Heat Treatment of Nonferrous Metals

Aluminum Alloys

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.