Aluminium

Contaminants in aluminium

Contaminants in liquid aluminium

Aluminium charge can enter the foundry from two completely different sources:
• primary aluminium
• aluminium melted from scrap – recycled (secondary) aluminium.

The contaminants present in liquid metal from these sources vary significantly and can affect the molten metal processing methods used [1]:

Recycled aluminium typically has higher levels of inclusions, hydrogen, calcium and solid oxides that form during high temperature scrap smelting processes.
On the other hand, primary metal is associated with higher levels of sodium, aluminium carbide, and non-metallic inclusions resulting from the addition of large amounts of alloying elements.

Table 1 shows the levels of impurities present in the metal for both categories of aluminium.

Table 1 – Typical impurity levels in metal from smelter and remelt sources [1]*

*PoDFA – Porus disk filtration apparatus.

Contaminations in finished aluminium products

To evaluate the overall molten metal processing productivity currently required in a foundry, Fig. 1 shows the levels of impurities that must be achieved for various aluminium products. Comparison of these levels with the levels of impurities coming with the initial charge (Table 1) makes it possible to determine methods and equipment for removing impurities and contaminants from liquid metal in the manufacture of specific aluminium products.

Fig. 1 – Typical impurity concentrations in some aluminium products [1]

Practical conclusion from Figure 1:

  • In practice, most extruded aluminum products do not require filtering of the aluminium melt.

Inclusions in secondary aluminium

Inclusions in molten aluminium

Inclusions are mainly solid particles suspended in molten aluminium. The number and size of these particles depends on many factors, in particular on the initial quality of the scrap being melted and the impurities contained in this scrap. Inclusions are non-metallic particles, usually less than 100 microns in size. They are composed primarily of oxides, although several other types of compounds are also present.

There are two main classes of inclusions:

  • exogenous
  • indigenous.

Exogenous inclusions

Exogenous inclusions are particles that already exist as a separate phase before melting. The best known examples are small pieces of furnace refractory breaking off into the melt; pieces of oxide or dirt attached to the scrap is another. Exogenous inclusions consist almost entirely of oxide and are much larger than most indigenous inclusions. Because of this, their presence in aluminium is more harmful than indigenous inclusions. However, their larger size makes them easier to remove.

Indigenous inclusions

Indigenous inclusions or in situ inclusions are formed as a result of chemical reactions occurring in the melt. An example is the reaction of dissolved oxygen with molten aluminium to form aluminium oxide Al2O3.

As can be seen from Table 1, inclusions in remelted aluminium scrap differ from inclusions in the primary metal. Scrap by its nature contains more dirt and oxides than the primary metal, and oxide films formed during melting further increase the number of inclusions. As a result, inclusion removal is much more important in the production of recycled aluminium.

The significance of inclusions

Figure 2 shows the significance of different types of inclusions in molten aluminium [2]Ж

  • The shaded area is the unofficial quality limit for the concentration of inclusions of various sizes in cast aluminum.
  • In deformable aluminium (extruded, forged, rolled products), these limits are lower [2].
  • Curve C is for inclusions formed during casting, i.e. oxide films and slag inclusions; exogenous inclusions also fall on this curve.
  • Curve B is for indigenous inclusions formed during melting and processing of the melt.

Figure 2 – Concentrations of different types of inclusions in molten aluminium by par­ticle size [2]

Iron as impuruty element

The most common impurity element

Iron is the most common impurity element in aluminium alloys, sourced from bauxite and steel tools used in both primary and secondary production. Iron usually forms secondary phases in aluminum alloys, such as Al3Fe, α-AlFeSi and β-AlFeSi. This is due to the low equilibrium solubility of iron in solid aluminum (max. 0.05%). These iron-rich phases noticeably deteriorate the mechanical properties of the alloy. Ductility and tensile strength decrease progressively with increasing Fe content.

Iron in wrought aluminium alloys

The production of most aluminum wrought alloys requires tight control of the iron composition. For example [3]:

Iron levels above 0.15% by weight are not acceptable in aerospace alloys such as 7050, 7055 and 7475.
Automotive high performance alloys such as 5474 and 6111 also limit both iron and silicon to 0.40 wt%.

Iron in cast aluminium alloys

The deterioration of the ductile properties of cast aluminium alloys with excessive iron content is associated with the formation of iron-rich needle-shaped intermetallic inclusions, such as those shown in Figure 3.

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Figure 3 – Needles of iron-containing inclusions FeSiAl5 in the AlSi12 alloy (silumin) for casting into multiple molds [4]

The some benefits of iron

  • However, iron is present in most traditional die casting alloys as a beneficial impurity.
  • For high pressure casting alloys (such as e.g. 380, A380, C380, A383, A384, 360, A360), a minimum of 0.8% Fe is desirable to prevent adhesion of the alloy to the steel matrix [3].
  • Standards generally allow significantly higher amounts of Fe to be present in die cast alloys compared to sand cast alloys [3].
  • If ductility requirements are specified for the casting material for high-pressure casting, then the iron content in the alloy is limited to the range from 1 to 1.3% [4].

Sources:

  1. A Technical Perspective on Molten Aluminum Processing / Peter Waite // Essential Readings in Light Metals – Vol. 3 – eds. J. F. Grandfield and D.G. Eskin – 1986
  2. Aluminum Recycling / Mark E. Schlesinger – 2014
  3. 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
  4. Cast alloys and products // Aluminium Automotive Manual – European Aluminium Association, 2002