General Properties of Aluminum and Aluminum Alloys


Aluminum is the most widely used nonferrous metal, being second only to steel in world consumption. Aluminum is produced in both “pure” and alloyed forms. Aluminum is commercially available up to 99.8% pure. The most common alloying elements are copper, silicon, magnesium, manganese, and zinc, in varying amounts up to about 5%. The principal advantages of aluminum are its low density, good strength-to-weight ratio (SWR), ductility, excellent workability, castability, and weldability, corrosion resistance, high conductivity, and reasonable cost. Compared to steel it is 1/3 as dense (0.10 lb/in3 versus 0.28 lb/in3), about 1/3 as stiff (E = 10.3 Mpsi {71 GPa} versus 30 Mpsi {207 GPa}), and generally less strong. If you compare the strengths of low-carbon steel and pure aluminum, the steel is about three times as strong. Thus the specific strength is approximately the same in that comparison. However, pure aluminum is seldom used in engineering applications. It is too soft and weak. Pure aluminum’s principal advantages are its bright finish and good corrosion resistance. It is used mainly in decorative applications.

The aluminum alloys have significantly greater strengths than pure aluminum and are used extensively in engineering, with the aircraft and automotive industries among the largest users. The higher-strength aluminum alloys have tensile strengths in the 70 to 90 kpsi (480 to 620 MPa) range, and yield strengths about twice that of mild steel. They compare favorably to medium-carbon steels in specific strength. Aluminum competes successfully with steel in some applications, though few materials can beat steel if very high strength is needed. Figure 1 shows tensile-test engineering stress-strain curves for three aluminum alloys. Aluminum’s strength is reduced at low temperatures as well as at elevated temperatures.
Figure 1: Tensile Test Stress-Strain Curves for Three Aluminum Alloys

Some aluminum alloys are hardenable by heat treatment and others by strain hardening or precipitation and aging. High-strength aluminum alloys are about 1.5 times harder than soft steel, and surface treatments such as hard anodizing can bring the surface to a condition harder than the hardest steel.

Aluminum is among the most easily worked of the engineering materials, though it tends to work harden. It casts, machines, welds (The heat of welding causes localized annealing, which can remove the desirable strengthening effects of cold work or heat treatment in any metal), and hot and cold forms easily. It can also be extruded. Alloys are specially formulated for both sand and die casting as well as for wrought and extruded shapes and for forged parts.

WROUGHT-ALUMINUM ALLOYS are available in a wide variety of stock shapes such as I-beams, angles, channels, bars, strip, sheet, rounds, and tubes. Extrusion allows relatively inexpensive custom shapes as well. The Aluminum Association numbering system for alloys is shown in Table 1. The first digit indicates the principal alloying element and defines the series. Hardness is indicated by a suffix containing a letter and up to 3 numbers as defined in the table. The most commonly available and most used aluminum alloys in machine-design applications are the 2000 and 6000 series.

The oldest aluminum alloy is 2024, which contains 4.5% copper, 1.5% magnesium, and 0.8% manganese. It is among the most machinable of the aluminum alloys and is heat treatable. In the higher tempers, such as -T3 and -T4, it has a tensile strength approaching 70 kpsi (483 MPa), which also makes it one of the strongest of the aluminum alloys. It also has high fatigue strength. However, it has poor weldability and formability compared to the other aluminum alloys.
The 6061 alloy contains 0.6% silicon, 0.27% copper, 1.0% manganese, and 0.2% chromium. It is widely used in structural applications because of its excellent weldability. Its strength is about 40 to 45 kpsi (276 to 310 MPa) in the higher tempers. It has lower fatigue strength than 2024 aluminum. It is easily machined and is a popular alloy for extrusion, which is a hot-forming process.
The 7000 series is called aircraft aluminum and is used mostly in airframes. These are the strongest alloys of aluminum with tensile strengths up to 98 kpsi (676 MPa) and the highest fatigue strength of about 22 kpsi (152 MPa) @ 10^8 cycles. Some alloys are also available in an alclad form which bonds a thin layer of pure aluminum to one or both sides to improve corrosion resistance. 
Table 1: Aluminum Association Designations of Aluminum Alloys

CAST-ALUMINUM ALLOYS are differently formulated than the wrought alloys. Some of these are hardenable but their strength and ductility are less than those of the wrought alloys. Alloys are available for sand casting, die casting, or investment casting.

Titanium Though discovered as an element in 1791, commercially produced titanium has been available only since the 1940s, so it is among the newest of engineering metals. Titanium can be the answer to an engineer’s prayer in some cases. It has an upper servicetemperature limit of 1 200 to 1400°F (650 to 750°C), weighs half as much as steel (0.16 lb/in3 {4429 kg/m3}), and is as strong as a medium-strength steel (135 kpsi {930 MPa} typical). Its Young’s modulus is 16 to 18 Mpsi (110 to 124 GPa), or about 60% that of steel. Its specific strength approaches that of the strongest alloy steels and exceeds that of medium-strength steels by a factor of 2. Its specific stiffness is greater than that of steel, making it as good or better in limiting deflections. It is also nonmagnetic. Titanium is very corrosion resistant and is nontoxic, allowing its use in contact with acidic or alkaline foodstuffs and chemicals, and in the human body as replacement heart valves and hip joints, for example. Unfortunately, it is expensive compared to aluminum and steel. It finds much use in the aerospace industry, especially in military aircraft structures and in jet engines, where strength, light weight, and high temperature and corrosion resistance are all required. Tables bellow show mechanical properties of some aluminum alloys.

Table 2: Mechanical Properties of Some Aluminum Casting Alloys

Table 3: Mechanical Properties of some Wrought Aluminum Alloys