Additional digits may be added to T1 through T10 to indicate a variation in treatment, which significantly alters the characteristics of the product.

Aluminum-alloy sheets are marked with the specification number on approximately every square foot of material. If for any reason this identification is not on the material, it is possible to separate the heat-treatable alloys from the non-heat-treatable alloys by immersing a sample of the material in a 10 percent solution of caustic soda (sodium hydroxide). The heat-treatable alloys turn black due to the copper content, whereas the others remain bright. In the case of clad material, the surface remains bright, but there is a dark area in the middle when viewed from the edge.

The terms “Alclad and Pureclad” are used to designate sheets that consist of an aluminum-alloy core coated with a layer of pure aluminum to a depth of approximately 51⁄2 percent on each side. The pure aluminum coating affords a dual protection for the core, preventing contact with any corrosive agents, and protecting the core electrolytically by preventing any attack caused by scratching or from other abrasions.

There are two types of heat-treatments applicable to aluminum alloys: solution heat-treatment and precipitation heat-treatment. Some alloys, such as 2017 and 2024, develop their full properties as a result of solution heat-treatment followed by about 4 days of aging at room temperature. Other alloys, such as 2014 and 7075, require both heat-treatments.

The alloys that require precipitation heat-treatment (artificial aging) to develop their full strength also age to a limited extent at room temperature; the rate and amount of strengthening depends upon the alloy. Some reach their maximum natural or room temperature aging strength in a few days, and are designated as –T4 or –T3 temper. Others continue to age appreciably over a long period of time.

Because of this natural aging, the –W designation is specified only when the period of aging is indicated, for example, 7075–W (1⁄2 hour). Thus, there is considerable difference in the mechanical and physical properties of freshly quenched (–W) material and material that is in the –T3 or –T4 temper. The hardening of an aluminum alloy by heat-treatment consists of four distinct steps:

The first three steps above are known as solution heat-treatment, although it has become common practice to use the shorter term, “heat-treatment.” Room temperature hardening is known as natural aging, while hardening done at moderate temperatures is called artificial aging, or precipitation heat-treatment.

The temperatures used for solution heat-treating vary with

to obtain specified properties.

If the temperature is too low, maximum strength is not obtained. When excessive temperatures are used, there is danger of melting the low melting constituents of some alloys with consequent lowering of the physical properties of the alloy. Even if melting does not occur, the use of higher than recommended temperatures promotes discoloration and increases quenching strains.

The time at temperature, referred to as soaking time, is measured from the time the coldest metal reaches the minimum limit of the desired temperature range. The soaking time varies, depending upon the alloy and thickness, from 10 minutes for thin sheets to approximately 12 hours for heavy forgings. For the heavy sections, the nominal soaking time is approximately 1 hour for each inch of cross-sectional thickness. [Figure 7-7]

Choose the minimum soaking time necessary to develop the required physical properties. The effect of an abbreviated soaking time is obvious. An excessive soaking period aggravates high-temperature oxidation. With clad material, prolonged heating results in excessive diffusion of copper and other soluble constituents into the protective cladding and may defeat the purpose of cladding.