Formation of intermetallic phases in Al-coated hot-stamped 22MnB5 sheets in terms of coating thickness and Si content

AlSiFe coatings with differing thicknesses and Si contents were applied to steel sheets by hot dipping. The steel sheets were austenitized at TAUS = 920 °C for different dwell times and then quenched in water. Phase formation as a function of coating thickness and Si content at the steel substrate/coating interface was investigated by ex-situ phase analysis with synchrotron radiation and by electron backscatter diffraction (EBSD). X-ray diffraction (XRD) and EBSD investigations confirmed the formation of AlFe-rich intermetallics at the steel/coating interface as a result of a strong diffusion of the elements Al and Fe. Within the first minute, Fe diffusion into the partially melted Al-base coatings promotes the formation of intermetallics of type Al8Fe2Si, Al13Fe4, and Al5Fe2. After the coating has transformed completely into Al-Fe intermetallics, Al diffusion into the steel substrate becomes more pronounced, thus reducing the Al content in the Al-Fe intermetallics and promoting formation of the phases of type Al2Fe and AlFe in the coating and formation of an Al-rich bcc layer in the steel substrate. The transformation kinetics of the resulting Al-, Fe-rich intermetallics are influenced by the coating thickness and the chemical composition of the Al-base coating. On the one hand, faster saturation of Fe in the Al-base coating is promoted by a shorter diffusion path and therefore by a thinner coating thickness. Otherwise, Si influences the diffusivity of the elements Al and Fe in the Al-, Fe-rich intermetallics and promotes the formation of Si-richer intermetallics, which then act as nuclei for Fe-richer intermetallics.