7075 is a heat-treatable aluminium-zinc-magnesium-copper alloy that sits at the top of the strength range among commercially available wrought aluminium grades. Its mechanical performance approaches that of some steels while its density remains close to one third that of steel, so the grade is specified wherever weight reduction and load capacity must be balanced. What a finished component actually delivers in service depends far less on the alloy designation alone than on the temper, that is, the specific combination of solution heat treatment, quenching and controlled ageing applied to the metal.
This guide explains why 7075-T6 and its stress-relieved counterpart 7075-T651 are the tempers most widely stocked and supplied, how temper selection changes corrosion behaviour and fatigue life, and how the grade compares with general-purpose aluminium alloys.
Understanding the Temper Designations Applied to 7075
The letter and digits that follow the alloy number describe a complete production route rather than a single heat-treatment step. For 7075 the practical shortlist is small:
O (annealed) - the softest condition, used mainly for forming operations that are followed by a later heat treatment; tensile strength is roughly 230 MPa.
T6 - solution heat treated and then artificially aged. This is the highest-strength condition commonly available and the standard reference temper for the alloy.
T651 - T6 applied to plate that is then stretched by a controlled amount, typically 1.5 to 3 percent, to relieve internal residual stress. The resulting plate stays flat during machining.
T73 and T7351 - an over-aged condition in which artificial ageing continues beyond the peak-strength point. Tensile strength drops by roughly 10 to 15 percent, but resistance to stress-corrosion cracking improves substantially.
T76 and T7651 - intermediate over-ageing that trades a smaller strength loss for improved resistance to exfoliation corrosion.
Because the ageing cycle governs the precipitate structure, the same alloy chemistry can be supplied in conditions that differ by more than 40 percent in yield strength while sharing identical density and elastic modulus.
Why 7075-T6 and 7075-T651 Are the Most Common Tempers
Yes, T6 and T651 are by a wide margin the tempers most often specified and held in stock. Four practical reasons explain that dominance:
Peak strength per kilogram. No other widely traded temper of the alloy reaches the same combination of tensile and yield strength, which is decisive for airframe fittings, housings and high-load structural brackets.
Designation coverage. T6 and T651 are the conditions listed most completely in wrought aluminium designation systems and aerospace material specifications, so designers can call them out without a bespoke qualification programme.
Machining stability. Stretched T651 plate machines with minimal distortion, which matters for thin ribs, long spars and precision fixtures.
Supply breadth. Extrusion, bar, sheet and plate are all routinely produced in these conditions, giving short lead times and consistent availability.
Extruded and forged parts are usually ordered in T6, while rolled plate above roughly 12 mm is normally ordered in T651 so that the stretching step removes quench-induced residual stress.
Typical Mechanical Properties and Chemical Composition
The values below are typical for 7075 in the T6 and T651 conditions at room temperature. Actual figures vary with product form, thickness and testing direction.
| Property | Typical value (T6 and T651) |
|---|---|
| Ultimate tensile strength | 572 MPa (83 ksi) |
| Tensile yield strength | 503 MPa (73 ksi) |
| Elongation at break | 11 percent |
| Modulus of elasticity | 71.7 GPa (10.4 Msi) |
| Density | 2.81 g/cm3 |
| Hardness | about 150 HB |
| Fatigue endurance limit | about 159 MPa at 5 x 10^8 cycles |
The alloy owes this performance to a tightly controlled composition. Zinc and magnesium form the strengthening precipitates, copper adds further strength, and chromium refines the grain structure.
| Element | Composition range (percent by weight) |
|---|---|
| Zinc | 5.1 - 6.1 |
| Magnesium | 2.1 - 2.9 |
| Copper | 1.2 - 2.0 |
| Chromium | 0.18 - 0.28 |
| Iron | 0.50 max |
| Silicon | 0.40 max |
| Titanium | 0.20 max |
| Other elements, each | 0.05 max (0.15 total) |
| Aluminium | remainder |
Corrosion Resistance, Fatigue Life and Protective Treatments
7075 offers only moderate resistance to atmospheric corrosion and is notably susceptible to stress-corrosion cracking when it is held under sustained tensile stress in a humid or chloride-bearing environment. Two strategies are used to manage that risk.
The first is temper selection. Over-aged conditions such as T73 and T7351 move the microstructure away from the peak-strength state and raise stress-corrosion resistance markedly, at the cost of roughly 10 to 15 percent of tensile strength. Components that see continuous load in marine or humid service are frequently specified in these tempers even when the strength penalty is unwelcome.
The second is surface protection. Anodising, chemical conversion coating, metallic cladding and paint systems all isolate the surface from the environment. Anodised layers also improve wear resistance on sliding surfaces, while clad sheet places a thin layer of a more corrosion-resistant aluminium alloy over the core.
Fatigue performance is good at room temperature, but both corrosive media and elevated temperature reduce the endurance limit. Where a part experiences cyclic loading in such conditions, designers normally combine an over-aged temper with a protective coating and apply generous fatigue margins. Service life is otherwise long: with correct protection and maintenance, structural components routinely operate for several decades, provided sustained stress and aggressive environments are kept within design limits.
How 7075 Compares with General-Purpose Aluminium Alloys
Strength. 7075-T6 reaches roughly 572 MPa tensile strength, against about 310 MPa for 6061-T6 and about 470 MPa for 2024-T3, so it leads clearly when load capacity governs.
Weight. Density is 2.81 g/cm3, only marginally above the 2.70 g/cm3 of 6061, so the strength gain carries almost no weight penalty.
Formability. 7075 is less ductile than the 6xxx and 3xxx families and cannot be bent to tight radii in the T6 condition; forming is normally carried out in the annealed condition and followed by heat treatment.
Weldability. Fusion welding of 7075 in T6 is generally avoided because it destroys the ageing response in the heat-affected zone; riveted, bolted and bonded joints are standard practice instead.
Selection logic. Where stiffness, corrosion resistance and cost matter more than peak strength, 6061 remains the easier and more economical choice. Where every kilogram counts and loads are high, 7075-T6 or T651 is the material of choice.
Frequently Asked Questions
Q: Is 7075-T6 the most common temper of 7075 aluminum?
Yes. T6 and the stress-relieved T651 are the conditions most widely stocked and supplied, and together they account for the great majority of extruded, forged and rolled product shipped in this grade.
Q: What is the difference between 7075-T6 and 7075-T651?
Both are solution heat treated and artificially aged. T651 plate receives an additional controlled stretch of about 1.5 to 3 percent that relieves residual quenching stress, so it stays flatter and machines with less distortion.
Q: Which temper should be used in a marine or humid environment?
An over-aged condition such as T73 is normally preferred, because it resists stress-corrosion cracking far better than T6, and a protective coating or anodised layer is usually applied as well.
Q: Does 7075 aluminum alloy corrode easily?
It has only moderate corrosion resistance and is especially sensitive to stress-corrosion cracking, so cladding, anodising, conversion coating or painting is standard for demanding service conditions.
Q: How long does a 7075 aluminum component last?
With appropriate surface protection and maintenance, structural parts can remain in service for several decades. Unprotected parts exposed to chlorides or sustained tensile stress may deteriorate much sooner.
Q: Can 7075-T6 be welded?
Fusion welding is generally avoided because it removes the ageing response in the heat-affected zone and sharply reduces strength. Mechanical fastening and adhesive bonding are the usual joining methods.
Q: Is 7075 stronger than steel?
Pound for pound, yes. Its specific strength is higher than that of common structural steels, even though the absolute tensile strength of steel remains greater.
