Guide
What are the properties of 7075-T6 aluminum, and when is T6 the wrong temper?
7075-T6 aluminum is the solution heat-treated and artificially aged temper of 7075, a high-strength Al-Zn-Mg-Cu alloy, and it gives the highest strength of the 7075 tempers but also the lowest toughness and the lowest resistance to stress corrosion cracking. Use it where static strength rules and the part carries no sustained tensile stress across the grain; where it does, call out T73 or T7351 and accept the lower strength.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers and buyers who have 7075 on a drawing, or are about to put it there, and need to know which numbers to design to and which temper note to write. Property values come from the open military handbook MIL-HDBK-5J (2003), stress corrosion ratings from that handbook and NASA MSFC-STD-3029A, and anodizing rules from MIL-A-8625F. Whether 7075 is worth its price over 6061 is a separate question, answered in our 6061 vs 7075 guide; this page stays on 7075 itself.
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What strength does 7075-T6 bar actually have?
Per MIL-HDBK-5J, 7075-T6 and T651 bar up to 1.000 in carries an A-basis design tensile strength of 77 ksi and a yield of 66 ksi in the longitudinal direction.
Those are design allowables, not averages. The handbook values in the table are statistically set minimums, and typical mill test results usually run a few ksi above them. A drawing or stress report should name which kind of number it relies on, because a sales datasheet quoting typical strength and a handbook quoting a design allowable can both be right about the same bar.
Strength in 7075 also depends on direction and section size. Per MIL-HDBK-5J, the long-transverse values for rolled bar fall as the section gets thicker, and the handbook warns that extrusion properties should be read for the thickness at quench, not the thickness after machining. A thin web machined out of a thick bar keeps the properties of the thick bar, so pick the table column from the stock size, not the finished part.
| Property | T6 / T651, up to 1.000 in, A-basis | T6 / T651, up to 1.000 in, B-basis | T73 / T7351, 0.375-2.000 in, S-basis |
|---|---|---|---|
| Ultimate tensile strength, Ftu (ksi) | 77 | 79 | 68 |
| Tensile yield strength, Fty (ksi) | 66 | 68 | 56 |
| Ultimate shear strength, Fsu (ksi) | 46 | 47 | 42 |
| Elongation, S-basis (%) | 7 | – | 10 |
| Tensile modulus, E (10³ ksi) | 10.3 | 10.3 | 10.3 |
| Density (lb/in³) | 0.101 | 0.101 | 0.101 |
What do A-basis, B-basis and S-basis mean on a 7075 datasheet?
They are three ways of stating a minimum: A-basis is the value at least 99 percent of the material should exceed, B-basis is the value at least 90 percent should exceed, and S-basis is the specification minimum.
The definitions come from the statistical basis section of MIL-HDBK-5J. For a commercial part that is not flight hardware, the useful rule is simpler: the purchase specification (for bar, AMS 4122, AMS 4123, AMS 4186 or AMS-QQ-A-225/9 for T6 per MIL-HDBK-5J) sets the minimums that the mill certificate must show, and those are the numbers incoming inspection can check. If you design to a typical value from a sales sheet, nothing on the certificate tells you whether a given lot meets your margin.
In the RFQ, write the alloy, temper and material specification together, for example “7075-T651 bar per AMS-QQ-A-225/9, material certificate required”. A bare “7075” leaves temper and specification open, and a supplier quoting from stock may pick whichever is cheapest that week.
Why is 7075-T6 a stress corrosion risk?
Because MIL-HDBK-5J gives 7075-T6 rod and bar a D rating for stress corrosion cracking in the transverse directions, the lowest rating, meaning cracking has occurred in service or would be expected under any sustained tensile stress.
NASA reaches the same verdict: MSFC-STD-3029A lists 7075-T6 among the aluminum alloys with low resistance to stress corrosion cracking in sodium chloride environments, while 7075-T73 appears in its high-resistance table. The stress does not have to come from the service load. Per MIL-HDBK-5J, residual stress from quenching, straightening, fit-up and clamping is enough, and pin-hole flaws in a protective coating are enough to let the corrosion start.
The handbook names the features to avoid on T6 and T651 parts: press or shrink fits, taper pins, clevis joints where bolt tightening bends the lugs, and straightening or assembly steps that leave surface tension, especially in the short-transverse direction. A machined bar part with a pressed-in bushing, a tapered thread or a slotted clamp is exactly that case.
| Temper | L direction | LT direction | ST direction | What it means for the drawing |
|---|---|---|---|---|
| 7075-T6, T651 | A | D | D | No sustained tension across the grain: no press fits, taper pins or clamped lugs |
| 7075-T73, T7351 | A | A | A | Preferred where fits, threads or assembly stress load the part for years |
When should the drawing call out 7075-T73 or T7351 instead of T6?
Call out T73 or T7351 whenever the part will hold a sustained tensile stress across the grain, sees salt or humid service, or carries an interference fit; accept that per MIL-HDBK-5J the T73 bar yield drops to 56 ksi from the 66 ksi A-basis T6 value.
