Vacuum Technology Book, Volume II

3.2.1.3 Aluminum

Aluminum is mainly used in the low and high vacuum range, usually as an alloy, in special cases it is also used as pure aluminum. Components such as ISO-KF pipe components are often made of cast aluminum alloys with reworked flange faces. When selecting the material, shrinkage and porosity must be taken into consideration. For centering and supporting sealing rings, the components are made from bar stock. For metallic seals in the form of profile seals or wires, annealed aluminum-silicon alloys are preferred.

The vapor pressure of aluminum is low, and is only about 6 · 10-9 hPa at the melting point of 660°C. The large thermal expansion, the high thermal conductivity and stable aluminum oxide layer make it difficult to weld aluminum. There is a risk of the formation of pores and cracks, combined with a with great distortion. Uniform heating before welding reduces these risks. However, in practice this is often not possible.

Aluminum is not magnetizable. Aluminum flange connections can only be used for metallic sealing UHV connections to a limited extent, as their hardness is often too low. Although special bimetal flanges, consisting of an aluminum base and a stainless steel plating or aluminum flanges with hardened sealing surfaces, have been developed, their use often fails due to the relatively high price, critical processability or the limited application possibilities.

To increase their abrasion resistance, e. g. for use in cleanrooms or to increase the corrosion protection, the aluminum surfaces are often anodized. This results in a several µm thick, porous oxide layer, which is of only limited suitability for vacuum applications. Gas molecules increasingly become deposited on such surfaces leading to high desorption rates. In addition, gas molecules on the surface area can tunnel under seals and create leaks. There are various anodic treatments available. In deciding whether and, if so which method, to use, the limitations must be considered and weighed against the benefits.

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