Microstructural Refinement and α‐Dispersoid Evolution in Direct‐Chill Cast Al–Mg–Si–Fe Alloy

Abstract

High Fe levels, coarse grains, microstructural inhomogeneities, and coarse dispersoids for recrystallization suppression remain problems for industrial application of Al–Mg–Si (6xxx) alloys. Herein, Al–0.81%Mg–0.81%Si–0.7%Fe alloy billets are fabricated by direct chill (DC) casting at different casting speeds. The results indicate that high‐speed (300 mm min−1) DC casting can provide fine grains, refined Fe‐containing phases, relatively high solid–solute content, and improved distribution of Mn and Cr. After homogenization at 500 °C for 6 h, the α‐AlFeMnCrSi dispersoids in the high‐speed cast billet decrease in size and increase in number density, compared with the conventional cast billet (100 mm min−1). The low homogenization temperature (440 °C) also contributes to the size reduction and increases the number density of dispersoids. Air cooling after homogenization (holding temperature ≥500 °C) results in Q‐AlCuMgSi precipitation, which depends on the precipitated substrate of the α‐AlFeMnCrSi dispersoids. Moreover, the precipitation processes of α‐dispersoids and Q phases are discussed according to transmission electron microscopy (TEM) and high‐angle annular dark‐field scanning transmission election microscope (HAADF‐STEM) results. These results mean that high‐speed DC casting can effectively improve the as‐cast microstructure by increasing the cooling rate, and the as‐cast microstructural evolution promotes the precipitation of fine and dense α‐dispersoids from the α‐Al matrix.