Iron needles

Intermetallic phase (compound) of the type Al5FeSi (also known as β-Al5FeSi or β-AlFeSi) occurring in Al-Si alloys (see Aluminum casting) and containing ferrous impurities.

Three-dimensionally, this phase has a flake-like structure which, however, is usually ground in transverse direction in the metallographic specimen, therefore appearing needle-like (Fig. 1). Thus, iron needles are always understood to be the flake-like β-Al5FeSi phases.

Iron is contained in many aluminum alloys as an impurity. As early as in 1926, D. Stockdale and I. Wilkinson found that the ferrous phases forming in the Al-Si alloy during solidification have a highly negative impact on mechanical properties. Today, we know that the flake-like β-Al5FeSi phases have a much more disadvantageous effect on properties than the usually accompanying α-Al15(Fe,Mn)3Si2 phase (often also referred to as α-AlFeMnSi phase, see Fig. 2) which exhibits a script-like morphology. Especially at high Fe levels, iron needles are present as a structural anomaly. However, stray and small needles are indeed typical in the structure of die casting alloys containing iron (Fig. 2).

The formation of the ferrous phases and their morphology are mainly dependent on the chemical composition and solidification kinetics (see Solidification rate). It is known that the addition of manganese promotes formation of ferrous phases in the shape of script-like or polyhedral structures which is particularly influenced by the correct Fe/Mn ratio (see also comparison between Fig. 4a with 1% Fe and Fig. 4b with 1% Fe + 0.5% Mn).

Since classic die casting alloys have a rather high iron content (to reduce the soldering tendency) and solidification occurs relatively fast, the formation of iron needles cannot be prevented. Generally, from Fe levels of 0.9%, increasing amounts of iron needles are present in the structure, having a negative effect especially on the elongation at failure. The reason for this is the formation of a ternary Al-Si-Al5FeSi eutectic instead of the otherwise binary Al-Si eutectic. Large needles preferably form as a proeutectic, while smaller needles are rather present as a co-eutectic. Fig. 3 shows the phases forming during solidification of AlSiCuFe alloys, the time and temperature of their formation and their morphology. At long holding times and insufficient holding temperature, the Fe can set in polygonal AlFeMn phases called gravitational segregations (see Segregation factor).

The fact that the β-Al5FeSi phases are three-dimensionally present as a cross-linked flake structure has been described by C. M. Dinnis et al. and recorded by removing layers from metallographic specimens and then performing a 3-D reconstruction of the sectional images (Figs. 4 to 6).

In sand and gravity die casting alloys (see Aluminum gravity die casting alloys), the iron content is usually very limited so that they hardly contain any ferrous phases. For use in pressure die casting, a number of low-iron alloys have been developed (e.g. the Al Si9MnMg die casting alloy) in which the Mn level has been increased in particular. From a level of 0.6% Mn, the soldering tendency also decreases; due to the lack of iron needles in the structure, it is then also possible to achieve very high elongations at failure and good ductility for pressure die castings.

Additional references:
Intermetallic phase
Hard spots
Segregation factor
Gravitational segregation
Segregation factor
Segregation
Crystal segregation
Micro segregation

Literature references:
Stockdale D., Wilkinson I., Properties of modified aluminum-silicon alloys, J. Inst. Met., 1926.
Dinnis C. M., Taylor J. A., Dahle A. K., As-cast morphology of iron-intermetallics in Al-Si foundry alloys, Scripta Materialia 53 (2005) 955-958.
Shabestari S. G., The effect of iron and manganese on the formation of intermetallic compounds in aluminum-silicon alloys, Mat. Science & Eng. A 383 (2004) 289-298.

  • Fig. 1: Massive structural damage due to Al5FeSi iron needles in the structure of an AlSi7Mg alloy containing 1.5% iron, source: H. Rockenschaub, FT&E
  • Fig. 2: Structure of an Al Si9Cu3(Fe) die casting alloy with the phases typically present (500:1, etched with Keller-Wilcox), according to H. Rockenschaub, FT&E, based on the original image in Metallographic Atlas of Cast Aluminium Alloys β-Al5FeSi – needle-shaped α-Al15(Fe,Mn)3Si2 – Chinese script-like Al2Cu – coil-like Si – precipitated as eutectic Si in the (Al+Si) eutectic, granular Bright zones – α-Al solid solution
  • Fig. 3: Phases in hypoeutectic AlSiCuFe systems, their time and temperature of formation and morphology, according to H. Rockenschaub, FT&E
  • Fig. 4: Structural sections for reconstruction of the 3-dimensional structure of ferrous phases by means of sequential grinding according to C. M. Dinnis et al. a) AlSi9, 1% Fe, section for reconstructing the β-Al5FeSi iron needles b) AlSi9, 1% Fe + 0.5% Mn, section for reconstructing the α-Al15(Fe,Mn)3Si2 phase
  • Fig. 5: 3-dimensional reconstruction of the structural section in Fig. 4a, α-Al5FeSi flake network in all 3 spatial directions (C. M. Dinnis et al.)
  • Fig. 6: 3-dimensional reconstruction of the structural section in Fig. 4b, α-Al15(Fe,Mn)3Si2 coil in all 3 spatial directions(C. M. Dinnis et al.)
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