Microstructure Characterization of Al–Si Coatings on Hot Stamping Steel under Different Heat Treatment Processes

Abstract

The microstructures of Al–Si coatings under different heat treatment processes were characterized by scanning electron microscopy and transmission electron microscopy. The research shows that with the increase of heat treatment temperature, the Fe element in the matrix continuously diffuses into the Al–Si coating, the higher the heat treatment temperature, the more sufficient the diffusion, which promotes the evolution of the intermetallic compounds in the Al–Si coating. The EBSD Kikuchi pattern calibration and the transmission electron microscope multi-band axis calibration method can accurately determine the various phases of the Al–Si coating at different heat treatment temperatures, especially the phases with the same or similar crystal structure. The phases of Al–Si coating without heat treatment are Al, Si, Al13Fe4(θ) and Al167.8Fe44.9Si23.9(τ5); when the heat treatment temperature increased to 500°C, the phase of the Al–Si coating remained basically unchanged, and only part of Al167.8Fe44.9Si23.9(τ5) evolved into Al9Fe2Si2(τ6); when the heat treatment temperature increased to 600°C, Al5Fe2(η) was newly formed in the Al–Si coating and Al2Fe3Si3(τ1) was precipitated on its edge; when the heat treatment temperature increased to 700°C, the Al2Fe3Si3(τ1) precipitated evolved into AlFe in the Al–Si coating; when the heat treatment temperature increased to 800°C, the diffusion layer (α-Fe) is formed in the Al–Si coating next to the iron substrate; When the heat treatment temperature increased to 900°C, the phase of the Al–Si coating is basically the same as the former, but its morphology changes significantly, the diffusion layer is significantly thicker, and the band-shaped AlFe phase is obviously discretized.