Aluminium casting

Iron in cast aluminium

 

Sources of iron in aluminum

Iron is the most common impurity element in aluminum alloys, sourced from bauxite and steel tools used in both primary and secondary production. Iron typically 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 noticeable 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, plasticity decreases very sharply.

However, iron is present in most traditional die casting alloys as an impurity, but a very useful impurity. A minimum of 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 content phases in aluminum alloys containing silicon are the β phase and the α phase.

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

The lamellar β phase is usually identified as β-AlFeSi. Among all iron-rich phases, the β-AlFeSi phase is considered the most harmful. The β-AlFeSi phase has an undesirable plate-like morphology, as shown in Fig. 1 and fig. 3. It is a brittle phase, a stress concentrator and a point of weak coherence [1]. Typically, higher iron content and slower cooling rates result in larger β-phase particle sizes. The dominance of lamellar β phases leads to severe loss of strength and ductility in Al-Si cast alloys.


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


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


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

The effects of iron

Iron is added deliberately to some alloys. For example [1]:

  • Iron is usually added to Al-Cu-Ni alloys to improve heat resistance.
  • Iron is added to Al-Fe-Ni alloys to reduce corrosion in high temperature water vapor.
  • Iron is added to aluminum conductors to increase strength without significant loss of conductivity.
  • Industry standards generally allow significantly higher amounts of Fe to be present in permanent and die casting alloys compared to sand cast alloys. This is because the cooling rate under these casting conditions is higher and therefore the size of the microstructured structural components is smaller.
  • In industrial die casting alloys, the Fe content exceeds 0.8 wt.%, and the precipitating eutectic Al-Si-Fe 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 usually 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 what can be seen from two-dimensional observation. This morphology explains why they are so detrimental to the mechanical properties of aluminum [1].

Cast aluminum

3-D morphology

The mechanical properties of cast aluminum alloy are generally degraded 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 what can be seen from 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 that is responsible for the low level of toughness of conventional aluminum alloys.
  • The iron-containing phase that forms from the liquid in high-silicon aluminum alloys is β-FeSiAl5.
  • β-FeSiAl particles are commonly referred to as acicular or acicular (Fig. 5), although they are actually platelets.
  • Additions of manganese to an alloy in an amount 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-like, but somewhat reminiscent of a written font.
  • It is not as harmful as the acicular one, although it still remains in the embrittlement phase (Fig. 6).

zhelezo-v-liteynom-aluminievom-splaveFig. 5– ß-AlFeSi needles [2]

zhelezo-alyuminievyy-splav-a357Fig. 6 – Alpha-Fe Script Phase (Fe,Mn)3Si2Al15 in 357 alloy.
Less harmful than ß-AlFeSi needles but still embrittling [2]

Скорость охлаждения и размеры игл β-FeSiAl

  • Длина игл β-FeSiAl является функцией скорости охлаждения. Мерой скорости охлаждения служит междендритное расстояние в структуре сплава. Чем выше скорость охлаждения, тем меньше междендритное расстояние.
  • С увеличением содержания железа в сплаве длина игл β-FeSiAl также увеличивается (Fig. 6).

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

β-FeSiAl needles

The presence of β-FeSiAl needles in the microstructure of aluminum alloys reduces their mechanical properties.
What β-FeSiAl needles reduce most is the toughness of aluminum alloys, especially the toughness of secondary alloys with 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, Alcan) [2]

Sources:

  1. 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
  2. European Aluminium Association, 2002