National University of Science and Technology “MISiS”, Moscow, Russia:

E. A. Naumova, Principal Engineer, Metal Forming Department, e-mail: jan73@mail.ru
N. A. Belov, Chief Researcher, Metal Forming Department
M. A. Vasina, Graduate Student, Metal Forming Department
V. V. Doroshenko, Engineer, Metal Forming Department

The study is devoted to determining the optimal concentration ranges of highstrength aluminum alloys based on the Al – Zn – Mg – Ca – Fe – Si system. A complex distribution of alloying elements between phases has been established. In particular, the possible number of phases can reach nine, among which the three phases contain calcium: (Al, Zn)₄Ca, Al₁₀CaFe₂, Al₂CaSi₂. By using calculated projection of the liquidus surface (in the Thermo-Calc program) of the system at constant zinc and magnesium content, it was shown that with increasing calcium concentration, the area of primary crystallization of the aluminum solid solution (Al) significantly narrows. This leads to the risk of formation of primary crystals of the intermetallic phases Al₁₀CaFe₂ and Al₂CaSi₂. Using the methods of electron scanning microscopy, as well as electron microprobe analysis, the structures of model Al – 8% Zn – 3% Mg – 0.5% Fe – 0.5% Si alloy with 1% Ca and 2% Ca have been investigated in as-cast state. It has been established that the Al₂CaSi₂ compound has a coarser structure than Al₁₀CaFe₂. The fraction and size of silicon-containing inclusions increases significantly with increasing calcium and silicon content. The analysis of the tendency of alloys to hardening has been studied by constructing dependences of hardness (HV) as a function of aging temperatures in the range from 100 to 250 ^оС. It is found out that the hardness is minimal for alloys with a high calcium content, which is caused by excessive dissolution of zinc in the Al₄Ca phase. At the same time, silicon with a content of up to 0.5 wt% has little effect on the hardening of the alloys. The maximum hardness of alloys during aging exceeds 180 HV, which, according to the results of thermodynamic calculations, is due to the formation of metastable precipitates of the MgZn₂ phase (η'). Taking into account the experimental obtained data, the optimum content of ~1% of calcium, ~0.5% of iron, and ~0.2% of silicon was proposed for the base alloy Al – 8% Zn – 3% Mg. The alloy with such chemical composition allows creating a structure consisting of an aluminum solid solution with a total concentration of Zn and Mg of about 10 wt% and Ca containing eutectic, which also includes Fe and Si. The alloy also showed an acceptable level of manufacturability during casting and metal forming.
This work was financially supported by the Ministry of Education and Science of the Russian Federation within the implementation of the Agreement on subsidies No. 14.578.21.0220 (unique identifier of the project RFMEFI57816X0220).

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