Iron Overlayers Facilitate Conversion of Al-Si Coatings to Intermetallics during Hot Stamping

Aluminum-silicon (Al-Si)-coated steel is a mainstay material for manufacturing ultrahigh strength automotive parts through hot stamping. The coating protects the blanks from oxidation and decarburization as the steel is austenitized in a furnace. It also reacts with the steel to form solid Al-Fe-Si intermetallic phases that provide long-term corrosion protection. However, the coating extends the required heating times due to its high reflectance, and, in its molten state, it can also impregnate the furnace rollers, leading to their failure. This work demonstrates a strategy to avoid these issues by depositing an Fe-rich layer on the Al-Si coating. The introduction of a second Fe source both increases the blank's ability to absorb thermal irradiation and introduces a secondary mechanism that accelerates the reactions that convert the metallic coating into the intermetallic layer. Cross-sectional Raman microscopic mapping reveals that the intermetallic phases grow from the steel/coating interface into the Al-Si coating only after the binary phase θ (Al₁₃Fe₄) converts to η (Al₅Fe₂) at 620 °C. With the Fe-rich overlayer, however, speciation maps show that the coating can be converted completely to solid-state intermetallic phases at temperatures only slightly above the Al-Si melting point. This strategy provides a promising new method to mitigate furnace roller contamination in industrial hot stamping manufacturing lines.