Commercially pure aluminum is a white, lustrous metal, light in weight and corrosion resistant. Aluminum combined with various percentages of other metals (generally copper, manganese, magnesium, and chromium) form the alloys that are used in aircraft construction. Aluminum alloys in which the principal alloying ingredients are either manganese, magnesium, or chromium, or magnesium and silicon, show little attack in corrosive environments. On the other hand, those alloys in which substantial percentages of copper are used are more susceptible to corrosive action. The total percentage of alloying elements is seldom more than 6 or 7 percent in the wrought aluminum alloys.

TYPES, CHARACTERISTICS, AND USES. -Aluminum is one of the most widely used metals in modern aircraft construction. It is vital to the aviation industry because of its high strength/weight ratio, its corrosion-resisting qualities, and its comparative ease of fabrication. The outstanding characteristic of aluminum is its light weight. In color, aluminum resembles silver, although it possesses a characteristic bluish tinge of its own. Commercially pure aluminum melts at the comparatively low temperature of 1,216F. It is nonmagnetic, and is an excellent conductor of electricity.

Commercially pure aluminum has a tensile strength of about 13,000 psi, but by rolling or other cold-working processes, its strength may be approximately doubled. By alloying with other metals, together with the use of heat-treating processes, the tensile strength may be raised to as high as 96,000 psi, or to well within the strength range of structural steel.

Aluminum alloy material, although strong, is easily worked, for it is very malleable and ductile. It may be rolled into sheets as thin as 0.0017 inch or drawn into wire 0.004 inch in diameter. Most aluminum alloy sheet stock used in aircraft construction ranges from 0.016 to 0.096 inch in thickness; however, some of the larger aircraft use sheet stock that may be as thick as 0.0356 inch.

One disadvantage of aluminum alloy is the difficulty of making reliable soldered joints. Oxidation of the surface of the heated metal prevents soft solder from adhering to the material; therefore, to produce good joints of aluminum alloy, a riveting process is used. Some aluminum alloys are also successfully welded.

The various types of aluminum maybe divided into two classes-casing alloys (those suitable for casting in sand, permanent mold, and die castings) and the wrought alloys (those that may be shaped by rolling, drawing, or forging). Of the two, the wrought alloys are the most widely used in aircraft construction, being used for stringers, bulkheads, skin, rivets, and extruded sections. Casting alloys are not extensively used in aircraft.

WROUGHT ALLOYS. -Wrought alloys are divided into two classes-nonheat treatable and heat treatable. In the nonheat-treatable class, strain hardening (cold-working) is the only means of increasing the tensile strength. Heat-treatable alloys may be hardened by heat treatment, by cold-working, or by the application of both processes.

Aluminum products are identified by a universally used designation system. Under this arrangement, wrought aluminum and wrought aluminum alloys are designated by a four-digit index system.

The first digit of the designation indicates the major alloying element or alloy group, as shown in table 1-2. The lxxx indicates aluminum of 99.00 percent or greater; 2xxx indicates an aluminum alloy in which copper is the major alloying element; 3xxx indicates an aluminum alloy with manganese as the major alloying element; etc. Although most aluminum alloys contain several alloying elements, only one group (6xxx) designates more than one alloying element.

In the 1xxx group, the second digit in the designation indicates modifications in impurity limits. If the second digit is zero, it indicates that there is no special control on individual impurities. The last two of the four digits indicate the minimum aluminum percentage. Thus, alloy 1030 indicates 99.30 percent aluminum without special control on impurities. Alloys 1130, 1230, 1330, etc., indicate the same aluminum purity with special control on one or more impurities. Likewise, 1075, 1175, 1275, etc., indicate 99.75 percent aluminum.

Table 1-2.-Designations for Aluminum Alloy Groups

In the 2xxx through 8xxx groups, the second digit indicates alloy modifications. If the second digit in the designation is zero, it indicates the original alloy, while numbers 1 through 9, assigned consecutively, indicate alloy modifications. The last two of the four digits have no special significance, but serve only to identify the different alloys in the group.

The temper designation follows the alloy designation and shows the actual condition of the metal. It is always separated from the alloy designation by a dash.

The letter F following the alloy designation indicates the "as fabricated condition, in which no effort has been made to control the mechanical properties of the metal,

The letter O indicates dead soft, or annealed, condition.

The letter W indicates solution heat treated. Solution heat treatment consists of heating the metal to a high temperature followed by a rapid quench in cold water,

This in an unstable temper, applicable only to those alloys that spontaneously age at room temperature, Alloy 7075 may be ordered in the W condition.

The letter H indicates strain hardened, cold-worked, hand-drawn, or rolled. Additional digits are added to the H to indicate the degree of strain hardening. Alloys in this group cannot be strengthened by heat treatment, hence the term nonheat-treatable.

The letter T indicates fully heat treated. Digits are added to the T to indicate certain variations in treatment.

Greater strength is obtainable in the heat-treatable alloys. They are often used in aircraft in preference to the nonheat-treatable alloys. Heat-treatable alloys commonly used in aircraft construction (in order of increasing strength) are 6061, 6062, 6063, 2017, 2024, 2014,7075, and 7178.

Alloys 6061, 6062, and 6063 are sometimes used for oxygen and hydraulic lines and in some applications as extrusions and sheet metal.

Alloy 2017 is used for rivets, stressed-skin covering, and other structural members.

Alloy 2024 is used for airfoil covering and fittings. It may be used wherever 2017 is specified, since it is stronger.

Alloy 2014 is used for extruded shapes and forgings. This alloy is similar to 2017 and 2024 in that it contains a high percentage of copper. It is used where more strength is required than that obtainable from 2017 or 2024.

Alloy 7178 is used where highest strength is necessary, Alloy 7178 contains a small amount of chromium as a stabilizing agent, as does alloy 7075.

Nonheat-treatable alloys used in aircraft construction are 1100, 3003, and 5052. These alloys do not respond to any heat treatment other than a softening, annealing effect. They may be hardened only by cold- working.

Alloy 1100 is used where strength is not an important factor, but where weight, economy, and corrosion resistance are desirable. This alloy is used for fuel tanks, fairings, oil tanks, and for the repair of wing tips and tanks.

Alloy 3003 is similar to 1100 and is generally used for the same purposes. It contains a small percentage of manganese and is stronger and harder than 1100, but retains enough work ability that it is usually preferred over 1100 in most applications.

Alloy 5052 is used for fuel lines, hydraulic lines, fuel tanks, and wing tips. Substantially higher strength without too much sacrifice of workability can be obtained in 5052. It is preferred over 1100 and 3003 in many applications. 

Alclad is the name given to standard aluminum alloys that have been coated on both sides with a thin layer of pure aluminum. Alclad has very good corrosion-resisting qualities and is used exclusively for exterior surfaces of aircraft. Alclad sheets are available in all tempers of 2014, 2017, 7075, and 7178.