Effect of coating thickness on the in-situ reflectance and surface roughness of Al-Si coated 22MnB5 steel

A major challenge inherent to hot stamping aluminized 22MnB5 blanks involves the liquefaction of the protective Al-Si coating, followed by its transformation into an intermetallic Al-Fe-Si layer. The intermediate liquid state contaminates the furnace rollers and causes the blanks to migrate within the furnace, while the evolving surface topography of the intermetallic coating impacts the radiative properties of the blanks as well as the subsequent quenching rate. This study investigates how the heating profile and coating thickness impact the solidification process and surface roughness using in-situ laser-based reflectance measurements on blanks with two coating thicknesses. Temporally resolved reflectance measurements demonstrate that the duration over which the coating is liquid is approximately halved for blanks with the thinner coating. Moreover, in-situ light scattering measurements show that blanks with the thinner coating reach a higher maximum surface roughness of approximately 2.75 mu m compared to 2.65 mu m for blanks with the thicker coating. These results are also connected to ex-situ scanning electron microscope images and energy-dispersive X-ray spectroscopy measurements, which depict the growth of iron-rich intermetallic compounds Al4,5FeSi, Al7Fe2Si, Al5Fe2, and Al2Fe during heating.