Variables Affecting Growth and Morphology of the Intermetallic Layer (Fe2Al5) of Steel Aluminized in Al-Cu Alloys

Abstract

The effects of the Cu concentration in Al (2~15 wt. %) on the growth and morphology of the intermetallic layer, phase constituents, hardness, oxidation and adhesion properties of an aluminized steel have been examined. In addition, the effects of variation in aluminizing temperature (675~950 °C) and dipping time (1~10 minutes) on the growth of the intermetallic layer were investigated. The techniques employed during the investigation included optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and thermal gravimetric analysis. The intermetallic layer formed during aluminizing in pure Al was thick and exhibited a finger-like morphology. While in the Al-Cu alloys, thickness of the intermetallic layer decreased gradually by increasing the Cu content from 2-11%. This decrease in thickness was attributed to the formation of tetragonal phases, Al2Cu and Al7Cu2Fe, on the surface of aluminized steel. The addition of the Cu content beyond 11% did not reduce thickness of the intermetallic layer further; however, the finger-like morphology of the intermetallic layer transformed into smoother one. The thickness of the intermetallic layer increased by increasing the aluminizing temperatures from 675-775 °C in the pure Al and the Al-Cu alloys, while above this range it was found constant. Likewise, it increased rapidly by increasing dipping time up to 4 minutes, after that with increasing time it increased very slowly. Conversely, the variations in aluminizing temperature and dipping time did not influence morphology of the intermetallic layer. The hardness of the coating achieved with the addition of the Cu content in Al was higher than pure Al and Al-Si alloys. However, the oxidation and adhesion properties of the steel aluminized in the Al-Cu alloys were comparable with that aluminized in pure Al and Al-Si alloys.