Cleansing and degassing of light metals

Abstract

Aluminium and magnesium show much resemblance in physical properties but are melted and processed in widely differing ways. The approaches used reflect differences between the liquid metals in chemical reactivity to air and refractories, solvent power for iron, and the drop in hydrogen solubility on solidification. Optimum technical procedures for removing gas and inclusions are essentially the same: purging with chlorine or with an inert gas in the presence of a salt flux to effect degassing, salt-flux treatment to remove oxide, and filtration or baffle action to control intermetallic particles not wetted by fluxes. Application of these principles to aluminium melted in large reverberatory furnaces presents problems, unless the steps are performed outside the furnace in a separate unit; the advent of effective in-line metal treatment systems enables this to be done and, moreover, at the right place – i.e. close to the casting point. A few suggestions are offered as to how some of these principles might be applied in an aluminium castings foundry. Finally, attention is directed to the importance of the mode of solidification in aluminium block, since this can affect both the forming properties and the anodizing response of the final sheet.