Heat treatment of nonferrous metals (aluminum alloys)

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Annealing and normalizing

Aviation Structural Mechanic

Annealing

HEAT TREATMENT OF NONFERROUS METALS (ALUMINUM ALLOYS)

Aluminum is a white, lustrous metal, light in weight and corrosion resistant in its pure state. It is ductile, malleable, and nonmagnetic. Aluminum combined with various percentages of other metals, generally copper, manganese, and magnesium, form the aluminum alloys that are used in aircraft construction. Aluminum alloys are lightweight and strong, but do not possess the corrosion resistance of pure aluminum and are generally treated to prevent deterioration. "Alclad" is an aluminum alloy with a protective coating of aluminum to make it almost equal to the pure metal in corrosion resistance.

Several of the aluminum alloys respond readily to heat treatment. In general, this treatment consists of heating the alloy to a known temperature, holding this temperature for a definite time, then quenching the part to room temperature or below. During the heating process, a greater number of the constituents of the metal are put into solid solution. Rapid quenching retains this condition, which results in a considerable improvement in the strength characteristics.

The heating of aluminum alloy should be done in an electric furnace or molten salt bath. The salt bath generally used is a mixture of equal parts of potassium nitrate and sodium nitrate. Parts heated by this method must be thoroughly washed in water after treatment. The salt bath method of heating should never be used for complicated parts and assemblies that cannot be easily washed free of the salt.

Heat Treating Procedures

There are two types of heat treatment applicable to aluminum alloys. They are known as solution and precipitation heat treatment. Certain alloys develop their full strength from the solution treatment, while others require both treatments for maximum strength. The NA 01-1A-9 lists the different temper designa-tions assigned to aluminum alloys and gives an example of the alloys using these temper designations.

SOLUTION HEAT TREATMENT. The solution treatment consists of heating the metal to the temperature required to cause the constituents to go into a solid solution. To complete the solution, often the metal is held at a high temperature for a sufficient time, and then quenched rapidly in cold water to retain this condition. 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. 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 the heat treatment is low, maximum strength will not be obtained.

PRECIPITATION (AGE) HARDENING. The

Artificially 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 underaging.

Natural aging alloys can be artificially aged; however, it increases the susceptibility of the material to intergranular corrosion. If used, it should be limited to clad sheet and similar items.

Quenching

The basic purpose for quenching is to prevent the immediate re-precipitation of the soluble constituents after heating to solid solution. 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. There are three methods employed for quenching. The one used depends upon the item, alloy, and properties desired.

COLD WATER QUENCHING. Small parts made from sheet, extrusions, tubing, and small fairings are normally quenched in cold water. The temperature before quenching should be 85F or less. Sufficient cold water should be circulated within the quenching tanks to keep the temperature rise under 20F. This type of quench will ensure good resistance to corrosion, and is particular y important when heat-treating 2017 and 2024 alloys.

HOT WATER QUENCHING. Large forgings and heavy sections can be quenched in hot 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.

SPRAY QUENCHING. Water sprays are used to quench parts formed from alclad sheets and large sections of most alloys. Principal reasons for using this method are 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.