Primary casting aluminum alloys made from primary aluminum, usually, ingots or bars, which is remelted with the addition of alloying elements necessary for a given alloy.
Secondary cast aluminum alloys made (cheaper, than primary) from aluminum scrap, which, after careful sorting, is remelted with the addition of the necessary alloying elements.
Iron in secondary aluminum
Secondary alloys have a relatively high content of impurities, especially iron. Almost all aluminum scrap is almost always contaminated with iron or steel, for example, bolts and washers. The iron in aluminum is harmful in many ways, and as in liquid, and in solid states. for instance, iron adversely affects the casting properties of aluminum alloys, and also on their solid state viscosity. Therefore, the iron content of cast aluminum alloys is kept as low as possible.. Note, that the only casting method, which is not afraid of iron - this is high pressure casting.
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.
Iron in alloys for sand casting and multiple casting
1) For castings, which have high requirements for material viscosity, the iron content in the alloy should be no more 0,20 %. It means, that in this case it is necessary to use only primary alloys.
2) When material viscosity is not critical, the iron content in the alloy may be higher. However, in this case, the critical parameters are the fluidity of the alloy and its ability to be machined., for example, cutting. Iron content up to 0,5 %, and with a technical justification even higher.
Iron in die casting alloys
1) In this case, iron is a "useful impurity", as it reduces the tendency of aluminum alloys to adhere to the surface of the mold.
2) For this reason, the iron content is specified in these alloys, usually, no less 0,6 %. Therefore, these aluminum alloys may well be secondary..
3) If viscosity requirements are specified for the material of the castings, then the iron content in the alloy is limited to the interval from 1 to 1,3 %.
A source: European Aluminium Association, 2002