Aluminum Alloys


Of the various groups of alloys, the aluminum alloys are most readily forged into precise, intricate shapes. The five most significant reasons are:

     •     They are very ductile at optimum forging temperatures
     •     They can be forged in steel dies that can typically be heated to almost the same temperature as the workpiece
     •     They do not develop scale during heating or forging
     •     They require low forging pressures
     •     They may be forged at high or low strain rates

Aluminum alloys are designated by the four-digit numerical system that is an industry standard for wrought alloys. The numbers are systematically assigned, but do not have any quantitative significance. The first digit indicates the major alloying element, and the last three distinguish the various alloys in the group. For example, the major alloying element in the 2xxx series is copper, in the 5xxx series magnesium, in the 6xxx series magnesium and silicon, and in the 7xxx series zinc.

The major factors influencing the forgeability of aluminum alloys are the solidus temperature and deformation rate. Most of the alloys are forged at approximately 55°C (100°F) below the corresponding solidus temperature.

Most aluminum alloys can be forged in any type of equipment that is used for other metals. However, some grades, such as the 7xxx series, are more deformation rate sensitive and tend to have reduced forgeability when deformation rates are high. Therefore these grades require special care when forged on hammers and high speed presses. Good, continuous lubrication is required with all aluminum grades because the alloys do not form scale and will seize, gall or cause pressure welding to the die steel if they come in direct contact with it.

Some grades of the 5xxx series may be used in the as-forged condition. All other alloys are generally strengthened slightly in the hot-working range, then subjected to solution heat treating, quenching (usually in water or water-based synthetic quenchants) and subsequently aging at temperatures between 120 and 175°C (250 and 350°F). The aging processes vary from the as-quenched condition to normal aging (T6), or to overaging (T7x), which is done to enhance the stress-corrosion and impact toughness properties with some loss of strength.

Another series of supplemental temper designations denotes when small compressive or tensile deformations are imparted to the forgings after solution treatment. In this case, the supplemental designations are often of a three digit variety (e.g. T-652, T-651). This practice, while adding operations, and hence cost, improves resistance to stress corrosion cracking while not reducing the strength of the forgings.

The strength properties of aluminum alloys are affected by alloy composition, forging process variables and the final heat treatment. Corrosion resistance is affected primarily by alloy composition and the final aging cycle. For example, the 2xxx series, with significant amounts of copper, are generally more prone to atmospheric corrosion, pitting, stress corrosion and galvanic reactions than are zinc-magnesium 7xxx alloys with very low levels of copper.

Aluminum forging alloys 2xxx and 7xxx are used extensively in aerospace applications and airframe structures, due to their favorable high fatigue strength and low density. The 2xxx, 5xxx and 6xxx grades are selected for automotive and piping applications, with the 6xxx grades specified where superior resistance to corrosion is required.

Additional information about the remaining alloys in this category can be obtained from:

Aluminum Association
900 19th Street N.W.
Washington, D.C., 20006