Dewey Wu General Manager & senior mechanical engineer at EPOC CRAFTER, 15 years in design engineering, quality, and metallurgy. Hands-on across CNC machining, metalwork, sheet metal, and prototyping (subtractive + 3D printing).
6061-T6, T651, and T6511 are three tempers of one 6061 aluminum alloy that share nearly the same strength, about 310 MPa (45 ksi) tensile and 276 MPa (40 ksi) yield in typical ASM Handbook Vol.2 data, so the temper you order is decided by residual stress and stock form. This choice matters most for machined parts where the tolerance is set by flatness and post-machining stability. On the machine it becomes a dimensional problem: relieve the internal stress and a plate part moves under 0.002 in after a facing cut, while as-quenched T6 releases 0.010 to 0.030 in on the same cut in industry-reported ranges. What decides your order is stock form (plate, sheet, or extrusion), section thickness against the 0.25 in T6/T651 split in the ASTM B209 tables, and how tight your flatness callout runs. This guide covers what each temper changes, a three-gate rule for ordering the right one, and how to specify and verify it on the drawing.
1. What the T6, T651, and T6511 tempers change
6061-T6 is solution treated and aged with no stress relief. T651 adds a controlled 1.5 to 3 percent stretch before aging to relieve residual stress in plate, per AMS 4027. T6511 applies that stretch plus straightening for extrusions. The three share the same strength; what changes between them is how much locked-in stress the metal carries into your setup. If you are still deciding whether 6061 is the right alloy before you lock the temper, our 6061 alloy properties these tempers share covers the base properties these tempers share.
1.1 T6, solution treated and aged with no stress relief
6061-T6 is solution treated, quenched, and artificially aged to about 310 MPa (45 ksi) tensile and 276 MPa (40 ksi) yield in ASM Handbook Vol.2 typical data, with no stress-relief step. The quench locks in residual stress that aging does not remove. Machine a balanced, symmetric envelope and that stress stays in equilibrium; cut away one face and the part relaxes and moves. On thin-wall or single-sided parts you see bow or twist that forces a second setup or scraps the blank, which adds machine time and lead time.
1.2 T651, stretched plate that holds shape after machining
6061-T651 is solution-treated plate given a controlled 1.5 to 3 percent permanent stretch before aging, per AMS 4027. The stretch evens out quench stress across the section, so material removal releases far less movement. You pay a price premium over plain plate and buy back flatness: stretch-relieved plate holds post-machining movement under 0.002 in in industry reports, against 0.010 to 0.030 in for T6. On a flat, thin, or tight-tolerance part, that premium returns its cost the first time it saves a rework loop.
1.3 T6511, stretched and straightened extrusion
6061-T6511 is the extrusion version of T651: solution treated, stretched to relieve stress, then straightened after the stretch. ASTM B221 classifies T6510 and T6511 as stress-relieved tempers, and the trailing 11 marks the straightening pass that T6510 skips. Extruded 6061 in the T6 and T6511 group carries a lower guaranteed minimum, 38.0 ksi tensile against 42.0 ksi for plate, comparing ASTM B221 with ASTM B209. Order T6511 when your part starts from bar, rod, or profile stock; calling out plate-grade T651 on an extruded section is not a valid order, and the supplier will re-quote or substitute, costing you a day or more of lead time.
Engineering note. For a plate part with a flatness callout tighter than about 0.005 in over its length, specify T651; plain T6 will not hold that callout because the quench stress releases during machining and bows the part. The exception is a thick, symmetric block cut evenly on both faces, where balanced stock removal keeps T6 stable enough that the T651 premium returns little.
| T6 | Solution treated and aged, no stress relief | Sheet under 0.25 in, small bar | High, as-quenched | 42.0 ksi UTS / 35.0 ksi YS, sheet (ASTM B209) | Thin sheet, flatness not tightly toleranced |
| T651 | Adds 1.5 to 3% stretch before aging | Plate 0.25 in and thicker | Relieved | 42.0 ksi UTS / 35.0 ksi YS (ASTM B209) | Flat, thin-wall, or tight-tolerance plate parts |
| T6511 | Stretch plus straightening (extrusion) | Extruded bar, rod, profile | Relieved | 38.0 ksi UTS / 35.0 ksi YS (ASTM B221) | Part starts from extruded stock |
Note: ASM values above are typical; ASTM and AMS values are specified minimums. This guide keeps the two separate so you can quote to the minimum your drawing guarantees.
