Chemical and Physical Properties

The chemical compositions of unalloyed atomized aluminum powders are shown in Table 1. Iron and silicon are the major contaminants for both regular and high-purity powders. In many cases, atomizers have access to molten metal from a nearby aluminum smelter for regular purity metal grades. For high-purity powder, metal is typically bought in the form of purified ingots (sows) and prepared in furnaces that are suitably lined.

Table 1 Typical chemical analyses of atomized aluminum powders

Type of powder Composition, wt%_Other metallics

_Aluminum Iron Silicon Each Total

Atomized powders_

Maximum . . . 0.25(a) 0.15(a) 0.05 0.15(aF High-purity atomized powders_

Iron plus silicon, 0.30 wt% max

High-purity aluminum is resistant to attack by acids, but it dissolves in a mixture of nitric and hydrochloric acids. Solutions of alkali hydroxides rapidly attack aluminum, with evolution of hydrogen and formation of the corresponding soluble aluminate. Aluminum reacts vigorously with bromine and iodine to form aluminum halides. Dry, oxygen-free chlorine or hydrogen chloride gas also reacts with aluminum to form aluminum chloride. Finely divided aluminum can react violently when exposed to halogenated hydrocarbons, such as methylene chloride and carbon tetrachloride, especially if the operation is carried out under pressure.

Aluminum is stable in air because of its thin, natural oxide film. In finely divided powder form, however, aluminum is more chemically reactive and hydrates when exposed to moisture as described above. Powders also react with water to liberate hydrogen and form aluminum hydroxide and release heat in the process.

Physical Properties. The physical properties of aluminum and pure aluminum powders are listed in Table 2. The real density of aluminum powder approaches that of the base metal, but both apparent density and tap density vary as a function of particle size distribution.

Table 2 Typical physical properties of atomized aluminum powders

Wrought density (metal), kg/m3

2700

Melting point, °C (°F)

660 (1220)

Boiling point, °C (°F)

2430 (4410)

Surface tension at 800 °C (1470 °F), N/m

0.865

Apparent density, kg/m3

800-1300

Tap density, kg/m3

1200-1500

Melting point of oxide, °C (°F)

2045 (3720)

Oxygen content, wt%

0 0

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