Contamination in aluminum: quality criteria

Purity level of primary and secondary aluminum

Contaminants in liquid aluminum

Aluminum charge can enter the foundry from two completely different sources:
• primary aluminum
• aluminum, melted down from scrap (scrap) – secondary aluminum.

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

Recycled aluminum usually has a higher level of inclusions, hydrogen, calcium and hard oxides, which are formed during high-temperature scrap melting processes.
On the other hand, primary metal is associated with higher levels of sodium inclusions, aluminum carbide, as well as non-metallic inclusions, resulting from the addition of large amounts of alloying elements.

Table 1 impurity levels are given, present in the metal, for both categories of aluminum.

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

*PodDFA – Porus disk filtration apparatus.

The level of contamination in finished aluminum products

To evaluate the overall performance of molten metal processing, which is currently required in the foundry, on rice. 1 impurity levels shown, to be achieved for various aluminum products. Comparing these levels with impurity levels, coming with the initial charge (Table. 1), makes it possible to determine the methods and equipment for removing impurities and contaminants from liquid metal in the manufacture of specific aluminum products.

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

From drawing 1, in particular, it can be concluded, that in practice most extruded aluminum products do not require filtration of the aluminum melt.

Secondary foundry alloys

Inclusions in secondary aluminum

Inclusions are mostly solid particles, suspended in molten aluminum. The number and size of these particles depends on many factors., in particular on the initial quality of the remelted scrap and impurities, contained in this scrap. Inclusions are non-metallic particles, usually smaller than 100 m. They are mostly oxides., although several other types of connections are also presented. There are two main classes of inclusions: exogenous and indigenous.

Exogenous inclusions are particles, already existing as a separate phase before melting. The best known examples are small pieces of furnace refractory, broken off into the melt; pieces of oxide or dirt, attached to scrap, – this is different. Exogenous inclusions consist almost entirely of oxide and are much larger than most indigenous included. Because of this, their presence in aluminum is more harmful., than indigenous included. At the same time, their larger size makes them easier to remove.

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

As can be seen from the table 1, inclusions in remelted aluminum scrap differ from inclusions in primary metal. Scrap inherently contains more dirt and oxides, than the primary metal, and oxide films, formed during melting, further increase the number of inclusions. As a result, the removal of inclusions is of much greater importance in the production of recycled aluminum..

The figure 2 shows the importance of different types of inclusions in molten aluminum (Altenpohl, 1998) [2]. The shaded area is an unofficial quality limit for the concentration of inclusions of various sizes in cast aluminum.. In wrought aluminum (extruded, kovanoy, rolled products) these limits are lower [2]. Curve C is for inclusions, formed during casting, that is, 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 aluminum by particle size [2]

Iron

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, 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 markedly degrade the mechanical properties of the alloy.. Ductility and tensile strength decrease progressively with increasing Fe content.

Wrought Aluminum Alloys

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

Higher iron levels 0,15% by weight unacceptable in aerospace alloys, such as 7050, 7055 and 7475.
High Performance Automotive Alloys, such as 5474 and 6111, also limit the content of both iron, and silicon up 0,40 wt.%.

Casting aluminum alloys

The deterioration of the ductile properties of cast aluminum alloys with an excessive iron content in them is associated with the formation of iron-rich needle-shaped intermetallic inclusions, such as are shown in the figure 3.

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

Nonetheless, iron is present in most traditional die casting alloys as a useful impurity. For high pressure die casting alloys (such as, for example, 380, A380, C380, A383, A384, 360, A360), it is desirable to have at least 0,8% Fe, to prevent sticking of the alloy to the steel matrix [3].
Standards generally allow significantly more Fe to be present in die-cast alloys than alloys, sandcast [3].
If the casting material for high pressure casting has requirements for viscosity, then the iron content in the alloy is limited to the interval from 1 to 1,3 % [4].

Sources:

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
Aluminum Recycling / Mark E. Schlesinger – 2014
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
Cast alloys and products // Aluminium Automotive Manual – European Aluminium Association, 2002