Guide
What is the difference between 6061-T6 and 6061-T651?
6061-T651 is 6061 aluminum in the T6 condition that has also been stress-relieved by stretching: after solution heat treatment the producing mill pulls the plate or bar to a small, controlled permanent set, and it is then aged. The alloy, the chemistry and the minimum strength are the same as 6061-T6. What changes is how much locked-in quench stress the stock carries, and that decides how far a part moves when you mill it.
By Saola Manufacturing engineering team · Published · Updated
This guide is for mechanical engineers and buyers who meet both callouts on drawings, quotes and material certificates and want to know when the difference matters. The temper definitions come from ANSI H35.1, the American standard for aluminum alloy and temper designations, as reproduced in the Department of Defense handbook MIL-HDBK-5J. The strength figures come from the same handbook, and the product-form rules come from the ASTM mill-product specifications that cite H35.1.
A note on sources. ANSI H35.1 and the ASTM specifications are sold by their publishers, so this page only prints figures taken from a public document linked beside them. The current edition of the standard your drawing names still governs, and design allowables for flight hardware belong to the handbook’s successor, MMPDS, not to this page.
On this page
What does the 51 in T651 mean?
The digits 51 mean stress-relieved by stretching: under ANSI H35.1, a T_51 product is stretched a set amount after solution heat treatment and receives no further straightening afterward.
The basic T6 temper describes two steps only: solution heat treatment, then artificial aging. H35.1 allows extra digits after a T temper when a variation in processing significantly changes the product, and the stress-relief digits are the variations most engineers meet. The temper table in MIL-HDBK-5J, Table 3.1.2 sets them out, including the permanent set each product form must receive.
| Temper | What the mill did | Product forms | Permanent set from stretching or compressing |
|---|---|---|---|
| T6 | Solution heat-treated and artificially aged; any flattening or straightening is not recognized in the property limits | All wrought forms | Not specified |
| T651 | As T6, plus stress relief by stretching; no straightening after stretching | Plate; rolled or cold-finished rod and bar; die or ring forgings and rolled rings | Plate 1½–3%; rod and bar 1–3%; forgings and rings 1–5% |
| T6510 | As T6, plus stress relief by stretching; no straightening after stretching | Extruded rod, bar, shapes and tube; drawn tube | Extrusions 1–3%; drawn tube ½–3% |
| T6511 | As T6510, but minor straightening is allowed after stretching to meet standard tolerances | Extruded rod, bar, shapes and tube; drawn tube | Extrusions 1–3%; drawn tube ½–3% |
| T652 | As T6, plus stress relief by compressing | Products compressed after solution heat treatment | 1–3% |
| T62 | Solution heat-treated from the annealed or F temper and artificially aged, including heat treatment done by the user | Any form re-heat-treated after delivery | None |
Two details in those definitions are easy to miss. A plain T6 product may still have been roller-straightened or flattened at the mill; H35.1 simply does not count that work toward the property limits, and it is not a controlled stress relief. And T6510 and T6511 differ only in whether the producer may touch up straightness after stretching, which is why T6511 bar arrives straighter but has had a little cold work put back into it.
Is 6061-T651 as strong as 6061-T6?
Yes: MIL-HDBK-5J lists the same tensile minimums for 6061 in T6 and T651, so stretching neither adds nor removes strength.
The handbook tabulates 6061 sheet in T6, plate in T651, and rolled or cold-finished bar in both tempers. For sheet, for plate up to 4 inches thick and for bar, the tensile ultimate is 42 ksi and the tensile yield 35 ksi in every temper listed, per MIL-HDBK-5J Tables 3.6.2.0(b1), (b2) and (d).
| Form and temper | Thickness, in | Basis | Ultimate (Ftu) | Yield (Fty) |
|---|---|---|---|---|
| Sheet, T6 | 0.010–0.249 | A | 42 (LT) | 35 (LT) |
| Plate, T651 | 0.250–2.000 | A | 42 (LT) | 35 (LT) |
| Plate, T651 | 3.001–4.000 | S | 42 (LT) | 35 (LT) |
| Plate, T651 only | 4.001–6.000 | S | 40 (LT) | 35 (LT) |
| Rolled or cold-finished bar, T6 | up to 8.000 | S | 42 (L) | 35 (L) |
| Rolled or cold-finished bar, T651 | 0.500–8.000 | S | 42 (L) | 35 (L) |
Two readings of the table matter more than the numbers themselves. The first is that the choice between T6 and T651 is not a strength decision, so an engineer who wants a stronger part is looking at the wrong suffix and should look at the alloy instead. The second is the thick-plate row: the heaviest plate carries a lower ultimate minimum, and the handbook notes that the heaviest sizes are listed in T651 only.
