Aluminium encyclopaedia

Powder

Depending on the designated use, aluminium powder is produced using various processes and in different sizes and shapes. The starting material is mostly unalloyed aluminium with a minimum purity of 99.5 per cent aluminium.

  • Granules, rounded grains, three to 20 millimetres in diameter, solidify from drops of molten aluminium that fall onto a rotating, water-cooled dish. Used in steel production.
  • Grit consists of sharp-edged grains, 0.4 to four millimetres in diameter, mostly produced by tamping thin strip. Used in aluminothermics.
  • Flake a tenth of a millimetre thick and several millimetres long and wide are produced by allowing drops of molten aluminium to fall between water-cooled rotating rolls. Used in composite materials.
  • Leafing paste comprises fine to extremely fine flakes produced by ball milling foil with steel balls. It is used for paints and as filler in plastics (it imparts higher strength and thermal conductivity and reduces the electrostatic charge).
  • Powder comprises extremely fine grains, with diameters between a few hundred-thousandths and a tenth of a millimetre (10-5 to 10-1 millimetre). The most important method of production is "atomisation": molten aluminium flows downwards as a thin jet out of a nozzle and is atomised by compressed air introduced horizontally. This produces grains of irregular shape (because they solidify in the air and cover themselves with an oxide layer) and different sizes. They are collected and sieved according to size, for example between two and six tenths of a millimetre. Sufficient quantities of fine dust have to be removed continuously from the atomisation device that its concentration does not exceed 30 gram per cubic metre air (based on an average diameter of 40 thousandths of a millimetre), which is the lower explosion limit for mixtures of fine aluminium dust and air. Aluminium powder is used in paints, as catalysts in chemistry, for the manufacture of medication, for sintering and in explosives. Fine aluminium powder, mixed with oxygen providers, explodes at an ignition temperature of 250 to 300 °C with a strong explosive effect. Additions of aluminium powder to TNT make this a very safe to handle "permitted explosive". If the atomisation is not conducted using air but with an inert gas such as helium or argon, an oxide skin cannot form, the powder is far more readily inflammable and is used as rocket fuel in space travel and defence technology.
  • One can obtain even finer powder if an ultrasonic wave field is applied during atomisation. Such powder is needed for the manufacture aluminium"lithium alloys, for example for use in space travel.
  • Melt spinning is also possible with aluminium. Molten metal is allowed to fall through a nozzle (for example 0.8 millimetres wide) onto a rapidly rotating, water-cooled roll. It solidifies to produce long threads up to 40 to 50 thousandths of a millimetre thick, 1.5 millimetres wide and up to 20 millimetres long, which are used in composites.