Aluminum is mainly used in low and high vacuum ranges, usually as an alloy and, in special cases, as pure aluminum. Components such as ISO-KF pipe fittings are usually made of aluminum alloy with a reworked flange face. Shrinkage and porosity must be taken into account when selecting materials. When used for centering ring and matching sealing ring, the parts are made of rod material. Annealed al - Si alloys are preferred for gaskets or linear metal seals.
Aluminum has a very low vapor pressure, only about 6·10-9hPa at a melting point of 660°C. High thermal expansion coefficient, high thermal conductivity and alumina layer stability make aluminum difficult to weld. There is a risk of blowholes and cracks, which can easily distort. Uniform heating before welding reduces these risks. In practice, however, this is often not possible.
Aluminum is not magnetized. To a limited extent, aluminum flange connections can only be used for metal-sealed UHV connections as they are usually too hard. Although special bimetallic flanges have been developed, including aluminum base and stainless steel plate, or aluminum flanges with tough sealing surface, their application has not been successful due to their relatively high price, certain risk of machining performance and limited application. Aluminium surfaces are often anodized to improve their wear resistance, for example, in clean rooms, or to increase corrosion protection. This results in an increase in thickness of several microns and the presence of a porous oxide layer, which is of very limited applicability in vacuum applications. Gas molecules are deposited gradually on such surfaces, resulting in higher desorption rates. In addition, gas molecules in the surface area can form channels under the seal, causing leakage. Various anodizing treatments are available. Limitations must be considered and benefits weighed when deciding whether to use such an approach.
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