The handbook also warns, for 6061 extrusions, that properties should be chosen by the thickness at the time of quenching, not the final machined thickness, because the thinner figure can be unconservative. The same caution is a sensible default for any thin part machined out of heavy plate or bar: the certificate describes the stock, not the finished feature.
Why does T651 plate move less when you mill it?
Stretching evens out the residual stress that the quench leaves in the metal, so cutting away material from one side has much less locked-in stress to release.
Solution heat treatment ends with a rapid quench, which is what lets the aging step develop T6 strength. The outside of a plate cools and contracts first while the middle is still hot; when the middle cools and shrinks in turn, it pulls the skin into compression and leaves the core in tension. Those stresses balance each other, so the quenched plate looks flat. Mill a deep pocket from one face and you remove part of that balance, and the remaining section bows or twists until the stresses balance again.
Stretching the whole cross-section past its yield point by a small permanent amount lets the stress pattern relax before the part is ever machined. The handbook states the principle in its own notes: products that do not receive a mechanical stress-relieving process have the potential for induced residual stresses (MIL-HDBK-5J, notes to Table 3.1.2.1.6). It also advises that any straightening be done while the material is freshly quenched, to minimize the residual stress it induces.
In practice the difference shows up on particular geometries: flat plates with deep pockets on one side, thin floors, long thin ribs, and parts with a flatness callout over a large face. On a turned part made from round bar, or a small block where material comes off evenly on all sides, the gap between T6 and T651 stock is usually small. Stretching also does nothing about stress that machining itself puts in. Heavy roughing passes, worn tools and hard clamping add their own, which is why a common approach for tight flatness is to rough every face, release the clamps so the part can relax, and then take light finishing passes. The CNC milling page shows the prismatic part families run at our lead CNC plant, where this sequence applies.
Which 6061 product forms come as T6, T651 or T6511?
In MIL-HDBK-5J, 6061 sheet appears as T6, plate as T651, rolled or cold-finished bar in both T6 and T651, and extruded products with the T6510 and T6511 stretch variants alongside T6.
The handbook lists no plain T6 temper for 6061 plate, only T651, plus T62 for material heat-treated by the user. So when a drawing says “6061-T6” for a part milled from plate, the supplier will in practice quote T651 stock, and the drawing should say that this is acceptable.
The product specifications follow the same split by form. ASTM B209 covers sheet and plate (ASTM B209/B209M-21a), ASTM B211 covers rolled or cold-finished bar, rod and wire (ASTM B211/B211M-19), and ASTM B221 covers extruded bars, rods, wire, profiles and tubes (ASTM B221-21). Each states that its alloy and temper designations follow ANSI H35.1. On the aerospace side, the handbook lists AMS 4027 and AMS-QQ-A-250/11 among the 6061 sheet-and-plate specifications. Dimensional tolerances for the mill product itself, including flatness of plate as supplied, sit in a companion standard, ANSI H35.2, which the Aluminum Association maintains alongside H35.1 (Aluminum Association, standards).
How should a drawing call out 6061-T6 or 6061-T651?
Name the alloy, the temper and the product specification together, and say whether a stress-relieved temper is required or merely allowed.
A few callouts that leave nothing to interpretation:
- “6061-T651 plate per ASTM B209” for a flat milled part where flatness after machining matters.
- “6061-T6 or T651 bar per ASTM B211” for a turned part where either is fine.
- “6061-T6511 per ASTM B221” when the part is cut from extruded bar or a profile.
- “6061-T6 (T651 acceptable)” on legacy drawings you do not want to revise fully, so a supplier buying plate is not forced into a nonconformance.
