Commercially pure aluminum has a tensile strength of about 13,000 psi, but rolling or other cold-working processes may approximately double its strength. By alloying with other metals, or by using heat-treating processes, the tensile strength may be raised to as high as 65,000 psi or to within the strength range of structural steel.

Aluminum alloys, although strong, are easily worked because they are malleable and ductile. They 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 range from 0.016 to 0.096 inch in thickness; however, some of the larger aircraft use sheet stock that may be as thick as 0.356 inch.

The various types of aluminum may be divided into two general classes:

Of these two, the wrought alloys are the most widely used in aircraft construction, being used for stringers, bulkheads, skin, rivets, and extruded sections.

Aluminum casting alloys are divided into two basic groups. In one, the physical properties of the alloys are determined by the alloying elements and cannot be changed after the metal is cast. In the other, the alloying elements make it possible to heat-treat the casting to produce the desired physical properties.

A letter preceding the alloy number identifies the casting alloys. When a letter precedes a number, it indicates a slight variation in the composition of the original alloy. This variation in composition is simply to impart some desirable quality. For example, in casting alloy 214, the addition of zinc to improve its pouring qualities is indicated by the letter A in front of the number, thus creating the designation A214.

When castings have been heat-treated, the heat-treatment and the composition of the casting is indicated by the letter T, followed by an alloying number. An example of this is the sand casting alloy 355, which has several different compositions and tempers and is designated by 355-T6, 355-T51, or C355-T51.

Aluminum alloy castings are produced by one of three basic methods: sand mold, permanent mold, or die cast. In casting aluminum, it is important to note that in most cases different types of alloys must be used for different types of castings. Sand castings and die-castings require different types of alloys than those used in permanent molds.

Sand and permanent mold castings are parts produced by pouring molten metal into a previously prepared mold, allowing the metal to solidify or freeze and then removing the part. If the mold is made of sand, the part is a sand casting; if it is a metallic mold (usually cast iron), the part is a permanent mold casting. Sand and permanent castings are produced by pouring liquid metal into the mold, the metal flowing under the force of gravity alone.

The two principal types of sand casting alloys are 112 and

The permanent mold process is a later development of the sand casting process, the major difference being in the material from which the molds are made. The advantage of this process is that there are fewer openings (called porosity) than in sand castings. The sand and the binder, which is mixed with the sand to hold it together, give off a certain amount of gas, that causes porosity in a sand casting.

Permanent mold castings are used to obtain higher mechanical properties, better surfaces, or more accurate dimensions. There are two specific types of permanent mold castings: permanent metal mold with metal cores, and semi-permanent types containing sand cores. Because finer grain structure is produced in alloys subjected to the rapid cooling of metal molds, they are far superior to the sand type castings. Alloys 122, A132, and 142 are commonly used in permanent mold castings, the principal uses of which are in internal combustion engines.

Die-castings used in aircraft are usually aluminum or magnesium alloy. If weight is of primary importance, magnesium alloy is used, because it is lighter than aluminum alloy. However, aluminum alloy is frequently used because it is stronger than most magnesium alloys.

Forcing molten metal under pressure into a metallic die and allowing it to solidify produces a die-casting; then the die is opened and the part removed. The basic difference between permanent mold casting and die-casting is that in the permanent mold process, the metal flows into the die under gravity. In the die-casting operation, the metal is forced under great pressure.

Die-castings are used where relatively large production of a given part is involved. Remember, any shape that can be forged, can be cast.

Wrought aluminum and wrought aluminum alloys are divided into two general classes: non-heat-treatable alloys and heat-treatable alloys.