Glossary

What is meant by solid particles here are aluminium-carbide and aluminium-oxide which "float" (like noodles in a soup, to make a simple comparison) in the melt. Today the most widely used auxiliary substances used for degassing are nitrogen or argon. Although chlorine is more effective, it is hardly used any more in foundries because of its high toxicity. The degassing process can be carried out by means of a rotor, lance, holding treatment or the addition of chemical tablets. The way in which degassing treatment works is as follows:

• Hydrogen is purged from the melt by the nitrogen. This prevents the formation of hydrogen bubbles (or pores) later on in the casting and therefore improves the quality of the casting.
• Solid constituents are also purged, in this case because the small nitrogen bubbles stick to the solid particles and bring them with them to the surface. There the solid constituents settle to form a thick film (or dross) which can be skimmed off. The removal of the solid particles eliminates the possibility when the castings are machined some time later of the tool breaking or becoming blunt by hitting such particles which are generally extremely hard.