Influence of high-purity and commercial-purity Al with Fe and Mn additions on the age-hardening of Al-Mg-Si alloys

The effects of high-purity (99.99Base) and commercial-purity (99.7Base) Al, along with Fe and Mn additions, on the age-hardening behavior of Al-Mg-Si alloys were investigated using micro-Vickers hardness testing, optical microscopy, transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDS). Four alloys were examined: a high-purity 99.99-base alloy, a commercial-purity 99.7-base alloy, and 99.7-base alloys with individual Fe and Mn additions. The as-quenched hardness followed the Hall-Petch relationship, confirming that grain boundary strengthening contributes to the initial hardness of the alloys. The 99.99-base alloy exhibited the highest peak hardness (~94 HV) and age-hardening ability (ΔH ≈ 61 HV), attributed to a clean matrix that maximizes solute availability for β″ precipitation. In contrast, the 99.7-based alloys showed reduced hardening due to the formation of intermetallic compounds (IMCs) and dispersoids. Although a general correlation between precipitate number density and age-hardening was observed, the 99.7Mn alloy showed a significant deviation, exhibiting low hardness (ΔH ≈ 37 HV) despite a comparable precipitate density (~580 μm⁻²). Detailed analysis showed that this behavior originates from a multi-scale solute depletion mechanism. Coarse IMCs and fine α-Al(Fe,Mn)Si dispersoids consume significant Si, while dispersoid-induced precipitate-free zones (LPZs) locally suppress β″ formation. The LPZ width increases with dispersoid size, leading to reduced effective precipitation strengthening. These results demonstrated that age-hardening is governed not only by precipitate density but also by solute availability and spatial distribution, providing new insight into optimizing recycled Al-Mg-Si alloys.