Iron-containing intermetallic phases in Al-Si based casting alloys

This article is made available through the Brunel Open Access Publishing Fund. Copyright @ 2012 Elsevier B.V.

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Effect of iron on the microstructure and mechanical property of Al–Mg–Si–Mn and Al–Mg–Si diecast alloys

The effects of Mn on the microstructure and mechanical properties of Type 319 aluminum casting alloys have been examined. It is shown that, as the Mn content is increased up to 0.65 wt.pct (corresponding to an Fe/Mn ratio of ∼1.2) in the baseline alloy (Al–7 wt.%Si–3.8 wt.%Cu–0.5 wt.%Fe), the plate-like β intermetallic phase is completely converted to the Chinese script α phase resulting in improved tensile properties. Excess amounts of Mn, however, deteriorate the mechanical properties by increasing the total amount of iron-containing intermetallic phases. The porosity and volume percent of intermetallic phases are correlated with the tensile properties in order to determine the role of Mn on Type 319 casting alloys.

The effects of Mn additions on the microstructure and mechanical properties of Al–Si–Cu casting alloys

The microstructures of Al-11.5Si-0.4Mg alloys with various Fe and Mn contents have been studied to investigate the potential influence of oxide films on the precipitation of Fe-rich phases from the liquid metal. Oxide films are incorporated into melts by an entrainment process. This is an enfolding mechanism of incorporation. Folded oxide films in melts have two sides: the dry unbonded inner surfaces and their wetted exterior surfaces. The wetted outer interfaces of oxide films appear to act as preferred substrates for the nucleation and growth of some Fe-rich phases. It is expected that there will be good atomic contact between the wetted side of the film and Fe-rich phases or aluminum matrix. However, the gap between the two dry sides of a double film constitutes a crack, thus explaining the commonly observed cracks in and around such intermetallics and aluminum matrix. The calculation of planar disregistries between some typical oxides and α-Fe phase (primary or script type) indicated that MgO, MgO · Al2O3, γ-Al2O3, and α-Al2O3, might all be good substrates for the nucleation of α-Fe phase.

The nucleation of Fe-Rich phases on oxide films in Al-11.5Si-0.4Mg cast alloys

The castability and properties of Al—Si based foundry alloys are affected by the inevitable introduction of impurity elements during melting and casting. Iron has consistently emerged as the most common impurity element that generally imparts negative influences on the soundness and mechanical properties of these alloys. It leads to the formation of iron-bearing intermetallics whose type, size, morphology and amount depend on the alloy composition, melt thermal history, cooling rate and post-casting heat treatments. These compounds, particularly their size and morphology, are the main culprits responsible for the significant influence of iron on the castability and mechanical properties. The most common of these compounds are β-Al5FeSi with a needle/platelet morphology, α-Al15Fe3Si2 with a Chinese script or polyhedron morphology and π-Al8Mg3FeSi6 also with a Chinese script morphology. The iron in the α phase can be substituted by Mn or Cr whereas the π phase forms when Mg is present. The β phase (the most...

Influence of iron on castability and properties of aluminium silicon alloys: literature review

The crystallization behavior of iron-containing intermetallic compounds in industrial grade 319 aluminum alloy has been investigated by means of thermal analysis and metallography. In the absence of manganese, the iron compound crystallizes in theβ phase, at all cooling rates ranging from 0.1 °C/s to 20 °C/s under normal casting temperatures (750 °C). However, when the melt is superheated to a high temperature (about 200 to 300 degrees above the liquidus temperature), the iron compound crystallizes in the α phase at high cooling rates. This is due to the fact that γ alumina, which forms at low melt temperatures (≤750 °C), acts as a nucleus for crystallization ofβ phase. When the melt is superheated to high temperature (≥85O °C), the γ alumina transforms to a alumina. This is a poor nucleus for the β-phase crystallization, and as a result, a phase forms. The importance of nucleation and growth undercooling for the crystallization of iron compounds is highlighted. In the presence of manganese, the iron compound crystallizes in a phase at low cooling rates and in both the α andβ phases at high cooling rates. This reverse crystallization behavior is explained in terms of phase diagram relationships.

Crystallization behavior of iron-containing intermetallic compounds in 319 aluminum alloy

Conventional heat treatment techniques in Al-Si alloys to achieve optimum mechanical properties are limited to precipitation strengthening processes due to the presence of second-phase particles and spheroidization of silicon particles. The iron intermetallic compounds present in the microstructure of these alloys are reported to be stable, and they do not dissolve during conventional (equilibrium) heat treatments. The dissolution behavior of iron intermetallics on nonequilibrium heat treatment has been investigated by means of microstructure and mechanical property studies. The dissolution of iron intermetallics improves with increasing solution temperature. The addition of manganese to the alloy hinders the dissolution of iron intermetallics. Nonequilibrium heat treatment increases the strength properties of high iron alloys until a critical solution temperature is exceeded. Above this temperature, a large amount of liquid phase is formed as a result of interdendritic and grain boundary melting. The optimum solution treatment temperature for Al-6Si-3.5Cu-0.3Mg-lFe alloys is found to be between 515 °C and 520 °C.

L. Anantha Narayanan

Papers

Crystallization behavior of iron-containing intermetallic compounds in 319 aluminum alloy

Dissolution of iron intermetallics in Al-Si Alloys through nonequilibrium heat treatment