2. The Three-Gate Temper Check
The Three-Gate Temper Check orders your temper in three questions: what stock form the part comes from, whether the section is thicker than 0.25 in, and how tight the flatness callout runs. Form picks the T651 or T6511 track, thickness picks T6 against T651, and the flatness number decides whether you pay for stress relief. Run them in that order and the temper falls out before you leave the RFQ.
2.1 Gate 1, stock form picks the stress-relief track
Start with the form your part is cut from, because it splits the temper options before anything else. Plate and sheet run on the T6 and T651 track; extruded bar, rod, and profile run on T6511. You cannot order plate as T6511 or an extrusion as T651, so getting the form wrong at the RFQ stage sends the quote back and costs a day of lead time. Name the stock form on the drawing so the supplier pulls from the right mill product.
2.2 Gate 2, section thickness picks T6 against T651
Measure the finished section thickness against the 0.25 in line in the ASTM B209 tables. Below 0.25 in, 6061 is supplied as T6 sheet; at 0.25 in and above, it is supplied as T651 plate, and AMS 4027 pairs the same split for aerospace stock. Ordering T651 for a 0.1 in part leaves the mill with no matching product, so you either accept T6 or pay to cut down custom plate. What is stockable is set by the thickness on your model, so confirm it before you write the temper on the drawing.
2.3 Gate 3, the flatness callout decides if you pay for stress relief
Read the flatness and tolerance callouts your part needs. A callout under about 0.005 in over the length, or any thin-wall part that loses symmetry during machining, needs a stress-relieved temper, so order T651 plate or T6511 extrusion. A loose bracket with open tolerances runs in T6 and saves the stretch premium. This gate is where you spend or save money, because stress relief you do not need is wasted cost, and stress relief you skip on a tight part is a scrapped blank.
Engineering note. Run the gates in order, because form and thickness cap what the mill can supply before the flatness callout applies. When a thin sheet part in T6 shows a tight flatness callout, T651 is not stocked under 0.25 in, so the fix is a design or process change or a move to thicker plate machined down. The exception is a part you can nest in thicker plate, where you buy T651 and accept the added stock cost to hold flatness.
| 1. Form | Plate/sheet or extruded stock (ASTM B209 vs B221) | Extrusion orders as T6511; plate and sheet stay on the T6/T651 track |
| 2. Thickness | Finished section against 0.25 in (ASTM B209, AMS 4027) | Under 0.25 in ships as T6 sheet; 0.25 in and over ships as T651 plate |
| 3. Flatness | Post-machining movement the part needs | Under about 0.005 in or thin-wall needs stress relief (T651/T6511); open tolerance runs in T6 |
3. Mechanical properties and post-machining movement
All three tempers reach about 45 ksi (310 MPa) tensile and 40 ksi (276 MPa) yield in typical ASM Handbook Vol.2 data, and their guaranteed minimums track product form. The number that separates them on a machined part is post-machining movement, an industry-reported 0.010 to 0.030 in for T6 against under 0.002 in for stress-relieved T651.
Put the three side by side and the strength lines up. T6 sheet and T651 plate both carry a 42.0 ksi tensile and 35.0 ksi yield minimum in ASTM B209, and cold-finished T651 bar holds the same 42.0 ksi in ASTM B211. Extruded T6511 sits lower at 38.0 ksi in ASTM B221, a difference that comes from the extrusion product form and applies to extruded T6 as well. Picking a temper for strength alone changes almost nothing on your part, so spend the decision on stability.
