Phosphate conversion process on Al–Si-coated steel: characterization and impact on the heat-treatment performance†

Energy- and time-efficient hot-stamping processes are essential for the production of lightweight, high-performance automotive components. Aluminum–silicon (Al–Si)-coated steel, widely used in hot-stamping, suffers from low heating efficiency due to its reflective surface, limiting process speed and energy efficiency. In this study, a phosphate conversion process including alkaline cleaning and subsequent phosphate conversion was applied to aluminum–silicon (Al–Si-)-coated steel sheets to improve the heating rate during heat treatment. Surface characterization of the sheets was performed using field-emission scanning electron microscopy (FE-SEM), X-ray fluorescence (XRF), powder X-ray diffraction (PXRD), Fourier-transform infrared (FT-IR) spectroscopy with an integrating sphere, and X-ray photoelectron spectroscopy (XPS). Alkaline cleaning increased the surface concentration of silicon, which in turn decreased reflectance compared to untreated sheets, as confirmed by total reflectance measurements. Subsequent phosphate conversion formed a hopeite (Zn₃(PO₄)₂·4H₂O) layer, further reducing the reflectance. In situ temperature curves, recorded using a thermocouple embedded in the sheets, revealed a significant reduction of the heating time for the cleaned and phosphated sheets. Longer phosphating durations progressively shortened the heating time by up to 50%, which correlated with an increase in surface emissivity of nearly 100%, thereby doubling the efficiency of the heating process. Characterization of the phosphated sheets after heat treatment revealed that the phosphate coating has reacted with aluminum from the Al–Si-layer to form ZnAl₂O₄ (gahnite) and AlPO₄ (tridymite-type). The dehydration of the hopeite layer was also studied under the heat-treatment conditions, which showed an amorphous, hydrated intermediate phase after 2 s at 920 °C and the formation of crystalline Zn₃(PO₄)₂ after 10 s.