3-22. Natural aging alloys can be artificially aged, however, It increases the susceptibility of the material to intergranular corrosion. If
utilized it should be limited to clad sheet, extrusions and similar items. For aging treatment, temperature and times see Table 3-11.
3-23. SOLUTION HEAT TREATMENT . As previously pointed out it is necessary that solution heat treatment of aluminum alloys be
accomplished within close limits in reference to temperature control and quenching. The temperature for heat treating is usually
chosen as high as possible without danger of exceeding the melting point of any element of the alloy. This is necessary to obtain the
maximum improvement in mechanical properties.
3-24. If the maximum specified temperature is exceeded eutectic melting will occur. The consequence will be inferior physical
properties, and usually a severely blistered surface. If the temperature of heat treatment is low, maximum strength will not be
3-25. Heating Time. The heating time commonly called the "soaking time" required to bring about solution increases with the
thickness of the section or part to be heat treated. Solution heat treatment should be held to the minimum time required to obtain the
desired physical properties. In many instances the above will require sample testing to determine the exact solution time. For the
recommended approximate soaking time for various alloys see Table 3-7.
3-26. The time at temperature (soaking time) is measured from the time the metal reaches the minimum limit of the temperature
range. In the case of thick material the controlling factor would be when the center (core) reached the minimum temperature. The
soaking period will vary from 10 minutes for thin sheet to approximately 12 hours for the thicker materials, such as heavy forgings. A
general guide to use is approximately one hour for each inch of cross-sectional thickness. It is recommended that thermocouple be
placed in the coldest part of the load to determine the period required to bring the load to the correct temperature (soaking
3-27. The soaking temperature required is selected to put all of the soluble elements into solid solution. With clad materials,
prolonged heating may defeat the purpose of the cladding by excessive diffusion of copper and other soluble elements into the
3-28. RE-SOLUTION HEAT TREATMENT . The bare heat-treatable alloys can be solution heat treated repeatedly without harmful
effects other than high temperature oxidation. The oxidation can be retarded by using either sodium or potassium fluoborate during
the heating cycle.
3-29. For clad sheet the number of solution heat treatment is limited due to the increased diffusion of the core and cladding. See
Table 3-12 for the recommended reheat-treatment times.
3-30. QUENCHING. The basic purpose for quenching is to prevent the immediate re-precipitation of the soluble constituents after
heating to solid solution.
3-31. To obtain optimum physical properties of aluminum alloys, rapid quenching is required. The recommended time interval
between removal from the heat and immersion is 10 seconds or less. Allowing the metal to cool before quenching promotes
intergranular corrosion and slightly affects the hardness. This is caused by re-precipitation along grain boundaries and in certain slip
planes. For specific quench delay see Table 3-10.
3-32. There are three methods employed for quenching. The one used depends upon the item, alloy and properties desired.
3-33. Cold Water Quenching. Small articles Lade from sheet, extrusions, tubing and small fairing are normally quenched in cold
water. The temperature before quenching should be 85°F or less. Sufficient cold water should be circulated within the quenching
tanks to keep the temperature rise under 20°F. This type of quench will insure good resistance to corrosion and particularly important
when heat-treating 2017 and 2024.
3-34. Hot Water Quenching. Large forgings and heavy sections can be quenched in(150°-180°F)or boiling water. This type of
quench is used to minimize distortion and cracking which are produced by the unequal temperatures obtained during the quenching
operation. The hot water quench will also reduce residual stresses which improves resistance to stress corrosion cracking.
3-35. Spray Quenching. Water sprays are used to quench parts formed from alclad sheet and large sections of most alloys.
Principal reasons for using this method is to minimize distortion and to alleviate quench cracking. This system is not usually used to
quench bare 2017 and 2024 due to the effect on their corrosion resistance. The parts quenched by this media should pass the
corrosion test required for the item involved; (see specification MIL-H-6088).
3-36. STRAIGHTENING OF PARTS AFTER SOLUTION HEAT TREATMENTS . It will be necessary to straighten some parts after
heat treating due to warping produced by the process. These parts are usually straightened by restriking or forming. It is desirable to
place these parts in refrigeration immediately after quenching to retard natural aging until such time straightening is accomplished. A
temperature of 32°F or below will delay or retard natural aging for approximately 24 hours, lower temperatures will delay the aging