The Morphology of Coating-Substrate Interface in Hot-Dip-Aluminized Steels

Abstract

In Hot-Dip-Aluminized steels, the morphology and the profile of the interface between the aluminum coating and the substrate steel, are affected both by the composition of the molten aluminum as well as by the composition, and even the microstructure, of the substrate steel. This effect has been investigated using optical and scanning electron microscopy, and X-ray diffraction. The reaction between the steel and the molten aluminum leads to the formation of Fe-Al intermetallic compounds on the steel surface. The thickness and the morphology of the interlayer vary with the silicon-content of the molten aluminum. In hot-dip-aluminizing with pure aluminum, the interlayer is ‘thick’ and exhibits a finger-like growth into the steel. With a gradual addition of silicon into the aluminum melt, the thickness of the interlayer decreases and the interface between the interlayer and the substrate successively becomes ‘smoother’. With an increase in the carbon-content of the substrate steel the growth of the interlayer into the steel is impeded by the pearlite phase, whereas the ferrite phase appears to dissolve more readily. X-ray diffraction and scanning electron microscopic studies showed that the interlayer formed in samples aluminized in pure aluminum, essentially consisted of orthorhombic Fe2Al5, while with a gradual addition of silicon into the aluminum melt, a cubic phase based on Fe3Al also started to form in the interlayer and replaced most of the Fe2Al5. It was further observed that the grains of Fe2Al5 phase exhibited a preferred lattice orientation, and also that even when other phases are present in the interlayer, the phase at the transformation front is always the Fe2Al5.

Bending experiments carried out on aluminized specimens showed that as the thickness of the inter-metallic layer increased the angle, at which the start of the cracking in the aluminum coating during bending was observed, decreased. Metallographic examination of bent samples showed that the interlayer developed cracking much earlier than the appearance of the cracks on the surface. These experiments suggested that the aluminized steel flats (or sheets) exhibit very limited formability.