The low temperature precipitation in commercial-purity aluminium sheets for foils

The solid solubility of elements Fe and Si in aluminium matrix, and the amount of precipitation of intermetallic compounds bearing Fe and Si from the matrix are crucial microstructural parameters, which are found to have marked effects on the formability of aluminium sheets and the mechanical properties of final foil products. The low temperature precipitation in commercial-purity aluminium sheets for foils was investigated by XRD, TEM, optical microscopy, and quantitative metallography. A method was proposed to quantify the precipitation fraction and solid solubility from matrix lattice parameter based on X-ray diffraction analysis. The results showed that the fine rod-like phase, β-FeSiAl5, with average size of about 0.2 μm, precipitates out from the Al matrix during low temperature additional annealing. The precipitation of β-FeSiAl5 was found to lead to an increase in the lattice constant of the Al matrix. Annealing at 210 °C is favourable to the precipitation of the compound containing Si and Fe from the matrix. After 15 h annealing at 210 °C, the solid solubility of Si is reduced by about 50%. Appropriate pre-anneal deformation promotes the precipitation and helps microstructure improvement.