Al–Mg–Si alloys are age-hardenable, heat-treatable materials widely used as 6000 series aluminum alloys, whose mechanical properties are strongly governed by precipitation during aging. When the atomic ratio of Mg to Si is 2:1, a quasi-binary Al–Mg₂Si system is formed, whereas alloys containing excess Si relative to this stoichiometric ratio are classified as excess-Si type alloys. Previous studies have reported that both the peak hardness and the time required to reach peak aging differ significantly between quasi-binary and excess-Si compositions [1].

For future recycling of 6000 series aluminum alloys, contamination by transition elements is unavoidable, depending on the purity of the base aluminum [2]. However, the effects of such transition elements on precipitation behavior and age-hardening characteristics have not yet been fully clarified. In this study, the influence of minor additions of Fe, Mn, and Cu on the age-hardening behavior and microstructure of excess-Si type Al–Mg–Si alloys was systematically investigated.

Four alloys were prepared using 99.99% pure aluminum: an Al–1.0Mg₂Si–0.2Si base alloy and the same alloy with additions of 0.30 wt% Fe, Mn, or Cu. After homogenization at 848 K for 10.8 ks, the ingots were hot rolled and subsequently solution heat-treated at 848 K for 3.6 ks, followed by quenching into ice water. Aging treatments were carried out at 473 K in an oil bath. The age-hardening response was evaluated by Vickers hardness testing. Microstructural observations were conducted using scanning electron microscopy (SEM) in the solution-treated condition and transmission electron microscopy (TEM) at the peak-aged condition.

The Cu-added alloy exhibited a higher peak hardness than the base alloy, indicating a beneficial effect of Cu on age hardening. In contrast, the Fe- and Mn-added alloys showed lower peak hardness compared with the base alloy. SEM observations revealed the presence of constituent particles formed by the added transition elements after solution treatment. TEM and STEM observations at the peak-aged condition were used to characterize the precipitates responsible for strengthening. These results demonstrate that minor additions of transition elements significantly modify the age-hardening response and precipitation behavior of excess-Si type 6000 series aluminum alloys, providing important insights for alloy design and recycling strategies of Al–Mg–Si alloys.

References

[1] K. Matsuda et al.; J. JPN INST LIGHT MET, 1995, 45, 95-100.

[2] M. Isao, J. JPN INST LIGHT MET, Transition elements in alumminum alloys(2) 1974, 270-278.

Yukito Suzuki (B.Eng.)