Precipitation-hardening produces a great increase in the strength and hardness of the material with corresponding decreases in the ductile properties. The process used to obtain the desired increase in strength is therefore known as aging or precipitation-hardening.

The strengthening of the heat-treatable alloys by aging is not due merely to the presence of a precipitate. The strength is due to both the uniform distribution of a finely dispersed submicroscopic precipitate and its effects upon the crystal structure of the alloy.

The aging practices used depend upon many properties other than strength. As a rule, the artificially-aged alloys are slightly over-aged to increase their resistance to corrosion. This is especially true with the artificially-aged, high-copper content alloys that are susceptible to intergranular corrosion when inadequately aged.

The heat-treatable aluminum alloys are subdivided into two classes: those that obtain their full strength at room temperature and those that require artificial aging.

The alloys that obtain their full strength after 4 or 5 days at room temperature are known as natural-aging alloys. Precipitation from the supersaturated solid solution starts soon after quenching, with 90 percent of the maximum strength generally being obtained in 24 hours. Alloys 2017 and 2024 are natural-aging alloys.

The alloys that require precipitation thermal treatment to develop their full strength are artificially-aged alloys. However, these alloys also age a limited amount at room temperature, the rate and extent of the strengthening depending upon the alloys.

Many of the artificially-aged alloys reach their maximum natural or room temperature aging strengths after a few days. These can be stocked for fabrication in the –T4 or –T3 tempers. High zinc content alloys, such as 7075, continue to age appreciably over a long period of time. Their mechanical property changes being sufficient to reduce their formability.

The advantage of –W temper formability can be utilized; however, in the same manner as with natural-aging alloys; that is, by fabricating shortly after solution heat-treatment or retaining formability by using refrigeration.

Refrigeration retards the rate of natural aging. At 32 °F, the beginning of the aging process is delayed for several hours, while dry ice (−50 °F to −100 °F) retards aging for an extended period of time.

The temperatures used for precipitation-hardening depend

The time at temperature is dependent upon the temperature used, the properties desired, and the alloy. It ranges from 8 to 96 hours. Increasing the aging temperature decreases the soaking period necessary for proper aging. However, a closer control of both time and temperature is necessary when using the higher temperatures.

After receiving the thermal precipitation treatment, the material should be air cooled to room temperature. Water quenching, while not necessary, produces no ill effects. Furnace cooling tends to produce over-aging.

The annealing procedure for aluminum alloys consists of heating the alloys to an elevated temperature, holding or soaking them at this temperature for a length of time depending upon the mass of the metal, and then cooling in still air. Annealing leaves the metal in the best condition for cold-working. However, when prolonged forming operations are involved, the metal takes on a condition known as “mechanical hardness” and resists further working. It may be necessary to anneal a part several times during the forming process to avoid cracking. Aluminum alloys should not be used in the annealed state for parts or fittings.

Clad parts should be heated as quickly and carefully as possible, since long exposure to heat tends to cause some of the constituents of the core to diffuse into the cladding. This reduces the corrosion resistance of the cladding.

Aluminum alloy rivets are furnished in the following compositions: alloys 1100, 5056, 2117, 2017, and 2024.

Alloy 1100 rivets are used in the “as fabricated” condition for riveting aluminum alloy sheets where a low-strength rivet is suitable. Alloy 5056 rivets are used in the “as fabricated” condition for riveting magnesium alloy sheets.