TABLE 3-7 - HEAT TREATING (SOAKING) TEMPERATURES (CONT'D)
3-16. PHYSICAL PROPERTIES. Commercially pure aluminum weighs 0.098 pounds per cubic inch, corresponding to specific
gravity of 271. Data for standard alloys are shown in Table 3-6. The approximate weight for aluminum, including its alloys, is one
tenth of a pound per cubic inch.
3-17. HEAT TREATMENT OF ALUMINUM ALLOYS .
MIL-H-6088, Heat Treatment of Aluminum Alloys, will be the control document for heat treatment of
Aluminum Alloys used on aerospace equipment. For complete description of aluminum heat treat
requirements, refer to latest issue of MIL-H-6088.
3-18. GENERAL. There are two types of heat treatment applicable to aluminum alloys. They are known as solution and
precipitation heat treatment. Some alloys such as 2017 and 2024 develop their full mechanical properties as a result of solution heat
treatment followed by 96 hours (natural precipitation) aging at room temperature. Other alloys, such as 2014, 7075, and 7178
require solution heat treatment and aging (precipitation heat treatment) for specific length of time at a definite temperature (see Table
3-19. Solution heat treatment is a process where the alloying elements enter into solid solution in the aluminum at critical
temperatures. It has been found that those alloying elements which increase the strength and hardness are more soluble in solid
aluminum at high temperature that at low. To complete the solution often the metal is held at high temperatures for sufficient time; it
is then quenched rapidly in cold water to retain this condition. Immediately after quenching, the alloy is in an unstable condition,
because it consists of a supersaturated solid solution of the hardening agent. Upon standing at room temperature the hardening
constituent in excess of that which is soluble at room temperature precipitates. The precipitate is in the form of extremely fine
particles which due to their keying action, greatly increase the strength. This is in effect a method where the molecules of the
aluminum and alloying elements are realigned to increase the strength and hardness of some aluminum alloys.
3-20. PRECIPITATION (AGE) HARDENING . This phase of heat treatment consists of aging material previously subjected to
solution heat treatments by natural (occurs at room temperature) or artificial aging. Artificial aging consists of heating aluminum
alloy to a specific temperature and holding for a specified length of time. During this hardening and strengthening operation the
alloying constituents in solid solution precipitate out. As precipitation progresses, the strength of the material increases until the
maximum is reached. Further aging (overaging) causes the strength to decline until a stable condition is obtained. The
strengthening of material is due to the uniform alignment or formation of the molecule structure of the aluminum and alloying
3-21. Artificial aged alloys are usually slightly overaged to increase their resistance to corrosion. Especially the high copper content
alloys. This is done to reduce their susceptibility to intergranular corrosion caused by under-aging.
Change 9 3-16