Iron in cast aluminum

Sources of iron in aluminum

Iron is the most common impurity element in aluminum alloys., sourced from bauxite and steel tools, used as in the primary, as well as secondary production. Iron usually forms secondary phases in aluminum alloys due to its low equilibrium solubility in solid aluminum (max.. 0,05%). These phases are mainly Al3Fe, α-AlFeSi and β-AlFeSi. These iron-rich phases have a marked negative effect on the mechanical properties of the alloy.. Ductility and tensile strength decrease progressively with increasing Fe content. After exceeding a certain critical Fe content, the ductility decreases very sharply..

Nonetheless, iron is present in most traditional die casting alloys as an impurity, but very useful impurities. Minimum 0,8% Fe is useful for some high pressure casting alloys to prevent sticking to the steel mold [1].

β-phase and α-phase

Al-Fe-Si are the main high iron phases in aluminum alloys. The most important high iron phases in aluminum alloys, containing silicon, are β-phase and α-phase.

α-phase is usually identified as α-AlFeSi. This α-phase has a compact morphology, such as “chinese writing”, star-shaped and polygonal (Fig.. 1 and rice. 2). It is believed, that for the mechanical properties of aluminum alloys, the α-phase is much less harmful, than the lamellar phase β-Al-FeSi [1].

The lamellar β-phase is usually identified as β-AlFeSi. Among all phases, rich in iron, the β-AlFeSi phase is considered the most harmful. The β-AlFeSi phase has an undesirable lamellar morphology, as shown in pic. 1 and rice. 3. She is a fragile phase, stress concentrator and point of weak coherence [1]. Generally, higher iron content and slower cooling rate results in an increase in the particle size of the β-phase. The dominance of lamellar β-phases leads to a serious loss of strength and ductility in cast Al-Si alloys.

Fig. 1 Typical morphology of a-phase and β-phase in aluminum [1]

Fig. 2 Three-dimensional reconstruction of a-phase:
(a) original two-dimensional photo;
(b) three-dimensional a-phase with high convoluted arms observed

Fig. 3 Three-dimensional reconstruction of β-phase,
(a) original two-dimensional phases; (b) three-dimensional β-phase

Effect of iron on mechanical properties

In some alloys, iron is added intentionally.. For example [1]:

Iron is usually added to Al-Cu-Ni alloys to increase heat resistance..
Iron is added to Al-Fe-Ni alloys to reduce corrosion in high-temperature steam.
Iron is added to aluminum conductors to increase strength without significant loss of conductivity..
Industry standards generally allow significantly higher amounts of Fe in die casting and die casting alloys than alloys., which are cast in sand molds. This is because, that the cooling rate under these casting conditions is higher and, respectively, therefore the size of the microstructural structural components is smaller.
In alloys for industrial injection molding, the Fe content exceeds 0,8 wt.%, and the liberated Al-Si-Fe eutectic composition prevents molten alloys from sticking to the steel mold. In these cases, Fe is the alloying element.

The mechanical properties of cast aluminum alloy are generally degraded by the presence of iron.. Three-dimensional morphological designs of iron-rich intermetallic compounds show, that they have a much more complex and fragile morphology, than that, what can be seen in two-dimensional observation. This morphology explains, why they are so detrimental to the mechanical properties of aluminum [1].

aluminum foundry

3-D morphology

The mechanical properties of cast aluminum alloy generally deteriorate in the presence of iron.. Three-dimensional morphological designs of iron-rich intermetallic compounds show, that they have a much more complex and fragile morphology, than that, what can be seen in two-dimensional observation (Fig.. 4) [1]. This morphology explains, why they are so detrimental to the mechanical properties of aluminum [1].

Fig. 4 – Three-dimensional morphologies of Fe-rich intermetallic phase:
(Chinese script) morphologies of β-Al (Fe, Mn)3Si[1]

Iron is the main impurity, which is responsible for the low level of toughness of conventional aluminum alloys.
iron-containing phase, which is formed from a liquid in high-silicon aluminum alloys is β-FeSiAl₅.
β-FeSiAl particles are commonly referred to as acicular or acicular (Fig. 5), although in fact they are plates.
Additives in manganese alloy in quantity, equal to half the iron content, change the β-FeSiAl phase to the α-FeSiAl phase with the chemical formula (Fe, Mn)₃Si₂Al₁₅. This phase is no longer needle-shaped, but something resembles a written font. She is not so harmful, like a needle, although it still remains an embrittling phase (Fig. 6).

Fig. 5– ß-AlFeSi needles [2]

Fig. 6 – Alpha-Fe Script Phase (Fe,Mn)3Si2Al15 in 357 alloy.
Less harmful than ß-AlFeSi needles but still embrittling [2]

Cooling rate and size of β-FeSiAl needles

Β-FeSiAl needle length is a function of cooling rate. A measure of the cooling rate is the interdendritic distance in the alloy structure.. Higher cooling rate, the smaller the interdendritic distance.
With an increase in the iron content in the alloy, the length of the β-FeSiAl needles also increases (Fig. 6).

Fig. 6 – ß-AlFeSi needle length as function of secondary dendrite arm spacing
(Source: Biswal et al) [2]

Β-FeSiAl needles and mechanical properties

The presence of β-FeSiAl needles in the microstructure of aluminum alloys of aluminum reduces their mechanical properties.
Most β-FeSiAl needles reduce the viscous properties of aluminum alloys, especially the toughness of secondary alloys with a high iron content (Fig.. 7).

Fig. 7 – Comparison of low vs. high Fe 357 alloys (0.093% Fe vs. 0.055% Fe)
(Source: F. Major, Alkane) [2]

Sources:

Iron: Removal from Aluminum / L. Zhang, J. Gao, L. N. W. Damoah and D. G. Robertson // Encyclopedia of Aluminum and Its Alloys – Eds. G.E. Totten, M. Tiryakioglu, O. Kessler – 2019
European Aluminium Association, 2002