In the wrought form, commercially pure aluminum is known as 1100. It has a high degree of resistance to corrosion and is easily formed into intricate shapes. It is relatively low in strength and does not have the properties required for structural aircraft parts. High strengths are generally obtained by the process of alloying. The resulting alloys are less easily formed and, with some exceptions, have lower resistance to corrosion than 1100 aluminum.
Alloying is not the only method of increasing the strength of aluminum. Like other materials, aluminum becomes stronger and harder as it is rolled, formed, or otherwise cold worked. Since the hardness depends on the amount of cold working done, 1100 and some wrought aluminum alloys are available in several strain hardened tempers. The soft or annealed condition is designated O. If the material is strain hardened, it is said to be in the H condition.
The most widely used alloys in aircraft construction are hardened by heat treatment rather than by cold work. These alloys are designated by a somewhat different set of symbols: T4 and W indicate solution heat treated and quenched but not aged, and T6 indicates an alloy in the heat treated hardened condition.
- W — Solution heat treated, unstable temper
- T — Treated to produce stable tempers other than F, O, or H
- T2 — Annealed (cast products only)
- T3 — Solution heat treated and then cold worked
- T4 — Solution heat treated
- T5 — Artificially aged only
- T6 — Solution heat treated and then artificially aged
- T7 — Solution heat treated and then stabilized
- T8 — Solution heat treated, cold worked, and then artificially aged
- T9 — Solution heat treated, artificially aged, and then cold worked
- T10 — Artificially aged and then cold worked
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 heattreatable 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 will turn black due to the copper content, whereas the others will remain bright. In the case of clad material, the surface will remain bright, but there will be 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 5 1⁄2 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. One is called solution heat treatment, and the other is known as 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:
- Heating to a predetermined temperature.
- Soaking at temperature for a specified length of time.
- Rapidly quenching to a relatively low temperature.
- Aging or precipitation hardening either spontaneously at room temperature, or as a result of a low temperature thermal treatment.
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.