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requirement is that the element or compound that is put into solution during the solution heat treating operation must be capable of forming a fine precipitate that will produce lattice strains in the aluminum matrix. The precipitation of these elements or compounds progressively hardens the alloy until a maximum hardness is obtained. Alloys that are not aged sufficiently to obtain maximum hardness are said to be underaged, while those that are aged past peak hardness are said to be overaged. Underaging can be a result of not artificially aging at a high enough temperature or an aging time that is too short, while overaging is usually a result of aging at too high a temperature. An example of the aging

Fig. 2.11. Artificial Aging Curves for 2024-T4 Sheet3

behavior of artificially aged 2024-T4 is shown in Fig. 2.11. Note the large reduction in strength when it is overaged at 425° F.

Aluminum alloys that satisfy both of these conditions are classified as heat treatable, namely the 2XXX, 6XXX, 7XXX, and some of the 8XXX wrought alloys. The precipitation hardening process is conducted in three steps:

1. Heating to the solution heat treating temperature and soaking for long enough to put the elements or compounds into solution.

2. Quenching to room or some intermediate temperature (e.g., boiling water) to keep the alloying elements or compounds in solution; essentially this creates a supersaturated solid solution.

3. Aging at either room temperature (natural aging) or a moderately elevated temperature (artificial aging) to cause the supersaturated solution to form a very fine precipitate in the aluminum matrix.

The solution heat treating temperature is as high above the solid solubility curve as possible without melting the lowest melting point eutectic constituents. Therefore, close temperature control, normally ±10° F, is required for the furnaces used to heat treat aluminum alloys. If the alloy is heated too high and incipient grain boundary melting occurs, the part is ruined and must be scrapped. For example, 2024 is solution treated in the range of 910-930° F, while a low melting eutectic forms at 935° F, only 5° F higher than the upper range of the solution heat treating temperature. On the other hand, if the temperature is too low, solution will be incomplete and the aged alloy will not develop as high a strength as expected. The solution heat treating time should be long enough to allow diffusion to establish an equilibrium solid solution. The product form can determine the time required for solution treating, i.e. castings require more time than wrought products to dissolve their relatively large constituents into solution. The time required can vary anywhere from less than a minute for thin sheet to up to 20 h for large sand castings. While longer than required soak times are not usually detrimental, caution needs to be applied if the part is Alclad, since excessive times can result in alloying elements diffusing through the Alclad layer and reducing the corrosion resistance.

The oven used to conduct solution heat treating should be clean and free of moisture. The presence of moisture can cause hydrogen to be absorbed into the aluminum parts, while sulfur compounds can decompose the protective surface oxide, making the part even more susceptible to hydrogen absorption. Absorbed moisture can result in internal voids or surface blisters, and the 7XXX alloys are the most susceptible to this type of attack followed by the 2XXX alloys. Moisture can be minimized by thoroughly cleaning and drying parts and racks before placing them into the oven.

After the elements are dissolved into solution, the alloy is quenched to a relatively low temperature to keep the elements in solution. Quenching is perhaps the most critical step in the heat treating operation. The problem is to quench the part fast enough to keep the hardening elements in solution, while at the same time minimizing residual quenching stresses that cause warpage and distortion. In general, the highest strength levels, and the best combinations of strength and toughness, are obtained by using the fastest quench rate possible. Resistance to corrosion and SCC are usually improved by faster quenching rates; however, the resistance to SCC of certain copper-free 7XXX alloys is actually improved by slow quenching. While fast quenching rates can be achieved by cold water, slower quenching rates (e.g., hot or boiling water) are often used to sacrifice some strength and corrosion resistance for reduced warpage and distortion.

If premature precipitation during quenching is to be avoided, two requirements must be met. First, the time required to transfer the part from the furnace to the quench tank must be short enough to prevent slow cooling through the critical temperature range where very rapid precipitation takes place. The high strength 2XXX and 7XXX alloys should be cooled at rates exceeding 800° F/sec. through the temperature range of 750-550° F. The second requirement is that the volume of the quenching tank must be large enough so that the quench tank temperature does not rise appreciably during quenching and allow premature precipitation.

