Alloy 7075 and What a Temper Actually Changes
7075 is an Al-Zn-Mg-Cu alloy: zinc 5.1-6.1%, magnesium 2.1-2.9%, copper 1.2-2.0%, chromium 0.18-0.28%, with iron and silicon held low. The chemistry is fixed by the alloy specification, so the temper is the only variable a forging buyer can use to move strength, toughness and corrosion behaviour. Every temper starts from the same solution heat treatment, typically 465-480 C followed by a quench, which dissolves the strengthening phases. What differs is the ageing treatment afterwards and, for the stress-relieved tempers, the controlled deformation applied between quenching and ageing.
The Four Tempers and Their Heat Treatment Routes
T6 is a single-stage artificial age at roughly 120 C: it produces the highest strength of the family and it is the temper most often specified for heavily loaded fittings. T73 uses a two-stage overageing treatment that deliberately coarsens the grain boundary precipitates, giving up some strength in exchange for resistance to stress corrosion cracking in the short transverse direction. T7352 is the same ageing treatment applied to a forging that has been stress relieved by compression after quenching, which reduces residual stress and improves dimensional stability during machining. T74 is an intermediate overageing schedule developed for thick sections, where it provides corrosion performance close to T73 with higher strength than T73 can deliver at that section size.
Strength and Corrosion Comparison
The values below are typical certified properties for 7075 forgings and plate; the controlling specification for each temper must be quoted on the purchase order, since minimum values are stated there per product form and thickness.
| Temper | Typical tensile strength (MPa) | Typical 0.2% proof stress (MPa) | Elongation (%) | Stress corrosion behaviour |
|---|---|---|---|---|
| 7075-T6 | about 570 | about 505 | about 11 | Sensitive in the short transverse direction |
| 7075-T73 | about 505 | about 435 | about 12 | High resistance |
| 7075-T7352 | about 505 | about 435 | about 12 | High resistance, low residual stress |
| 7075-T74 | about 525 | about 455 | about 11 | Good resistance in thick sections |
Fracture toughness should be ordered on the basis of the applicable test method rather than assumed from the strength table, because overageing raises toughness while lowering strength, and because toughness falls as section thickness increases.
Specifications Used on Forging Purchase Orders
AMS 4126 covers 7075-T6 forgings and rolled rings, AMS 4141 covers 7075-T73 die forgings, AMS 4147 covers 7075-T7352 forgings, and AMS 4131 covers the 7075-T74 condition. Heat treatment of finished parts is controlled by AMS 2770, and the general requirements for aluminium alloy forgings, including permissible tolerances and inspection, are given in ASTM B247. Ultrasonic inspection of the finished forging is called out from the AMS 2630 series of specifications, and testing of plane-strain fracture toughness follows ASTM B645. A complete order therefore names the aerospace material specification and the drawing, plus any additional ultrasonic class, surface finish or grain flow requirement.
Forging Practice and Application Selection
Closed die forging produces the grain flow that makes these parts dependable: metal is worked so that flow lines follow the load path, which is why a forging outperforms a machined-from-plate part of the same temper under fatigue. Practical selection follows the service condition. Wing and fuselage fittings, actuator components and valve bodies that are loaded in tension and protected from sustained wet service are usually T6. Landing gear parts, wing attachments and other components held under sustained tensile stress in the short transverse direction are normally T73 or T7352, accepting the lower strength to remove the risk of stress corrosion cracking. Thick sections where T73 would force an overweight design are the natural home of T74, and parts that are machined to close tolerances after heat treatment benefit from the stress relief in T7352.
Frequently Asked Questions
Q: Is 7075-T6 stronger than 7075-T73?
A: Yes, by roughly 13% in tensile strength, but T6 is susceptible to stress corrosion cracking when loaded in the short transverse direction and exposed to moisture or chloride.
Q: When should T7352 be chosen instead of T73?
A: When the forging will be machined to tight tolerances after heat treatment. The compression stress relief reduces distortion during machining while the mechanical properties match T73.
Q: Can a forging be re-heat treated to change temper?
A: In principle yes, by re-solution treatment and re-ageing, but the operation must follow AMS 2770 and the part must be re-qualified mechanically, because repeated thermal cycles affect grain structure and properties.
Q: Does the temper change the alloy chemistry?
A: No. The composition of 7075 remains within the same limits in every temper; only the ageing treatment and any stress relief deformation differ.
Q: How is grain flow verified?
A: By macro-etch examination of a sectioned forging, usually on a sacrificial part or test piece from the same die, with acceptance criteria agreed on the drawing.
Q: What inspection is normally applied to aerospace 7075 forgings?
A: Ultrasonic inspection for internal discontinuities, dimensional and visual inspection, hardness checks as a heat treatment indicator, and mechanical testing of specimens cut from a prolongation or from a separately forged test piece.