatings are used to prevent scaling during the austitisation. AlSi is the most common protection. Zn coatings are also applied but are susceptible to microcracks. These microcracks can be avoided by using pre-cooling. In this investigation the formability and fracture of AlSi and Zn coating at several temperatures are investigated. Cross dies are formed and extensively analysed. At low temperatures the AlSi coating is very brittle and results in large amount of tool pollution. With respect to fracture and formability the Zn coating is stable in the whole temperature range and could be suitable for multi step hot stamping process. (paper)