Aluminum

Though light in weight, commercially pure aluminum has a tensile strength of about 13,000 psi. Cold working the metal approximately doubles its strength. In other attempts to increase strength, aluminum is alloyed with elements such as manganese, silicon, copper, magnesium, or zinc. The alloys can also be strengthened by cold working. Some alloys are further strengthened and hardened by heat treatments. At subzero temperatures, aluminum is stronger than at room temperature and is no less ductile. Most aluminum alloys lose strength at elevated temperatures, although some retain significant strength to 500F. Besides a high strength-to-weight ratio and good formability, aluminum also possesses its own anticorrosion mechanism. When exposed to air, aluminum does not oxidize progressively because a hard, microscopic oxide coating forms on the surface and seals the metal from the environment. The tight chemical oxide bond is the reason that aluminum is not found in nature; it exists only as a compound.

Aluminum and its alloys, numbering in the hundreds, are available in all common commercial forms. Aluminum-alloy sheet can be formed, drawn, stamped, or spun. Many wrought or cast aluminum alloys can be welded, brazed, or soldered, and aluminum surfaces readily accept a wide variety of finishes, both mechanical and chemical. Because of their high electrical conductivity, aluminum alloys are used as electrical conductors. Aluminum reflects radiant energy throughout the entire spectrum, is nonsparking, and nonmagnetic.

Wrought aluminum: A four-digit number that corresponds to a specific alloying element combination usually designates wrought aluminum alloys. This number is followed by a temper designation that identifies thermal and mechanical treatments.

To develop strength, heat-treatable wrought alloys are solution heat treated, then quenched and precipitation hardened. Solution heat treatment consists of heating the metal, holding at temperature to bring the hardening constituents into solution, then cooling to retain those constituents in solution. Precipitation hardening after solution heat treatment increases strength and hardness of these alloys.

While some alloys age at room temperature, others require precipitation heat treatment at an elevated temperature (artificial aging) for optimum properties. However, distortion and dimensional changes during natural or artificial aging can be significant. In addition, distortion and residual stresses can be introduced during quenching from the solution heat-treatment cycle. These induced changes can be removed by deforming the metal (for example, stretching).

Wrought aluminum alloys are also strengthened by cold working. The high-strength alloys — either heat treatable or not — work harden more rapidly than the lower-strength, softer alloys and so may require annealing after cold working. Because hot forming does not always work harden aluminum alloys, this method is used to avoid annealing and straightening operations; however, hot forming fully heat-treated materials is difficult. Generally, aluminum formability increases with temperature.

Recently developed aluminum alloys can provide nearly custom-engineered strength, fracture toughness, fatigue resistance, and corrosion resistance for aircraft forgings and other critical components. The rapid-solidification process is the basis for these new alloy systems, called wrought P/M alloys. The term wrought P/M is used to distinguish this technology from conventional press-and-sinter P/M technology. Grades 7090 and 7091 are the first commercially available wrought P/M aluminum alloys. These alloys can be handled like conventional aluminum alloys on existing aluminum-fabrication facilities.

Other significant new materials are the aluminum-lithium alloys. These lightweight metals are as strong as alloys now in use and can be fabricated on existing metalworking equipment. Although impressive structural weight reductions (from 7 to 10%) are possible through direct substitution, even greater reduction (up to 15%) can be realized by developing fully optimized alloys for new designs. Such alloys would be specifically tailored to provide property combinations not presently available. Producing an alloy that will provide these combinations is the object of second and third-generation low-density alloy development efforts.

Operating economy is still an important consideration in vehicle design despite fluctuating fuel prices. Downsizing to save fuel has reached its practical limits; now, reducing the weight of individual components is taking over. One significant change being implemented by designers of automobiles and military vehicles today is converting driveshafts, radiators, cylinder heads, suspension members, and other structural components to aluminum.

Cast aluminum: Aluminum can be cast by all common casting processes. Aluminum casting alloys are identified with a unified, four-digit (xxx.x) system. The first digit indicates the major alloying element. For instance, 100 series is reserved for 99% pure aluminum with no major alloying element used. The second and third digits in the 100 series indicate the precise minimum aluminum content. For example, 165.0 has a 99.65% minimum aluminum content. The 200-900 series designate various aluminum alloys, with the second two digits assigned to new alloys as they are registered. The fourth digit indicates the product form. Castings are designated 0; ingots are designed 1 or 2.

Letter prefixes before the numerical designation indicate special control of one or more elements or a modification of the original alloy. Prefix X designates an experimental composition. The material may retain the experimental designation up to five years. Limits for the experimental alloy may be changed by the registrant.

Commercial casting alloys include heat-treatable and nonheat-treatable compositions. Alloys that are heat treated carry the temper designations 0, T4, T5, T6, and T7. Die castings are not usually solution heat treated because the temperature can cause blistering.

Permanent-mold casting technology involves several variations having to do with how the metal gets into the mold cavity. Initially, molds were simply gravity filled from ladles, in the same manner as sand molds. Subsequently, low pressure on the liquid-metal surface of a crucible was used to force the metal up, through a vertical tube, into the mold cavity. This refinement produces castings with higher mechanical properties and is more economical than gravity filling because extensive gates and risers are unnecessary.

More recently, the process was modified to use a low level of vacuum drawn on the mold cavity, causing atmospheric pressure to force the molten metal up into the mold. This process variation, together with controlled and rapid solidification, increases properties further because it produces castings that are almost entirely free of porosity.

Although both variations improve properties and speed casting cycles, the added equipment complexities limit the casting size that can be handled. Consequently, all three permanent-mold processes are in use today, turning out aluminum castings weighing from less than one pound to several hundred pounds.

Aluminum matrix composites: Metal matrix composites (MMCs) consist of metal alloys reinforced with fibers, whiskers, particulates, or wires. Alloys of numerous metals (aluminum, titanium, magnesium and copper) have been used as matrices to date.

Recent MMC developments, however, seem to thrust aluminum into the spotlight. In the NASA space shuttle, for example, 240 struts are made from aluminum reinforced with boron fibers. Also, aluminum diesel-engine pistons that have been locally reinforced with ceramic fibers are eliminating the need for wear-resistant nickel-cast iron inserts in the automotive environment.

Fabrication methods differ for both products. Monolayer tapes in the space shuttle struts are wrapped around a mandrel and hot isostatically pressed to diffusion bond the layers. For the pistons, a squeeze-casting process infiltrates liquid metal into a fiber preform under pressure. Other fabrication methods for MMCs include: hot pressing a layer of parallel fibers between foils to create a monolayer tape; creep and superplastic forming in a die; and spraying metal plasmas on collimated fibers followed by hot pressing.

Superplastic aluminum: Superplastic forming of metal, a process similar to vacuum forming of plastic sheet, has been used to form low-strength aluminum into nonstructural parts such as cash-register housings, luggage compartments for passenger trains, and nonload-bearing aircraft components. New in this area of technology is a superplastic-formable high-strength aluminum alloy, now available for structural applications and designated 7475-02. Strength of alloy 7475 is in the range of aerospace alloy 7075, which requires conventional forming operations. Although initial cost of 7475 is higher, finished part cost is usually lower than that of 7075 because of the savings involved in the simplified design/assembly.