If the part will be welded or heat-treated again after machining, the stretch is gone. Material re-solution-treated and aged by the user takes the T62 designation under H35.1, and a T62 part has no mechanical stress relief, so write the final condition on the drawing rather than the stock temper. General tolerances on the machined features are a separate callout; the ISO 2768 and NADCA tolerance guide covers how to write them, and the RFQ preparation guide covers where the material line belongs in the package.
What should the mill certificate show?
It should show the exact temper you ordered, such as T651 rather than just T6, together with the governing specification, the product form and size, a lot number that traces to the stock, the chemistry and the tensile results.
Read the certificate against the drawing, line by line:
- Temper suffix. A certificate reading “T6” for plate deserves a question, because the handbook lists 6061 plate as T651.
- Specification and revision. The standard on the certificate should be the one the drawing names, or one you have accepted in writing.
- Chemistry. Compare the reported elements with the registered 6061 limits in the Aluminum Association Teal Sheets (Teal Sheets, revised December 2025). The same record lists 6061A, a variant registered in Europe with an extra element limit, so check which of the two the certificate names.
- Tensile results. Compare them with the minimums for the thickness of the stock, not the finished part.
- Traceability. The lot or heat number should match the marking on the plate or bar and the supplier’s job traveler. A certificate issued by the producing mill carries more weight than a distributor’s summary of it.
Does the temper matter when you source machined parts from Vietnam?
It matters exactly as it does at home, but it is easier to lose in translation, so put the temper and specification on the purchase order and ask for the mill certificate to travel with each lot.
A machining supplier in Southeast Asia may buy plate and bar from distributors whose stock was made to ASTM, EN or JIS standards, and quotes often shorten the material line to “6061” or “Al 6061-T6”. European certificates may name the grade by its chemical symbol, AlMg1SiCu, which the Teal Sheets cross-reference to 6061. None of that is a problem if the quote names the stock form and temper it is based on and the certificate proves it.
Three habits close the gap. Ask the quote to state the stock form, temper and specification. Ask for the mill certificate in English with the lot number, and for that number on the packing list. Where flatness matters, make a flatness check after machining part of the first article inspection, so a stock problem shows up on the first parts rather than the first container. Saola runs CNC milling and CNC turning at its lead CNC plant, T.K Viet Nam Precision Company Limited. To check a 6061 part, send the drawing for a quote with the temper written as you need it.
Frequently asked
Can I substitute 6061-T651 for 6061-T6?
For strength, usually yes, because the published minimums are the same. For a controlled drawing, only if the drawing allows it or you approve a written deviation.
Is 6061-T6511 the same as 6061-T651?
Both are T6 plus stretching. T6511 is the extruded-product version, and it allows minor straightening after stretching to meet standard tolerances; T651 is for plate and rolled or cold-finished bar and allows none.
Why did my T651 part still warp?
Stretching reduces quench stress but does not remove it entirely, and it does nothing about stress added by machining or clamping. Deep one-sided pockets and thin floors are the usual suspects; roughing, releasing and then finishing helps.
What happens if a T651 part is heat-treated again?
It is no longer T651. Material solution heat-treated and aged by the user is designated T62 under ANSI H35.1 and has no mechanical stress relief.
Does the temper change how 6061 anodizes?
The stretch step does not change the alloy chemistry, so T6 and T651 are specified for anodizing the same way. The finish comparison guide covers how to call out the finish itself.
Sources
- MIL-HDBK-5J, Metallic Materials and Elements for Aerospace Vehicle Structures, Department of Defense, 31 January 2003: Table 3.1.2 (temper designation system), Section 3.1.3, Section 3.6.2 and Tables 3.6.2.0(a) to (d)
- ASTM B209/B209M-21a, Aluminum and Aluminum-Alloy Sheet and Plate (scope), ASTM International, 2021
- ASTM B211/B211M-19, Aluminum and Aluminum-Alloy Rolled or Cold-Finished Bar, Rod, and Wire (scope), ASTM International, 2019
- ASTM B221-21, Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes (scope), ASTM International, 2021
- International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys (Teal Sheets), The Aluminum Association, revised December 2025
- Standards: ANSI Accredited Standards Committee H35 scope, covering H35.1 and H35.2, The Aluminum Association, accessed September 2026