The casting alloys are identified by a letter preceding the alloy number. This is exactly opposite from the case of wrought alloys, in which the letters follow the number. When a letter precedes a number, it indicates a slight variation in the composition of the original alloy. This variation in composition is made simply to impart some desirable quality. In casting alloy 214, for example, the addition of zinc, to increase its pouring qualities, is designated by the letter A in front of the number, thus creating the designation A214. When castings have been treated, the heat treatment and the composition of the casting are indicated by the letter T and 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, and A355-T51.

Aluminum alloy castings are produced by one of three basic methods-sand mold, permanent mold, and die cast. In casting aluminum, 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.

SHOP CHARACTERISTICS OF ALUMINUM ALLOYS. workable of all the common commercial metals. It can be fabricated readily into a variety of shapes by any conventional method; however, formability varies a great deal with the alloy and temper.

In general, the aircraft manufacturers form the heat-treatable alloys in the -0 or -T4 condition before they have reached their full strength. They are subsequently heat-treated or aged to the maximum strength (-T6) condition before installation in aircraft. By this combination of processes, the advantage of forming in a soft condition is obtained without sacrificing the maximum obtainable strength/weight ratio.

Aluminum is one of the most readily weldable of all metals. The nonheat-treatable alloys can be welded by all methods, and the heat-treatable alloys can be successfully spot welded. The melting point for pure aluminum is 1,216F, while various aluminum alloys melt at slightly lower temperatures. Aluminum products do not show any color changes when heated, even up to the melting point. Riveting is the most reliable method of joining stress-carrying parts of heat-treated aluminum alloy structures.