7075 is a heat-treatable Al-Zn-Mg-Cu alloy that delivers among the highest strengths available in a common wrought aluminum product. The alloy designation alone, however, does not define the properties that a design will experience. The temper, shown by the letter and digit suffix, describes the heat treatment and mechanical working history of the material, and it changes the balance between strength, ductility, dimensional stability and resistance to stress-corrosion cracking more than any other single decision.
Three service questions should be settled before a temper is selected: what loads and cycles the part must carry, what environment it will see, and how much machining distortion the finished component can tolerate. The answers normally point to one temper and eliminate the rest.
| Stress-corrosion resistance | ||||
|---|---|---|---|---|
| 7075-O | ||||
| 7075-T6 | High-strength parts in protected, non-corrosive service | |||
| 7075-T651 | T6 plus controlled stretching | Same as T6 | Similar to T6 | |
| 7075-T73 / T7351 | Overaged for corrosion resistance, T7351 also stretched | Slightly lower than T6 |
Strength, Stress-Corrosion Resistance and the T6 Versus T73 Trade-off
Where maximum strength is the governing requirement, 7075-T6 is normally the preferred option. Its tensile strength is on the order of 570 MPa with typical elongation around 11 percent in the longitudinal direction, which is why it appears in highly loaded airframe fittings, tooling, high-stress structural brackets and moulds that must resist wear.
The penalty of that strength is sensitivity to sustained tensile stress in a corrosive environment. T6 material can develop stress-corrosion cracking when it is held under continuous tensile stress in moist air or where chlorides collect. The T73 and T7351 treatments use an overaging step that deliberately gives up a small amount of strength, typically in the region of ten percent, in exchange for a large improvement in resistance to stress-corrosion cracking and exfoliation corrosion. Any part that will sit under permanent load outdoors, near the coast, or in contact with moisture should therefore be specified in a T73-type temper rather than T6.
Two practical notes follow. First, a T6 part can still be used in a corrosive location if it is fully protected by a coating system and the protection is maintained, but that is a maintenance commitment rather than a material solution. Second, mechanical properties vary with product form and direction: properties are specified in the longitudinal, long-transverse and short-transverse directions, and the short-transverse direction is always the weakest. Design allowable values should be taken from the direction that governs the part.
Machining Stability: T651, T6511 and Residual Stress
When dimensional stability during machining is the critical requirement, 7075-T651 is the more suitable choice. The T651 designation means the material is solution treated, then stretched by a controlled amount, and then artificially aged. Stretching relieves most of the internal residual stress that would otherwise be released as metal is removed, so thin ribs and long pockets stay flat instead of bowing after the final cut.
Extruded or rolled bar stock in a T6511-type condition provides the same principle in long sections. The sequence of operations also matters: rough machine with an allowance left on all faces, allow the part to stabilise, then finish machine in a light, balanced pass on both sides. Stress-relieved stock plus a two-stage machining sequence controls distortion far more reliably than either measure alone.
Typical properties of 7075 are worth stating alongside the temper. Density is about 2.81 g/cm³ and Young's modulus about 71.7 GPa, with a Poisson's ratio near 0.33. The melting range is approximately 477 to 635 ℃, and thermal conductivity is in the region of 130 to 150 W/m·K. Rockwell hardness in the T6 condition is roughly 87 HRB. The registered composition range is about 5.1 to 6.1 percent zinc, 2.1 to 2.9 percent magnesium, 1.2 to 2.0 percent copper and 0.18 to 0.28 percent chromium, with aluminum as the balance and minor additions of iron, silicon, manganese and titanium.
Welding deserves an explicit warning: 7075 is not normally considered weldable for structural service, because the heat-affected zone loses a large part of its strength and becomes prone to cracking. Joints should be designed as mechanically fastened or bonded assemblies rather than welded ones.
Before releasing an order, confirm four things: that the temper is quoted explicitly with the alloy and product form, that the inspection certificate reports tensile properties and composition for the actual heat, that the material is supplied to a recognised product specification such as ASTM B209 for sheet and plate or ASTM B211 for bar, and that ultrasonic or equivalent internal soundness testing is available where the part is highly stressed. A certificate that names the alloy but omits the temper cannot be verified against the design requirements.
Q: Is 7075-T6 suitable for outdoor marine use?
Not in an unprotected condition under sustained tensile load. Stress-corrosion cracking is the governing risk in a moist chloride environment, so a T73 or T7351 temper plus a protective coating is the usual specification.
Q: How much strength is lost when moving from T6 to T73?
Roughly ten percent in tensile and yield strength, depending on product form and direction. In return, resistance to stress-corrosion cracking and exfoliation corrosion improves substantially.
Q: What does the extra digit in T651 mean?
It indicates that the material was stretched a controlled amount after solution treatment to relieve internal residual stress, which is what makes it suitable for precision machining.
Q: Can 7075 be welded?
It is not normally weldable for structural applications because the weld and heat-affected zone lose a large proportion of their strength and are crack sensitive. Mechanical fastening or adhesive bonding is preferred.
Q: Does 7075 have good fatigue performance?
Its fatigue strength is high for an aluminum alloy in the aged tempers, which is one reason it is used in cycling and aerospace structures. Performance still depends on surface finish, notches and the presence of corrosion pits.
Q: How long will a 7075 component last in service?
Service life is governed by load, environment and maintenance rather than by a fixed figure. With protective coatings and prompt removal of corrosion, long service is achievable, but unprotected material under sustained stress in a wet environment can deteriorate much faster.
