Aluminum

 

Silvery white metal with a low melting point  and a high degree of oxygen affinity. If exposed to air, the material immediately forms an oxide film (Al2O3), both in solid and liquid state, which protects it from further oxidation.

SymbolAl
Atomic number13
Atomic weight26.97
Specific weight at 20 °C2.7 g/cm3
Melting point660 °C
Boiling point2400 °C
Melting heat396 kJ/kg
Specific thermal capacity at 20 °C0.90 kJ/(kg·K)
Thermal conductivity237 kJ/(m·K)
Therm. coefficient for linear expansion23·10-6/K

The raw material used for aluminum production is bauxite; alumina is recovered from bauxite due to alkaline digestion. Alumina is melted while adding flux (cryolite) and at a temperature of 950 °C it is electrolyzed using graphite electrodes. Liquid aluminum separates and deposits at the cathode at the bottom.

Aluminum in steel
Solubility of aluminum is 1.1 % in γ-iron and 36 % in α-iron. In γ-iron it is increased with higher carbon contents. In solid solutions, aluminum promotes significant increase of ferritehardness. In austenite hardness is also slightly increased due to solid-solution formation. Aluminum does not promote carbide formation, on the contrary, it has a graphitizing effect.

In its capacity as deoxidation agent aluminum is widely used in the production of steel since it combines with the oxygen from the dissolved iron oxides to form alumina, which often remains in the steel in evenly dispersed manner. In addition, aluminum has great affinity for nitrogen and combines with this substance to form aluminum nitride. Moreover, aluminum has the capacity to sweep dissolved gases out of the molten metal and thus serves for reducing cavities and segregations.
Of all elements that are added to steel for influencing the austenitegrain size, aluminum is the most effective. Recent considerations explain the influence of aluminum on the “characteristic” grain size of steel by the fact that aluminum nitrides deposit at the grain boundaries of the austenitestructure and thus prevent coalescence of the primarily formed austenite grains. Unalloyed steel treated with aluminum displays higher resistance against mechanical aging and improved impact resistance. The regular and homogeneous structure also results in enhanced processing and machining properties; however, creep strength is reduced in some cases. Deoxidation involving aluminum has a great impact on the type, size, and distribution of sulphidic inclusions. Within the scope of a study on the influence of aluminum on cast steel, it was found that aluminum additions, which were not sufficient for complete deoxidation, caused sulfide inclusions of various sizes and irregular distribution. Completely deoxidized steels, on the other hand, have inclusions that are present either in the form of chains of very small nodules or as a kind of eutectic discharge along the primary grain boundaries. If steel is deoxidized with an excess of aluminum, this results in coarse sulphides with irregular shapes and distribution.

Aluminum in cast iron
Upon addition of solid aluminum, it is possible to introduce up to 2 % Al into the molten cast iron without problems; however, for higher Al contents, the two metals are best mixed in molten condition. The viscous oxide layer that covers the surface of all aluminum-containing iron melts, has a tendency to permeate the metal. This may lead to problems in casting and casting defects due to aluminum oxide inclusions, which results in poor mechanical properties.

Aluminum contents of up to 4 % promote graphite formation in cast iron. In contrast to this, aluminum contents between 4 and 10 % stabilize the eutectoidcarbide phase and with contents between 10 and 18 % Al, carbides prevail. With aluminum contents between 18 and 24 %, graphite precipitation is initiated again; precipitation increases constantly with increasing aluminum content. With a content of 24 % Al, the entire carbon is present in the iron in the form of graphite. With aluminum contents above that value, carbide formation sets in again until no graphite is present any more at an aluminum content of 29 %.
Cast iron grades with high aluminum contents provide good resistance to growth and scaling. With increasing aluminum contents, particularly above 3 %, formation of scale is more and more reduced. The influence on growth tendency is only limited for aluminum contents below 3 %. Above this limit, growth is reduced with increasing aluminum contents, and is expected to reach a minimum between 8 and 20 % Al with little or no graphite present. An alloy known by the name CRALFER provides excellent resistance to growth and scaling at temperatures of up to 1000 °C and has the following contents of alloy elements: 7 to 7.5 % Al and 0.75 % Cr.

Cast iron grades that are nitrided, often contain between 1 and 1.5 % Al in addition to low contents of chromium and molybdenum. 

Aluminum is also present in ferrosilicon as well as other silicon-containing inoculation alloys.

In melleable cast iron, addition of 0.02 % Al causes formation of nodular graphite particles and thus facilitates tempering, probably due to the aluminum binding the nitrogen in the material.

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