The Role of Manganese and Liquid-Phase Electric Treatment on the Formation of Iron-Containing Phases in Predeutectic Alloys of the Al-Si-−Fe-Mn System
Processy litʹâ, 2020, Tom 142, №4, p.13-25
DOI:
https://doi.org/10.15407/plit2020.04.013Keywords:
aluminum-silicon alloys, iron-containing phases, alloying with manganeseAbstract
Received 05.10.2020
UDK 669.2/. 8.017 : 537.3.39
This work has developed an understanding of the mechanism of the formation of iron-containing phases and their forms of growth in hypoeutectic alloys of the Al−SI−Fe−Mn system, depending on the manganese content and the treatment of the melt with a unipolar pulse electric current. The study was carried out on basic alloys Al−8 % Si−0.9% Fe−0.3% Mn and Al− 8% Si− 0.9% Fe−0.5% Mn, the composition of which was chosen taking into account the concentration boundaries of silicon, iron and manganese in industrial silumins AK7, AK9, AK8M3, AK9M2 in accordance with DSTU 2839-94. Alloys are produced from scrap and waste of non-ferrous metals and are widely used as structural materials in various fields of mechanical engineering. Experimentally, using the methods of quenching microstructural, X-ray structural and local X-ray spectral analysis, it was shown that the mechanism of the influence of manganese on the growth forms of iron-containing phases consists in a change in the nature of phase transformations during crystallization, as a result of which, instead of the needle-shaped intermetallic β-FeSiAl5 with a monoclinic lattice, a branched hexagonal phase, which in the literature is designated as α-(Fe,Mn)3Si2Al15. The ratio of the phases α-(Fe,Mn)3Si2Al15 and β-FeSiAl5 in alloys and the features of their crystallization depend on the concentration of manganese. At 0.3% Mn in the second stage of crystallization, after the separation of the primary crystals of solid aluminum solution from the liquid, there is a eutectic transformation: Р → α-(Fe,Mn)3Si2Al15 + Alα. The formation of the α-(Fe,Mn)3Si2Al15 phase continues at the third and fourth stages following the reactions: Р → α-(Fe,Mn)3Si2Al15 + β-FeSiAl5 + Alα and Р → α-(Fe,Mn)3Si2Al15 + β-FeSiAl5 + Si + Alα. These eutectics can be formed both by the mechanism of cooperative growth of phases α-(Fe,Mn)3Si2Al15, β-FeSiAl5, Alα and α-(Fe,Mn)3Si2Al15, β-FeSiAl5, Si, Alα, and as a result of the formation of binary eutectics β-FeSiAl5+ Alα, α-(Fe,Mn)3Si2Al15 + Alα and Si + Alα, which nucleate and grow simultaneously in the same temperature range. Moreover, the volume fraction of the β-FeSiAl5 phase is very significant. An increase in the concentration of manganese to 0.5% leads to the suppression of the reactions by which the β-phase is formed. X-ray structural studies have established that the intermetallic compound α(Fe,Mn)3Si2Al15 is a solid solution of manganese in the compound α-Fe2SiAl8. It has a variable composition, which is primarily due to the content of manganese and iron in the alloy, and the forms of its growth depend on the amount of Fe + Mn in this phase. With an increase in their total concentration, the morphological features of the phase change in the sequence: finely differentiated eutectic crystals → large eutectic crystals with thickened branches → primary crystals.
Liquid-phase treatment of alloys doped with manganese with a unipolar pulsed electric current leads to significant changes in the microstructure and phase composition, which largely depend on the parameters of the electric current - its density and frequency. Under optimal processing conditions, the β-FeSiAl5 phase, which is the main factor of negative influence on the mechanical properties of aluminum alloys, is absent, even in the alloy with 0.3% Mn. Strengthening the action of manganese by treating the melt with an electric current makes it possible to limit its concentration in hypoeutectic silumins to 0.2-0.3 %, with obtaining a high level of mechanical properties.
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