Evolution of Fe-based intermetallic phases during homogenization of Al–Fe hypoeutectic alloy

Abstract

Homogenization heat treatment is the first step in the processing of aluminium alloys, which for most alloys is carried out before any deformation process. Homogenization is performed to counteract microsegregation, which is the result of non-equilibrium solidification caused by diffusion rate differences between solid and liquid states during solidification. Special attention must also be paid to the impact of homogenization on changes in the morphology of iron-based constituents which are known to adversely affect formability. For the purpose of this work, a hypoeutectic Al–Fe alloy containing 1.1 mass/% Fe was homogenized at 600 °C for up to 12 h in an electric furnace. After the homogenization heat treatment, differential scanning calorimetry (DSC) was performed, comparing the homogenized samples with the sample in the as-cast state. The results were compared with thermodynamic calculations using Thermo-Calc software in order to determine the differences between the microstructure in the equilibrium and non-equilibrium state. All samples were analysed by optical and electron microscopy, and the intermetallic phases were qualitatively evaluated by both methods. In addition, energy-dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) were required to determine the type of phases present in all experimental samples and a difference between the as-cast and homogenized state was found. The presence of metastable phases in the as-cast state and the transformation of such phases during the homogenization process was confirmed. A homogenization process, which lasted 12 h at 600 °C, was also performed with a DSC instrument. The experiment confirmed a greater heat transfer at the beginning of the homogenization process, which is consistent with all previous analyses. All results showed that after 10 h of homogenization the effects of non-equilibrium solidification in this alloy were sufficiently counteracted.