The gap that matters opens after the first cut. Stress-relieved T651 and T6511 hold post-machining movement under 0.002 in in industry reports, while as-quenched T6 can walk 0.010 to 0.030 in on the same part. On a datum face or a mating bore, 0.010 in of movement is a failed inspection and a recut, so on a tight part the stress-relief premium comes in under the cost of the second setup. EPOC runs T6 and T651 on the same precision CNC machining line, so choosing the stress-relieved temper changes your stock cost, not your cycle time.
According to Dewey Wu, most of that movement shows up in the first heavy roughing pass, so on a T6 plate part EPOC leaves 0.5 to 1 mm of finish stock, lets it rest, and re-indicates before the finish cut to catch what walked. The DFM mistakes that delay CNC prototypes guide covers the wall thickness, setup, and fixturing rules that keep even T6 within tolerance.
Engineering note. Match the temper to the tightest datum on the part, because one loose face does not lower the grade you need. When a plate part has open tolerances everywhere except a 0.001 in flatness datum, that single datum pulls the whole part to T651, since you cannot stress-relieve one feature in isolation. The exception is a part you can split into two operations, roughing in cheaper T6 and finishing after a separate stress-relief step, which only pays when T651 plate is not stocked in your size.
| Tensile strength, min | 42.0 ksi (290 MPa) | 42.0 ksi (290 MPa) | 38.0 ksi (262 MPa) | ASTM B209 sheet and plate, B221 extrusion |
| Yield strength, min | 35.0 ksi (240 MPa) | 35.0 ksi (240 MPa) | 35.0 ksi (240 MPa) | ASTM B209, B221 |
| Tensile strength, typical | 45 ksi (310 MPa) | 45 ksi (310 MPa) | 45 ksi (310 MPa) | ASM Handbook Vol.2 |
| Yield strength, typical | 40 ksi (276 MPa) | 40 ksi (276 MPa) | 40 ksi (276 MPa) | ASM Handbook Vol.2 |
| Fatigue strength at 5×10^8 cycles | 14 ksi (97 MPa) | 14 ksi (97 MPa) | 14 ksi (97 MPa) | ASM Handbook Vol.2 |
| Post-machining movement | 0.010 to 0.030 in | under 0.002 in | under 0.002 in | Industry-reported |
Note: ASTM B221 is inch-pound, so the extrusion SI value is converted. ASM Vol.2 lists typical values under T6 and T651; T6511 shares them within the alloy. Post-machining movement figures are industry-reported ranges, not EPOC measurements.
4. Thin-wall and large-flat parts, the warp trap
Thin-wall and large-flat 6061 parts are where T6 fails on flatness, so both call for stress-relieved T651 or T6511. A wall under about 3 mm or a plate over roughly 150 mm across has too little section to hold the quench stress in balance once machining removes one side.
A thin wall moves because you cut its symmetry away. As-quenched T6 carries balanced residual stress through the blank; mill a pocket or thin a wall and the remaining metal relaxes toward the cut, bowing the part by the industry-reported 0.010 to 0.030 in. On a 2 mm wall that bow is often larger than the wall tolerance itself, so the part leaves the machine already out of print. Order T651 or T6511 for these and the stretch has removed most of that stress before the first pass.
Large flat plates fail the same way over a longer span. A lid or baseplate skimmed on one face pulls into a dish, and the longer the part, the more total movement you see across the diagonal. T651 plate holds flat enough to re-fixture and finish, which is why optical brackets, precision jigs, and aerospace panels are specified in it. Running one of these in T6 to save on stock costs more in scrap and a second setup than the T651 premium saves.
