21.5: Contaminants in Aluminium Alloys
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Aluminium is the most widely used aerospace metal. As calculated above, it is highly energy intensive to produce, and recycling it is both economically and environmentally beneficial.
It is usually separated from scrap by the Eddy current separation method, as explained here. However, by chance very small amounts of ferrous metals and other contaminants will remain in the scrap when it finally reaches the melting stage.
This may not seem to be much of an issue – Aluminium is never used in the pure form. It is always alloyed with other metals and elements to give the exact mechanical and chemical properties for the desired application. The aluSelect website’s ‘alloy composition’ tool shows the exact compositions of these elements in the various alloys. Notice that iron is almost always <1%.
Why would having a few percent of e.g. ‘unplanned’ iron mixed in the aluminium melt be of consequence?
Look at the phase diagram below:
If iron were a contaminant in aluminium melt, it would be of a small percentage. The right hand of the diagram shows the behaviour of the system at this composition. When molten, iron is completely soluble in aluminium. What do you think will happen when the ingots are cooled in terms of the microstructure?
Although the microstructure of the ingots themselves is not of particular concern, the resulting behaviour of the Aluminium in its application is. The presence of the inter-metallic compounds (such as Al13Fe4) may reduce the ductility and machineability – since the presence of precipitates will interfere with dislocation motion (precipitation hardening ) and reduce ductility.
Aluminium alloys are also heat treated to optimize their properties. Obviously, the behaviour of an aluminium alloy when annealed , for example, cannot be predicted if its composition is not precisely known.
Even though recycling aluminium is highly attractive from an economic point of view, for some high-tech applications the aluminium used has to come from primary production. Development of methods to further refine the recycled aluminium at low energy costs (to keep recycling the lower energy process compared to production) are needed if a larger amount of aluminium used in aerospace applications is to come from recycled sources.