7075 aluminium alloy

Aluminium alloy 7075 is an aluminium alloy, with zinc as the primary alloying element. It is strong, with a strength comparable to many steels, and has good fatigue strength and average machinability, but has less resistance to corrosion than many other Al alloys. Its relatively high cost limits its use to applications where cheaper alloys are not suitable.

7075 aluminum alloy's composition roughly includes 5.6–6.1% zinc, 2.1–2.5% magnesium, 1.2–1.6% copper, and less than a half percent of silicon, iron, manganese, titanium, chromium, and other metals. It is produced in many tempers, some of which are 7075-0, 7075-T6, 7075-T651.

Basic properties

Aluminium 7075A has a density of 2.810 g/cm³^[1] (0.1015 lb/in³).

Mechanical properties

The mechanical properties of 7075 depend greatly on the temper of the material.^[2]

7075-0

Un-heat-treated 7075 (7075-0 temper) has maximum tensile strength no more than 40,000 psi (275 MPa), and maximum yield strength no more than 21,000 psi (145 MPa). The material has an elongation (stretch before ultimate failure) of 9–10%. It is very highly corrosion-resistant and has good strength.

7075-T6

T6 temper 7075 has an ultimate tensile strength of 74,000–78,000 psi (510–572 MPa) and yield strength of at least 63,000–69,000 psi (434–503 MPa). It has a failure elongation of 5–11%.^[3]

The T6 temper is usually achieved by homogenizing the cast 7075 at 450 °C for several hours, quenching, and then aging at 120 °C for 24 hours. This yields the peak strength of the 7075 alloy. The strength is derived mainly from finely dispersed eta and eta' precipitates both within grains and along grain boundaries.^[4]

7075-T651

T651 temper 7075 has an ultimate tensile strength of 83000 psi (572 MPa) and yield strength of 73000 psi (503 MPa). It has a failure elongation of 3–9%. These properties can change depending on the form of material used. Thicker plate may exhibit lower strengths and elongation than the numbers listed above.

The 51 suffix has no bearing on the heat treatment but denotes that the material is stress relieved by controlled stretching.

7075-T7

T7 temper has an ultimate tensile strength of 73,200 psi (505 MPa) and a yield strength of 63,100 psi (435 MPa). It has a failure elongation of 13%.^[5] T7 temper is achieved by overaging (meaning aging past the peak hardness) the material. This is often accomplished by aging at 100–120 °C for several hours and then at 160–180 °C for 24 hours or more. The T7 temper produces a micro-structure of mostly eta precipitates. In contrast to the T6 temper, these eta particles are much larger and prefer growth along the grain boundaries. This reduces the susceptibility to stress corrosion cracking. T7 temper is equivalent to T73 temper.^[4]

7075-RRA

The retrogression and reage (RRA) temper is a multistage heat treatment temper. Starting with a sheet in the T6 temper, it involves overaging past peak hardness (T6 temper) to near the T7 temper. A subsequent reaging at 120 °C for 24 hours returns the hardness and strength to or very nearly to T6 temper levels.^[4]

RRA treatments can be accomplished with many different procedures. The general guidelines are retrogressing between 180–240 °C for 15 min 10 s.^[6]

Uses

7000 series alloys such as 7075 are often used in transport applications, including marine, automotive and aviation, due to their high strength-to-density ratio.^[2][7] Their strength and light weight is also desirable in other fields. Rock climbing equipment, bicycle components, inlineskating-frames and hang glider airframes are commonly made from 7075 aluminium alloy. Hobby grade RC models commonly use 7075 and 6061 for chassis plates. 7075 is used in the manufacture of M16 rifles for the American military. In particular high quality M16 rifle lower and upper receivers as well as extension tubes are typically made from 7075-T6 alloy. Desert Tactical Arms, SIG Sauer, and French armament company PGM use it for their precision rifles. It is also commonly used in shafts for lacrosse sticks, such as the STX sabre, and camping knife and fork sets. It is a common material used in competition yo-yos as well.

Due to its high strength, low density, thermal properties, and its ability to be highly polished, 7075 is widely used in mold tool manufacture. This alloy has been further refined into other 7000 series alloys for this application, namely 7050 and 7020.

History

The first 7075 was developed in secret by a Japanese company, Sumitomo Metal, in 1943.^[8] 7075 was eventually used for airframe production in the Imperial Japanese Navy.

Trade names

7075 has been sold under various trade names including Zicral, Ergal, and Fortal Constructal.

Some 7000 series alloys sold under brand names for making molds include Alumec 79, Alumec 89, Contal, Certal, Alumould, and Hokotol.

References

Further reading

• "Properties of Wrought Aluminum and Aluminum Alloys: 7075, Alclad 7075", Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Vol. 2, ASM Handbook, ASM International, 1990, pp. 115–116.