Aluminium is produced from bauxite ore via Bayer process, which includes number of selective leaching sequences. In this process, aluminium ore is dissolved in leach solution while the impurity elements do not. Aluminium produced from bauxite ore is referred as primary aluminium.
Although high purity aluminium is produced in this process, energy consumed is ten times higher than recycling process. Due to the growing demand in aluminium in last decades and environmental concerns, recycling of aluminium is in interest so that in 2010 approximately one third of the total aluminium production is produced from scrap.
Aluminium produced from scrap is called as secondary aluminium.Although the production of secondary aluminium is way cheaper than the production of primary aluminium, secondary aluminium production cannot reach high purity levels as in primary aluminium production, which can reach the purity level of 99.9 wt% Al. This is because most of the aluminium scrap is contaminated by other metals like steel.
Another issue is that the removal of impurity elements present in secondary is hard to remove and costly. Commonly observed impurity elements present in secondary aluminium are Cu, Fe, Mg, Mn, Si and Zn. Final composition of secondary aluminium depends on the type of the scrap used in recycling.
Nevertheless, final purity of aluminium is lower than 97 wt. % Al and this level can decrease as low as to 83 wt%.Although solubility of iron in solid aluminium is very low, solubility of iron is high in liquid aluminium (5 wt% Fe at 800 °C). During the production of aluminium, iron from tools like dies dissolves in aluminium and the content of iron increases with each remelt cycle. As a result iron content of recycled Al may reach to 1.5 wt%, which is ten times higher as compared to primary aluminium.
High composition of impurity elements in aluminium reduces the mechanical properties of the aluminium alloys. Thus, some grades of aluminium have strict composition levels for these impurity elements, especially for iron level. Especially in aerospace industry and in automotive industry iron level must be lower than 0.15 wt% while Si is lower than 0.40 wt%. Therefore, secondary aluminium is diluted by addition of primary aluminium in order meet this level of impurity content. However, addition of primary aluminium increases the cost of the secondary aluminium.
Binary Iron-Aluminium System
Solid solubility of Fe in Al is very low at room temperature. Solid solubility of Fe reaches a maximum of 0.05 wt% Fe at eutectic temperature. Thus, in binary Al-Fe system Fe is always present as aluminides. In order to form ternary intermetallics in Al-Fe-Si system, higher concentrations of Si needed so that for 0.4 wt% Fe containing alloys minimum 0.2 wt% Si needed. Below this level Si dissolves in binary intermetallics without forming ternary phase.
For example, Si can be dissolved in FeAl6 up to 0.5 wt% Si. Minimum silicon content to form ternary phase depends on the intermetallics formed during the solidification. High iron content results in the formation of higher density of eutectic fibres. Although higher density fibres increase the tensile strength, ductility of alloy decreases. Coarse intermetallics act as a crack initiation sites and produce notches, which, consequently, reduce the formability and fatigue resistance.
Al-Fe binary phase diagram suggests a eutectic reaction with an iron composition of 1.8 wt%. Eutectic transformation results in the formation of stable Al3Fe which has a c-face centred monoclinic structure (Al3Fe is also denoted as Fe4Al13 or θ phase). Al3Fe has a morphology of needle like or angular shape. Although these morphologies increase the hardness of the alloy, these shapes has a detrimental effect on the formability and the fatigue resistance of the alloy.As aluminium rich part of Al-Fe phase diagram suggests, Al3Fe is the only equilibrium phase in Al-Fe alloys. However, non-equilibrium solidification promotes the formation of metastable phases. These metastable phases make aluminium at elevated temperatures so that up to 300 °C Al-Fe alloy can compete with titanium.
Thermodynamically metastable phases can also be formed due to non-equilibrium solidification conditions in binary Al-Fe system. These phases are FeAl6, FeAlx, FeAlm, Fe2Al9 and FeAlp. While AlmFe is most commonly observed in AA1xxx and AA5xxx wrought alloys, low cooling rates promote the formation of Al3Fe and Al6Fe.
FeAl6 has a c-face centred orthorhombic crystal structure. Manganese addition increases the thermodynamic stability of this phase by lowering the free energy of the intermetallic. Manganese addition can form MnAl6, which is an isomorph of FeAl6, or can substitute for Fe.FeAlm has a crystal structure of body centred tetragonal and forms higher cooling rates than both Al3Fe and FeAl6. FeAlx has a crystal structure of c-Centred Orthorhombic.
FeAlx is thought to be the Silicon modified version of FeAl6 because of the close stoichiometry and identical crystal structure of the phases. Fe2Al9 has a crystal structure of monoclinic. Similar to FeAl6, Cobalt lowers the free energy of FeAl6 and promotes the formation of isomorphous Co2Al9.Since Fe is always present in recycled aluminium alloys as intermetallics, which reduce the mechanical properties, strategies have been developed to reduce the deleterious effect of Fe.
Rapid solidification of iron containing aluminium alloys give an opportunity to modify the effect by increasing solid solubility of iron and forming finer metastable phases. These metastable phases improve the production of aluminium alloys with high temperature strength, excellent thermal stability and high elastic moduli.