Aluminium encyclopaedia

The processing of aluminium includes a multitude of processes that can be used to treat the surfaces of aluminium alloys and components in such a way that one achieves specific improvements to these surfaces. One differentiates between mechanical and chemical processes and coating.

In principle, the same tools are used for mechanical surface treatment - essentially the removal and smoothing out ("burnishing") of unevenness, for example on semi-finished products - as for other industrially used metals. One should note, however, that for reasons of corrosion protection one should not use the same tools as are used for processing steel or copper, and that unalloyed aluminium, refined aluminium and lower-strength magnesium-containing alloys have a tendency to smear.

A selection of the processes:

• Deflashing and fettling, for example of castings, using band and circular saws, files, milling cutters, chisels and abrasive discs.

• Brushing with rotating stainless steel brushes, in some cases also shot-blasting with fine-grained corundum particles, produces a lightly and uniformly roughened surface or a surface with a "satin" finish. Such silk-matt sheets and panels, which are protected against fingerprints by anodising, are used in the manufacture of industrial equipment.

• Polishing produces a high-gloss finish, which disappears with time, however, unless it is protected by a clear lacquer or anodising. Polishing is carried out with buffing wheels using a soft cloth and polishing paste, in ball drums or tumbling barrels (where small components are rotated together with polished stainless steel balls in a polishing liquid) or with glossy finish rolls in the final rolling pass.

• With embossing, a pattern in the shape of small deformations is formed in foil and strip using engraved or etched stainless steel rolls (with a backing roll made of synthetic resin bonded paper).

• It is also possible to produce very smooth surfaces on aluminium using machining (for example milling).

The processes used for chemical surface treatment differ according to the result required:

• Anodising creates an artificial oxide layer with high corrosion resistance, hard anodising a surface with high abrasion resistance.

• Lacquering (organic coating).

• Depending on the composition of the aluminium alloy and the objective, etching is carried out with different etchants (based on alkalis and acids). Etching of metallographic specimens is necessary in order to examine the grain structure. Text and graphics are etched into signs, printing plates and plates that are coated with a photographic layer, for example to permanently protect valuable drawings and plans. "Chemical milling" or deep etching is used in aircraft construction and in space travel, for example to etch out stiffening ribs in thick-gauge sheet. Machining would be too laborious and too expensive. A solution containing 15 per cent of sodium hydroxide at 80 to 85 °C has proven itself in practice.

• Pickling, for example with a solution containing ten per cent of sodium hydroxide at 50 °C, dissolves extremely small amounts of aluminium and cleans the surface. Pickling in solutions containing sodium hydroxide produces surfaces with a satin finish, which are used in building construction.

• Chromate or phosphate conversion coating of pickled aluminium components in appropriate baths causes chemical oxidation and leads to the formation of thin layers of oxide hydrates or phosphates, which act as bonding for lacquering and corrosion protection.