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Phase transformations during the crystallization of the AK7 alloy, neutralization of the harmful effect of iron by treating the melt with a pulsed electric current

Metalozn. obrobka met., 2020, vol. 26 (96), 17-29

A.G. Prigunova, Doctor of Technical Science, Professor, Head of Department, аdel_nayka@ukr.net

UDC 669.2/.8.017 : 537.3.39
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The method of quenching-microstructural analysis, the so-called "stop-quenching", was used to study phase transformations during crystallization of the AK7 casting alloy of the Al-Si-Mg system. The alloy is smelted from scrap and waste, therefore it contains a large amount of impurities, including iron, copper. It was found that the formation of the structure of the AK7 alloy is carried out in the course of the following reactions: 1. Р → Аlα. 2. P → α-(Fe, Mn) 3Si2Al15 + Alα. 3. P → β-FeSiAl5 + Alα. 4. P + β-FeSiAl5 → α-(Fe, Mn, Cu)3Si2Al15. 5. P → β-FeSiAl5 + Si + Alα. 6. P → β-FeSiAl5 + α-(Fe, Mn, Cu)3Si2Al15 + Si + Alα. At the last stages of crystallization, eutectics are formed, which include phases with magnesium and copper: Mg2Si, π-FeMg3Si6Al8, W-Cu2Mg8Si6Al and  θ-CuAl2. The transformation P → β + Si + Alα is realized both by the mechanism of cooperative growth of the β, Si, and Alα phases, and by the formation of double eutectics: P → Si + Alα and P → β + Alα, which grow simultaneously in the same temperature range from various centers of crystallization. The four-phase eutectic transformation P → β + α + Si + Alα is carried out according to the type of double or triple eutectic reactions: P → Si + Alα; P → α + Alα; P → β + Alα; β + Si + Alα. The main iron-containing phase in the AK7 alloy is the needle-shaped intermetallic compound β-FeSiAl5 - a stress concentrator, which leads to a decrease in mechanical properties. In this work, the neutralization of the harmful effect of iron was carried out by treating the melt with a unipolar pulsed electric current, under the influence of which the cluster structure of the melt changes. During crystallization, instead of β, a branched phase α is formed. The ratio of intermetallic compounds β and α depends on the processing modes. With optimal parameters of density and current frequency, at which the volume fraction of the branched phase α is the largest, the strength of the AK7 alloy increases by 32.1%, the hardness by 16.7%, and the relative elongation by 2.5 times, bringing its mechanical properties closer to the primary alloy AK7h.
Keywords: aluminum-silicon alloys, phase transformations, iron-containing intermetallic compounds, growth forms, treatment of the melt with an electric current.


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