Effect of Cooling Rate on Microstructure, Phases, and Properties of Al-Si Coated Hot-Press-Forming Steel Sheets

The evolution of various intermetallic layers and the formation of voids at the substrate/coating interface of hot-dip-aluminized (Al-9 wt% Si) hot-press-forming steel were systematically investigated using a field-emission scanning electron microscope and glow discharge optical emission spectrometer. Following austenitization at 900°C for 10 min, the substrate/coating interface of the air-cooled and quenched Al-Si coatings was characterized by the presence of Fe2SiAl2, Fe2Al5, and α-Fe(Al) layers. The results revealed that rapid cooling/quenching effectively restricts the diffusion of Al, Si, and Fe atoms, leading to the formation of fewer Kirkendall voids within the intermetallic and α-Fe(Al) layers, with a decreased thickness of the latter. Consequently, the quenched Al-Si coating exhibited increased Vickers hardness, indicating enhanced mechanical properties due to the higher cooling rate.