Limitations of Wrought and Cast Aluminum Alloys

There are several characteristics of aluminum alloys that require special attention in alloy selection or design:

Moduli of Elasticity.

As noted earlier, the elastic moduli of aluminum alloys are about onethird those of steel. In applications such as bridges, where some designs may be deflection critical, this is a disadvantage and consideration should be given to the fact that aluminum alloys will deflect about three times more than comparably sized steel members. To compensate for this, aluminum members subject to bending loads are usually made deeper or

thicker in their upper and lower extremities to reduce stresses and/or deflections.

Melting Temperature. 

Aluminum Alloys melt at about 1000 F (535 C), well below where steels melt, and so they should not be selected for applications such as flue pipes and fire doors where the low melting point may result in unsatisfactory performance. The useful limit of high-temperature structural application of aluminum alloys is about 500 F (about 260 C) for conventional alloys or about 600 F (315 C) for alumina-enhanced alloys. It is useful to note, however, that even in the most intense fires, aluminum alloys do not burn; they are

rated noncombustible in all types of fire tests and achieve the highest ratings in flame-spread tests. Further, as noted earlier, they are slower to reach high temperatures in fires than other metals such as steels because of their higher thermal conductivity and emmissivity. Nevertheless, they should not be used where service requirements include structural strength above 500 F (260 C) or exposure above 700 F (370 C).

Stress–Corrosion Susceptibility of Some Alloys.

Some aluminum alloys, notably the 2xxx and 7xxx alloys, when stressed perpendicular to the major plane of grain flow (i.e., in the short-transverse direction) may be subject to intergranular stress–corrosion cracking (SCC) unless they have been given a special thermal treatment to reduce or eliminate this type of behavior. If short transverse stresses are anticipated in relatively thick components, 2xxx alloys should only be utilized in the T6- or T8-type tempers (not T3- or T4-type tempers), and 7xxx alloys should only be used in the T7-type tempers (not T6- or T8-type tempers). Similarly, 5xxx alloys with more than 3% Mg should not be used in applications where a combination of high stress and high temperature will be experienced over a long period of time (more than several hundred hours at or above 150 F, or 65 C) because some susceptibility

to SCC may be encountered (i.e., the alloys may become ‘‘sensitized’’); for applications where temperatures above about 150 F (65 C) are likely to be encountered for long periods, the use of 5xxx alloys with 3% or less Mg, e.g., 5454, is recommended.

Mercury Embrittlement. Aluminum alloys should never be used when they may be in direct contact with liquid or vaporized mercury; severe grain-boundary embrittlement may result. This is an unlikely exposure for the vast majority of applications, but in any instance where there is the possibility of mercury being present, the use of aluminum alloys should be avoided.

READ MORE.......

J. G. Kaufman

Kaufman Associates

Columbus, Ohio

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.

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