In situ optical diagnostics for characterizing the coating transformation of aluminized steel during hot stamping

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Cosma International, one of Canada's largest automotive parts manufacturers, makes high strength automotive parts through hot stamping. In this procedure, ultrahigh strength steel blanks are first austenitized in a roller hearth furnace and then transferred to a water-cooled die, where the parts are simultaneously quenched and formed into shape. The blanks have a thin aluminum/silicon coating that prevents the steel from oxidizing and decarborizing in the furnace. The coating melts and reacts with iron from the steel to form a permanent Al-Si-Fe intermetallic layer that provides some long term corrosion protection, but may also impair welding assembly if it grows too thick. Both the radiative properties of the steel, which determines the blank heating rate, and the surface texture, which affects die quenching rates and paint adhesion, also evolves with heating time. The present state-of-knowledge about coating evolution comes almost entirely from ex situ analysis of coupons that have been heated in the furnace, extracted, and quenched, but the temporal resolution of this procedure is limited and quenching alters the coating texture. This research will deploy a new suite of optical diagnostics, including laser speckle analysis, Fourier-transform infrared spectroscopy, and 3D Raman spectroscopy, to provide continuous in situ measurements of the Al-Si coating as it transforms with heating. These measurements will be used to derive a coating evolution model that will allow Cosma to optimize furnace-based hot stamping operations, adopt their techniques to new materials, and develop entirely new manufacturing processes