Formability and Corrosion Behavior of Marine Aluminum-Magnesium Alloys

Research Achievement Share by Assistant Professor Yang-Chun Chiu, Department of Materials and Mineral Resources

Structural shapes of aluminum-magnesium ($\text{Al-Mg}$) alloys are primarily formed through severe cold working. Consequently, when stored at temperatures between $40^\circ\text{C}$ and $50^\circ\text{C}$, these alloy shapes undergo sensitization. With higher magnesium content and prolonged exposure, substantial amounts of the $\beta\text{-Mg}_2\text{Al}_3$ phase precipitate along the grain boundaries, causing severe intergranular corrosion (IGC) and strictly limiting their storage conditions. Thus, enhancing corrosion resistance is a crucial issue for $\text{Al-Mg}$ alloys. Currently, high-temperature process annealing ($300^\circ\text{C}$ to $450^\circ\text{C}$) recrystallizes the $\text{Al-Mg}$ alloy, relieves internal stored energy, stabilizes the alloy, and reduces $\beta\text{-Mg}_2\text{Al}_3$ phase precipitation, thereby significantly improving corrosion resistance. However, during recrystallization, anisotropic grain growth negatively impacts subsequent formability. Therefore, simultaneously improving both corrosion resistance and formability is a vital topic for $\text{Al-Mg}$ alloys.

Through alloy design and process annealing heat treatments, this study investigates the effects of alloying elements on the microstructure, formability, and corrosion properties of $\text{Al-Mg}$ alloys across different annealing temperatures to comprehensively demonstrate their characteristics. Since manganese ($\text{Mn}$), zirconium ($\text{Zr}$), and scandium ($\text{Sc}$) are key alloying elements affecting recrystallization in $\text{Al-Mg}$ alloys, three alloy compositions were subjected to heat treatments at various annealing temperatures to explore the effects of trace $\text{Mn}$, $\text{Sc}$, and $\text{Zr}$ on recrystallized microstructure, mechanical properties, and formability.

During recrystallization, as shown in Figure 1, the $\text{Al}_3\text{Zr}$ and $\text{Al}_3\text{Sc}$ particles in the $\text{Zr}$- and $\text{Sc}$-containing alloys demonstrate superior efficacy over the $\text{Al}_6\text{Mn}$ precipitates in the $\text{Mn}$-containing alloy in pinning (sub)grain boundary migration. This indicates that $\text{Zr}$- and $\text{Sc}$-containing alloys possess a stronger capacity to retard recrystallization compared to $\text{Mn}$-containing alloys. At a high temperature of $450^\circ\text{C}$, the grain growth inhibition effect of the $\text{Sc}$-containing alloy surpasses that of the $\text{Zr}$-containing alloy, revealing that $\text{Al}_3\text{Sc}$ particles exhibit better thermal stability than $\text{Al}_3\text{Zr}$. Furthermore, during the grain growth stage, the recrystallized grains of the $\text{Zr}$-containing alloy display isotropic grain growth, the $\text{Mn}$-containing alloy displays anisotropic grain growth, while the $\text{Sc}$-containing alloy shows no significant grain growth.

Based on the degree of recrystallization, microstructural analysis, and Nitric Acid Mass Loss Test (NAMLT) results, the following conclusions were reached: during the recovery stage of recrystallization in the $\text{Zr}$-containing alloy, the $\beta\text{-Mg}_2\text{Al}_3$ phase precipitates along subgrain boundaries (Figure 2), which mitigates intergranular corrosion. In contrast, during the early stage of recrystallization, continuous precipitation of the $\beta\text{-Mg}_2\text{Al}_3$ phase along grain boundaries leads to severe intergranular corrosion. For the $\text{Sc}$-containing alloy, the lack of a distinct grain growth stage results in continuous precipitation of the $\beta\text{-Mg}_2\text{Al}_3$ phase along grain boundaries, causing intergranular corrosion and a significant decline in corrosion resistance. By comparison, the $\text{Mn}$- and $\text{Zr}$-containing alloys exhibit notable grain growth. Following grain growth, magnesium atoms aggregate, causing the $\beta\text{-Mg}_2\text{Al}_3$ phase to precipitate discontinuously along grain boundaries. This alters the corrosion morphology to localized pitting rather than intergranular corrosion, thereby enhancing the alloy's overall corrosion resistance.