7075 Aircraft Aluminium: What Makes It Aerospace Grade

Why 7075 became an aircraft alloy

Aircraft design is a constant trade between weight and strength. 7075-T6 offers a typical tensile strength of about 572 MPa and yield of about 503 MPa at a density of 2.81 g/cm³, roughly one-third that of steel. In specific strength (strength per unit weight) it beats many structural steels and competes with titanium in moderately loaded parts, at a fraction of the cost and machining time.

The alloy was developed in the early 1940s and quickly adopted for wartime aircraft. Since then, it has been used for wing and fuselage structure, fittings, landing-gear parts and countless machined components. Modern airframes often use newer alloys such as 7050, 7150 and 7085 for thick, damage-tolerance-critical parts, but 7075 remains one of the most used aerospace alloys, especially for sheet, extrusions, bar and smaller forgings.

The weakness of 7075 is just as important to understand. In the peak-aged T6 temper it is susceptible to stress-corrosion cracking (SCC) and exfoliation corrosion, particularly in the short-transverse direction of thick sections. That is why aerospace engineers specify over-aged T73 or T76 tempers for many parts, and why thin 7075 sheet for skins is usually Alclad.

Commercial 7075 versus aerospace 7075

The alloy limits are the same in both cases. What differs is the specification the material is made to and the evidence that comes with it. A plate sold as "7075-T651 to ASTM B209" may be perfectly good material, but it is not acceptable for a part whose drawing calls up an AMS specification.

In practice, buyers of aerospace 7075 should check three things on the certificate: the exact specification and revision, the temper and product form it covers, and the test results it requires (including ultrasonic class where relevant). If any of these is missing, the material should not enter a flight-hardware supply chain.

7075 chemical composition (wt %)

7175 and 7475 are higher-purity derivatives of 7075 with tighter iron and silicon limits for better fracture toughness and fatigue. They are worth considering when damage tolerance governs. For thick parts that need both strength and SCC resistance, 7050 is often the better choice; see our 7050 vs 7075 comparison.

Aircraft tempers and their properties

Typical values per ASM data. Specified minimums vary with form and thickness and are set by the order specification; MMPDS gives design allowables.

A practical rule: T6 for thin or lightly loaded parts where stress corrosion is not a concern; T73 or T7351 whenever there is sustained tensile stress in the short-transverse direction, such as in press-fitted bushings, lugs and thick fittings; T76 where exfoliation, rather than SCC, is the main threat.

Common 7075 aerospace specifications by product form

Specifications are revised periodically. Always order to the specification and revision on the drawing; the list above is a guide to where to look, not a substitute for it.

Stress corrosion, exfoliation and Alclad protection

Most 7075 failures in service are not overload failures but corrosion-assisted ones. Two mechanisms matter. Stress-corrosion cracking is cracking under sustained tensile stress in a moist environment. In 7075-T6 it runs along the elongated grain boundaries, so it is worst when stress acts in the short-transverse direction, across the thickness of a plate or forging. Interference-fit bushings, clamped joints and residual stress from quenching or machining can all supply that stress without any external load.

Exfoliation is layered corrosion that lifts the surface in leaves, again following the flattened grain structure of rolled and extruded products. It shows up around fastener holes and cut edges where the end grain is exposed. The T76 temper was developed specifically to resist exfoliation while keeping more strength than T73.

Over-ageing works because it coarsens and spaces out the precipitates on grain boundaries and changes their electrochemistry, which interrupts the continuous anodic path along which cracks grow. The cost is strength. Aerospace specifications for T73 and T76 therefore include acceptance tests, typically a combination of electrical conductivity and tensile yield limits, to confirm the over-ageing has been done properly.

Note that Alclad reduces fatigue strength slightly, because the soft cladding is a weaker surface layer, and the cladding is removed wherever the part is machined. It is a sheet solution, not a cure for machined parts.

Where 7075 is used on an aircraft

A typical design path for a machined fitting is to start with 7075-T7351 plate or bar, because the part will carry sustained loads near bolt holes. If the fitting is thicker than about 75 – 100 mm, or toughness governs, engineers usually move to 7050-T7451. For thin formed brackets, 7075-T6 sheet or extrusion is common, with anodising and primer for corrosion protection.

Machining, forming, joining and finishing for flight parts

Buying 7075 for aerospace: a checklist

1. Specification and revisionQuote the exact AMS, AMS-QQ or customer specification, including the revision letter.
2. Form, temper and sizeFor example 7075-T7351 plate, 50 mm, or 7075-T73511 extrusion to drawing.
3. InspectionUltrasonic class, conductivity, grain direction marking and any first-article requirements.
4. Cutting and traceabilityCut pieces should carry the heat or lot number; ask for re-marking on every piece.
5. DocumentsEN 10204 3.1 mill certificate with chemistry, mechanical and other required test results, plus any certificate of conformity your quality system requires.

7075 aerospace products and guides

7075 aircraft aluminium (aluminum): FAQs