That loss of roughly a sixth of the yield strength is the price of moving from a D to an A rating in every direction. Per MIL-HDBK-5J, the T76 temper sits between the two, improving exfoliation resistance with only limited stress corrosion improvement, so do not treat T76 as a cheaper T73.
Two practical checks help. First, recompute the margin with the T73 allowable; many parts designed to 7075-T6 were sized by stiffness or by thread strength, and the stiffness does not change, because E is the same 10.3 × 10³ ksi in both tempers per MIL-HDBK-5J. Second, ask the supplier what bar sizes are available in T7351, since the availability of a given diameter in the overaged temper is a buying question, not a design one.
How does 7075-T6 behave when anodized?
7075 takes Type II sulfuric and Type III hard anodize, but per MIL-A-8625F the part must already be in its heat-treated temper, such as T6 or T73, before it is anodized.
MIL-A-8625F limits Type III to alloys with no more than 5 percent nominal copper and 8 percent nominal silicon unless the contract says otherwise, which 7075 satisfies, and it warns that hard coatings may reduce fatigue strength. For fatigue-critical parts the same specification points designers toward the thinner chromic and non-chromate Types I, IB, IC and IIB. Its Type I rule, which bars chromic anodizing on alloys whose total nominal alloying content exceeds 7.5 percent unless specified, is worth checking against your 7075 material specification before you write Type I on the drawing.
Appearance is the other surprise. Clear anodize on 7075 is not as clear as on 6061; the clear anodize guide covers the color by alloy, and the Type II vs Type III guide covers the choice of type. A coating with pin-holes is also the start point for stress corrosion described above, so anodizing is not a substitute for T73 on a stressed part.
Can 7075-T6 be welded or straightened after machining?
Treat 7075 as a machined-only alloy: MIL-HDBK-5J rates it C for inert-gas arc welding in all tempers, meaning crack sensitivity and loss of properties are likely, and it recommends against cold straightening T6 parts.
A C rating does not forbid welding, but the weld zone loses the T6 strength and the alloy is crack sensitive, so a 7075 drawing should not show a weld unless the joint has been qualified. If a design needs welding, 6061 in its T4 and T6 tempers is rated A for arc welding in the same handbook table and is the usual choice.
Straightening is the quieter issue. Per MIL-HDBK-5J, straightening and forming should be done in the freshly quenched condition or at elevated temperature to limit residual stress. A long, thin 7075-T6 part that moves after roughing should be stress-relieved stock (the T651 or T7351 tempers, which are stretched after quench) and machined with balanced stock removal, not bent back after the fact.
How should a 7075 part be written on the drawing and RFQ for Saola?
Write alloy, temper and material specification in the material block, add any stress corrosion or anodizing notes, and ask for the mill certificate; 7075 is not on the bar list of our CNC turning page, so the grade and stock form are confirmed at drawing review.
A complete material note reads like “7075-T7351 bar per AMS 4124, material certificate required, anodize per MIL-A-8625 Type II Class 1 after machining”. The material certificate is available on request on every order, and new parts get a first-article inspection report. Anodizing on aluminum is one of the eight finishes on our managed finishing menu, carried out at partner plants rather than at the CNC turning and CNC milling plant, so the anodizing note goes to the finisher exactly as written.
Flag in the RFQ any feature that loads the part across the grain, such as press-fit bores, tapered threads or clamp slots, so the temper question is settled before a quote is priced on T6 stock.
Frequently asked
Is 7075-T6 stronger than mild steel?
In tensile strength per unit weight, yes by a wide margin; per MIL-HDBK-5J its A-basis tensile strength in bar is 77 ksi. It is far less stiff than steel, so a part that is limited by deflection will not get lighter just by switching to 7075.
What is the difference between 7075-T6 and 7075-T651?
Both are solution treated and artificially aged. T651 is also stress relieved by stretching after quench, which leaves less residual stress and less movement during machining; MIL-HDBK-5J lists them with the same bar allowables.
Does 7075-T73 machine differently from T6?
Shops typically treat them alike for cutting. The practical differences are stock availability by size and the lower strength allowable you design to.
Can a T6 part be re-aged to T73 after machining?
T73 is an overaging treatment of solution-treated material, so it is a heat-treat process step with its own specification, not a quick rework. Buy T73 or T7351 stock instead, unless the heat treat is qualified.
Does anodizing protect 7075-T6 from stress corrosion?
No. Per MIL-HDBK-5J, pin-hole flaws in a protective coating are enough to allow stress corrosion cracking, so the temper choice, not the coating, controls the risk.
Sources
- MIL-HDBK-5J, Metallic Materials and Elements for Aerospace Vehicle Structures, sections 3.1 and 3.7.6, US Department of Defense, 31 January 2003
- MSFC-STD-3029A, Guidelines for the Selection of Metallic Materials for Stress Corrosion Cracking Resistance in Sodium Chloride Environments, NASA Marshall Space Flight Center, 24 February 2005
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 10 September 1993
- ASTM G64-99(2021), Standard Classification of Resistance to Stress-Corrosion Cracking of Heat-Treatable Aluminum Alloys (scope), ASTM International, 2021