Iron: impurity and alloying element
Iron is the main impurity, and most dangerous in most industrial casting aluminum alloys and deformed. Therefore, the iron content in aluminum alloys severely limit, in some alloys - up 0,05 %.
On the other hand, Iron is a major component in the material based on aluminum, which make use of new technologies. for instance, in amorphous and nanocrystalline alloys is the content of iron is greater than 10 %. These materials have strength to record level 1500 MPa.
Sources of impurities in aluminum
Regular aluminum and its alloys inevitably contain impurities, ie chemical elements, are not specifically introduced into the alloy composition. impurities, including iron, have different origins. They can get out of the ores, may include in the metal in the electrolysis process and is not always completely removed during the production and refining of primary aluminum. Impurities can occur during melting and casting due to contamination of charge, interaction with the metal lining and fluxes, and also because dissolution of elements of the casting equipment and the casting tool. Moreover, large amount of impurities may enter during remelting of aluminum waste.
Classification impurities in aluminum
Impurities in metals, including aluminum, can be roughly divided into:
- soluble and insoluble, and
- metallic and non-metallic.
Non-metallic impurities in the aluminum
The main non-metallic impurities in aluminum are oxygen and hydrogen. Oxygen has a low solubility in the liquid and solid aluminum, hydrogen and - a high solubility in liquid aluminum and extremely low solubility in solid aluminum. Oxygen forms oxide. Hydrogen, dissolved in liquid aluminum, released during its solidification leads to the formation of shrinkage porosity.
In practice, the harmful effect of gaseous impurities is suppressed by purifying the melt from hydrogen (usually by blowing with inert gases or chlorine) and by filtering it in order to remove oxide particles..
Metallic impurities in aluminum
Metallic impurities are also classified as impurities with low and high solubility in the aluminum solid solution. Their solubility in liquid aluminum is usually very high.
Impurities with a high solubility in the solid state generally have on the mechanical properties of a small effect, but greatly reduced electrical resistance and can affect the processes of recrystallization and aging by thermal treatment.
Insoluble impurities in aluminum
The negative effects of impurities with low solubility due to the formation of phases and / or eutectic with lowest melting point. phase particles, not soluble at homogenization annealing, generally have low ductility and are often elongated shape. These particles significantly reduce the technological plasticity, elongation and strength aluminum alloy. Moreover, you must take into account the possibility of connecting the main alloying elements in the insoluble phase. These phases may result in reduced, for example, the effect of age hardening.
Insoluble phase often have electrode potential, very different from the potential of the aluminum matrix, which reduces the corrosion resistance of the alloy. On the other hand, low-solubility metals have almost no influence on the electrical conductivity, which makes it possible to use them as alloying additives in electrical alloys: aluminum grades and aluminum alloys.
Primary and secondary impurity in aluminum
Impurities are separated as the primary, who were already in the primary metal, and secondary, which have been introduced or formed in subsequent stages of treatment of the alloy. Iron, like silicon, It belongs to a group of impurities, which have the highest content of the alloy.
Reducing the content of impurities in aluminum
The main methods for reducing the harmful effect of impurities are various technological methods:
- purification (refining) of the melt from impurities by keeping the alloy at a certain temperature (burning off impurities);
- vacuum treatment for volatile impurities;
- purging with inert gas or chlorine;
- processing with fluxes;
- electrolytic refining;
- zonnoe rafinirovanie.
Reducing the harmful effects of impurities in aluminum
However, the process operations are not always capable to reduce the impurity content to an acceptable level, and in some cases they are simply not cost effective. for instance, aluminum value after refining zone can be several times more, than the primary electrolytic aluminum. An alternative route is to control the impurities by additional doping and thermal treatment. Special doping changes the phase composition, properties and morphology excess phases, and the heat treatment may lead to dissolution, fragmentation and coagulation of particles with unfavorable morphology.
Iron as an impurity in aluminum
Iron belongs to the low solubility of metal impurities in aluminum. The concentration of iron in aluminum as a function of its purity aluminum may vary from hundredths to tenths of a percent. The main reason iron influence on the properties of aluminum alloys is in the phases, that it forms with other impurities, including silicon, as well as the basic alloying elements.
Iron aluminum as an alloying element
Iron, like silicon, It is the main impurity in aluminum, both in its amount, and volume problems, associated with it. At the same time, we know a lot of materials, in which iron is a necessary and a major alloying element. Among such materials - alloys, which are obtained by rapid solidification and mechanical alloying; composite materials, as well as some of the wrought and cast alloys, heat-resistant, electrically conductive and corrosion-resistant.
iron eutectic with aluminum
Iron is a transition metal, which forms a eutectic with aluminum and has a very low solubility in solid aluminum. Iron, as opposed to transition metals such as manganese, chromium, zircon, titanium and scandium, It has no tendency to form a solid solution. Moreover, a solid solution is not formed not only at industrial cooling rates (up to 1000 K / s), but even with the rapid solidification of the order of speeds 1000000 K / s. Moreover, phase, which forms iron, have the lattice parameters and structures, not such as of aluminum. Therefore, iron is not used as a grain refiner or a recrystallization retarder aluminum alloys.
The mechanism of iron-aluminum alloying
However, iron, as an unavoidable impurity in aluminum, very strongly influences the structure as cast, and the deformed metal. Therefore, use of iron as an alloying element in some alloys and materials related, usually, with special properties iron phases or special technologies, which are used in the production of such materials (melt quenching or mechanical alloying).
The phase diagram of aluminum-iron
For analysis of aluminum alloys, in which iron is an impurity or doping element, an indispensable tool is the state diagram (or phase diagram) of iron-aluminum. Figure - "Aluminum" region of the phase diagram of aluminum-iron
Alloys of aluminum rich eutectic characterized by reacting aluminum solid solution phase and Al3Fe:
L –> Al + Al3Fe.
Eutectic reaction takes place at 652-655 ° C when the iron concentration in the eutectic point 1,8 %.
Al phase3Fe (40,7 % Fe) occupies a vast homogeneous region from 37,3 to 40,7 %. Al phase5Fe2 It has an iron concentration 27,5-29 % Fe.
The solubility of iron in aluminum
The solubility of iron in aluminum is very insignificant:
- 0,052 % at 655 ºS;
- 0,043 % at 625 ºS;
- 0,034 % at 600 ºS;
- 0,021 % at 500 ºS;
- 0,005 % at 450 oC.
Source: Belov at al, Iron in Aluminium Alloys: Impurity and Alloying Element, 2002