Melting and casting: Oxidation
The positive effect of oxidation is highly undesirable during melting and casting of aluminium. An increased fraction of oxides in suspension in the melt can lead to all kinds of problems, of which the most important ones will be mentioned here:
- inferior fluidity
- inferior ability to fill the mould
- feeding is impeded
- greater mould wear in die casting
- inferior mechanical properties as a result of defects in the microstructure
- difficulties during mechanical processing (hard spots) and polishing
- inferior weldability
- inferior pressure tightness
Unalloyed aluminium undergoes the lowest amount of oxidation because an optimally compact oxide skin forms. Unalloyed aluminium thus also has very good resistance to attack, but it also has very poor mechanical properties. Thus, in practice, alloys are used almost exclusively in foundries.
To avoid oxide skin entering the melt, one should ensure that the ‘oxide pipe’ is maintained during casting operations. Unfortunately, this requirement is very often ignored in practice.
Certain alloying elements, such as sodium, calcium, lithium or magnesium , have a negative effect on the aluminium oxide skin, i.e. it becomes more permeable and oxygen can enter the melt significantly more easily and cause oxidation there, which leads to an increased melting loss due to dross formation. With aluminium-magnesium alloys, this effect is avoided by adding extremely small amounts of beryllium (a few ppm). The surface activity of the beryllium leads to effective oxidation protection during the melting, holding at temperature and casting of such alloys.
Water has a strong influence on the oxidation process. Dry oxygen reacts sluggishly with aluminium; it has to first dissociate in order to then be able to subsequently combine with the aluminium atomically. In practice, however, the oxidation process occurs mainly via water. As already mentioned, this water can come from different sources, such as water on the surface of the charge material, humidity, moisture in the combustion gases or water of hydration in the oxide skin of the charge material.