Key Takeaways & Executive Findings
- •• Increasing eutectic content thickens intergranular liquid films, which accommodate strain during solidification and reduce hot tearing susceptibility. • Prolonged solution treatment at 475 °C reduces residual eutectic fraction from 9.1% (10 h) to 0.35% (40 h) in Al-7Zn-3.5Mg-1.18Cu alloy. • Al-6Zn-3.0Mg-1.0Cu alloy achieves optimal mechanical properties after aging at 165 °C: tensile strength of 510 MPa and elongation of 6.4%. • Excess alloying elements beyond solubility limits form coarse residual intergranular phases, severely degrading mechanical properties; optimal addition levels are crucial.
Abstract
The 7xxx series aluminum alloys have emerged as a particularly promising class of lightweight structural materials. However, the inherent strength of these materials is primarily influenced by the content and type of alloying elements added during the manufacturing process, as well as casting defects. The present study investigated the effects of eutectics formed by solute atoms (Zn, Mg, and Cu), with equal mass ratios (Zn/Mg=2, Mg/Cu=3) but varying overall contents, on the liquid film thickness, crack propagation depth, and the mechanical properties of the Al-Zn-Mg-Cu alloy after heat treatment. The results from gravity casting indicate that the intergranular liquid film thickness increases with the increase of eutectic content. A thick intergranular liquid film in the casting can accommodate greater strain during grain contraction, thereby preventing liquid film rupture and subsequent hot tearing. Concurrently, during the solution treatment at 475 °C, the residual eutectic fraction in the Al-7Zn-3.5Mg-1.18Cu alloy diminishes from 9.1% at 10 h to 0.35% at 40 h. At 165 °C, the Al-6Zn-3.0Mg-1.0Cu alloy exhibits the optimal mechanical properties, with a peak aging tensile strength of 510 MPa and an elongation of 6.4%. The incorporation of lower concentrations of solute atoms (Zn, Mg, and Cu) serves to reduce the barrier to dislocation precipitation, thereby enhancing alloy plasticity. However, when the proportion of alloying elements exceeds the solubility limit of the α-Al matrix at specific heat treatment temperatures, coarse residual phases remain intergranular, thereby significantly impairing the mechanical properties of the alloy. This study provides a reference for the optimal addition level of the main strengthening elements in Al‑Zn‑Mg‑Cu alloys.
1. Introduction
Aluminum alloys are of particular interest due to their low density, high specific strength, and excellent corrosion resistance, which make them highly promising lightweight structural materials in aerospace, rail transportation, and automotive lightweighting applications [1-4]. Nevertheless, this series of alloys generally exhibits poor castability, and their mechanical properties mainly rely on the synergistic strengthening effect of major alloying elements such as Zn, Mg, and Cu. Nano-sized second phases can be precipitated from Zn and Mg after solution treatment and aging, thereby achieving a remarkable age-hardening effect [5-7]. Therefore, balancing the relationship between castability and mechanical properties has become a research hotspot in this field.
The ratio of primary alloying elements, such as Zn, Mg, and Cu, along with their solid solution behavior in the aluminum matrix, is a key factor determining the type, size, volume fraction, and spatial distribution of precipitation phases. These factors directly influence the overall properties of alloys [8-10]. Of particular significance is the equilibrium solubility limits of these elements in the Al matrix, which are pivotal to achieving effective solid solution strengthening. However, during the process of solidification, the presence of non-equilibrium solidification leads to the accumulation of solute atoms (e.g., Zn, Mg, Cu) at the solid-liquid interface. This results in the formation of a eutectic microstructure at the end of the solidification process. Consequently, these solute atoms are unable to achieve maximum solubility in the Al matrix. Therefore, a subsequent solution treatment is generally required to maximize the solubility of solute atoms (Zn, Mg, Cu) in the Al matrix and enhance the strength of alloys [11]. However, when elemental content exceeds this limit, excess elements tend to form coarse, insoluble secondary phases, leading to a sharp deterioration in mechanical properties. In the case of 7xxx alloys, the incorporation of alloying elements (e.g., Zr, Ti, Ce) and nanoparticles has been demonstrated to enhance mechanical properties by refining grain size and forming secondary phases [12-14]. As reported by Akuata et al. [15], the addition of Ag resulted in the formation of Ag-rich AlAgZnMgCu phases; however, it did not enhance the properties of the 7xxx alloys with a Zn/Mg ratio of 4. Nevertheless, given the streng...
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Peng-fei Li, Yu-dong Sui, Hai-ni Jin, An-kang Xiong, Wan-zeng Li, Hao Zhou, Ye-hua Jiang (2026). Effect of eutectic content on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys. China Foundry. https://doi.org/10.1007/s41230-026-5156-8
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Frequently Asked Questions
What is the effect of eutectic content on hot tearing in Al-Zn-Mg-Cu alloys?
Higher eutectic content increases the intergranular liquid film thickness, which can accommodate greater strain during grain contraction, thereby preventing liquid film rupture and reducing hot tearing susceptibility.
How does solution treatment duration affect residual eutectic fraction?
In Al-7Zn-3.5Mg-1.18Cu alloy, increasing solution treatment time at 475 °C from 10 h to 40 h reduces the residual eutectic fraction from 9.1% to 0.35%, indicating improved dissolution of eutectic phases.
Which alloy composition and aging condition yield optimal mechanical properties?
The Al-6Zn-3.0Mg-1.0Cu alloy aged at 165 °C exhibits the best combination of tensile strength (510 MPa) and elongation (6.4%), attributed to lower solute concentrations that reduce barriers to dislocation precipitation.
Why do excessive alloying elements degrade mechanical properties?
When alloying element content exceeds the solubility limit of the α-Al matrix at heat treatment temperatures, coarse residual phases remain at grain boundaries, which significantly impair the mechanical properties of the alloy.
What is the practical significance of this study?
The study provides a reference for determining the optimal addition levels of main strengthening elements (Zn, Mg, Cu) in Al-Zn-Mg-Cu alloys to balance castability and mechanical performance.
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