Effect of Temperature on Microstructure and Formability of Al-10 mass% Si Coatings

The type-1 aluminized (Al-10 mass% Si) coating on hot stamped steel is used in commercial applications. The effect of temperature on microstructure and formability of the Al-10 mass% Si coating was investigated. Hot-dip aluminized 22MnB5 steel was heated at 870, 900, 930, and 1050 °C for 5 min. After heat treatment at different temperatures, the microstructure of Al-10 mass% Si was characterized by field-emission scanning electron microscopy and energy-dispersive spectroscopy. The results show that when the Al-10 mass% Si coating was heated at 870 °C for 5 min, three types of intermetallic phases were formed: two ternary Fe-Si-Al phases (Fe2 SiAl7 + Fe2 Si2 Al5, and Fe2 SiAl2 + Fe2Si2Al5) and one binary Fe-Al phase (FeAl3). The phases in the coating became Fe2SiAl2 + Fe2Si2 Al5 and FeAl3, the binary Fe-Al phase near the interface became Fe2Al5 and the Si-rich layers were decreased with the increase of heating temperature. When the heat treatment was 1050 °C for 5 min, the Si-rich layer disappeared, the coating consisted of FeAl and Fe3Al, and the Kirkendall voids were formed. When the heating temperature was increased through 870, 900, and 930 °C, the coating hardness was reduced and cracks were formed in the coating after hot stamping. The formability of the coating with temperature change was confirmed by a hot stamping test. When heated at 1050 °C, the coating was formable but continuous Kirkendall voids were observed.