Both cold and hot water are common quenching media for aluminum alloys. Cold water, with the water maintained below 85° F, is used with the requirement that the water temperature does not rise by more than 20° F during the quenching operation. The quench rate can be further increased by agitation that breaks up the insulating steam blanket that forms around the part during the early stages of quenching. Parts that distort during quenching require straightening before aging, so hot water quenching, with water maintained between 150 and 180° F or at 212° F, is a less drastic quench resulting in much less distortion and is often used for products where it is impracticable to straighten after quenching. Polyalkylene glycol solutions in water are also used to quench aluminum alloys because they produce a stable film on the surface during quenching, resulting in more uniform cooling rates and less distortion. The time from removing from the solution treating furnace until placing in the quench media is also critical; maximum quench delays are on the order of 5 s for 0.016 in. thick material, 7 s for material between 0.017 and 0.031 in., 10 s for material between 0.031 and 0.090in., and 15 s for thicker material.

Aging is conducted at either room temperature (natural aging) or elevated temperature (artificial aging). The 2XXX alloys can be aged by either natural or artificial aging. The 2XXX alloys can be naturally aged to obtain full strength after 4-5 days at room temperature with about 90% of their strength being obtained within the first 24 h. Natural aging of the 2XXX alloys consists of solution heat treating, quenching, and then aging at room temperature to give the T4 temper. Naturally aged alloys are often solution treated and quenched (W temper), refrigerated until they are ready to be formed, and then allowed to age at room temperature to peak strength (T3 temper). To prevent premature aging, cold storage temperature needs to be in the range of -50 to -100° F. It should be noted that artificial aging of the 2XXX alloys to the T6 temper produces higher strengths and higher tensile-to-yield strength ratios but lower elongations than natural aging.

The T8 temper (i.e., solution treating, quenching, cold working, and then artificial aging) produces high strengths in many of the 2XXX alloys. Alloys such as 2011, 2024, 2124, 2219, and 2419 are very responsive to cold working by stretching and cold rolling; the cold work creates additional precipitation sites for hardening. The T9 temper is similar except that the cold work is introduced after artificial aging (i.e., solution treating, quenching, artificial aging, and cold working). Since the 7XXX alloys do not respond favorably to cold working during the precipitation hardening process, they are not supplied in the T8 or T9 tempers.

Artificial aging treatments are generally low temperature, long time processes; temperatures range from 240 to 375° F for times of 5-48 h. The 7XXX alloys, although they will harden at room temperature, are all given artificial aging treatments. The 7XXX alloys are usually aged at 250° F for times up to 24 h, or longer, to produce the T6 temper. Many of the thick product forms for the 7XXX

alloys that contain more than 1.25% copper are provided in the T7 oveaged condition. While overaging does reduce the strength properties, it improves the corrosion resistance, dimensional stability, and fracture toughness, especially in the through-the-thickness short transverse direction. There are a number of T7 tempers that have been developed that trade-off various amounts of strength for improved corrosion resistance. Most involve aging at a lower temperature to develop strength properties followed by aging at a higher temperature to improve corrosion resistance. For example, the T73 aging treatment consists of an aging temperature of 225° F followed by a second aging treatment at 315-350° F. The T76 temper is similar with an initial age at 250° F followed by a 325° F aging treatment. The T76 temper has a little higher strength than the T73 temper but is also a little less corrosion resistant and produces a lower fracture toughness. As shown in Table 2.9, the T77 aging treatment developed by Alcoa, which is a variation of the retrogression and re-aging treatment, produces the best combination of mechanical properties and corrosion resistance. Although it depends on the specific alloy, in the T77 treatment, the part is solution treated, quenched, and aged. It is then re-aged for 1 h at 390° F and water quenched, and finally aged again for 24 h at 250° F.

Verification of heat treatment is usually conducted by a combination of hardness and electrical conductivity.

2.5.2 Annealing

Cold working results in an increase in internal energy due to an increase in dislocations, point defects, and vacancies. The tensile and yield strengths increase with cold working, while the ductility and elongation decrease. If cold worked aluminum alloys are heated to a sufficiently high temperature for a sufficiently long time, annealing will occur in three stages: recovery, recrystallization, and grain growth. During recovery, the internal stresses due to cold work are reduced with some loss of strength and a recovery of some ductility. During recrys-tallization, new unstrained nuclei form and grow until they impinge on each other to form a new recrystallized grain structure. Although heating for longer times or at higher temperatures will generally result in grain growth, aluminum

Table 2.9 Effect of T7 Heat Treatments on 7XXX Aluminum Alloys3

Alloy and Temper

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