Engineering note. Below about 3 mm wall or 150 mm flat span, order T651 or T6511 and design in a stress-relief pass, because there is not enough section left to redistribute quench stress after roughing. The exception is a short, chunky part with walls over 6 mm, which stays stable in T6 and does not earn the stretched-stock premium. EPOC reviews wall thickness against temper in free DFM feedback before quoting, so the warp risk is caught on the model.
| Wall under 3 mm | Bows past wall tolerance on one-side cuts | T651 or T6511 with finish stock left |
| Flat plate over 150 mm | Dishes across the span after face skim | T651 plate |
| Short block, wall over 6 mm | Stays within tolerance | T6, no premium |
5. How to specify and verify the temper on the drawing
Write both the alloy-temper and the governing specification on the drawing, because the temper alone does not tell the mill which product form to pull. A complete callout reads 6061-T651 per ASTM B209 for plate, 6061-T6511 per ASTM B221 for extrusion, or 6061-T651 per AMS 4027 when the part needs aerospace certification. The specification number locks the form, the property minimums, and the stress-relief method in one line.
Lock the temper when stability governs the part, and leave it open when it does not. A drawing that reads 6061-T6 with no specification lets the supplier ship T6, T651, or T6511 to their stock, which is fine for a loose bracket and a scrap risk on a tight datum. Naming the temper and spec removes that choice and the inspection surprise that follows it.
You cannot confirm the temper on the bench, so verify it on paper. 6061-T6 and T651 hold the same 95 HB hardness and the same 45 ksi typical strength in ASM Vol.2 data, so a hardness check will not separate them. The temper comes from the mill test report, which ties the plate or bar to its heat lot and its stress-relief record, and lists the property minimums you can match against a 6061 material properties reference. Ask for the MTR at receipt and match the temper, specification, and lot to your drawing before the material hits the machine.
Engineering note. Require the MTR on any part with a flatness or fatigue callout, because the temper you paid for is only provable on the certificate. When a lot arrives with a T6 cert against a T651 drawing, reject it at receiving, since machining it first turns a paperwork mismatch into scrapped parts and lost lead time. Anodize interacts with temper on extrusions too, which the pre-anodize machining allowance guide covers for cosmetic and dimensional callouts.
| Plate | 6061-T651 per ASTM B209 | Locks stress-relieved plate and 42.0 ksi min |
| Extrusion | 6061-T6511 per ASTM B221 | Locks stretched and straightened profile |
| Aerospace | 6061-T651 per AMS 4027 | Adds certified 1.5 to 3% stretch and traceability |
Frequently asked questions
Do I need T651 for machining, or will T6 work?
Order T651 when the part has a flatness datum under about 0.005 in, a wall under 3 mm, or a flat span over 150 mm; T6 works for open-tolerance brackets and blocks. The split comes from residual stress. Both hold the same 45 ksi typical strength in ASM Vol.2 data, so strength does not decide it. As-quenched T6 walks 0.010 to 0.030 in after a one-sided cut in industry reports, while stress-relieved T651 stays under 0.002 in. On a tight datum, the T651 stock premium costs less than a scrapped blank and a second setup, often a full day of machine time lost to the recut.
Why does my 6061 part warp after machining?
6061-T6 warps because the quench locks balanced residual stress into the blank, and cutting one side lets the rest relax toward the cut. The bow runs an industry-reported 0.010 to 0.030 in, often larger than a thin-wall tolerance, so the part leaves the machine out of print. T651 and T6511 take a 1.5 to 3 percent stretch that removes most of that stress before you cut, per AMS 4027. If a T6 part keeps moving, switch the stock to T651, or leave 0.5 to 1 mm of finish stock and add a rest before the finish pass, then re-indicate to catch what moved.
What is the difference between T6 and T651?
T651 is T6 plate given a controlled 1.5 to 3 percent stretch before aging to relieve residual stress, per AMS 4027. Both reach the same 42.0 ksi tensile and 35.0 ksi yield minimum in ASTM B209 and the same 95 HB hardness in ASM Vol.2, so they match on strength and hardness. The difference is dimensional stability during machining. T651 holds post-machining movement under 0.002 in in industry reports, while plain T6 moves 0.010 to 0.030 in on the same cut. That gap decides datum faces, thin walls, and any part with a tight flatness callout, where the added stock cost buys back the flatness you would otherwise lose.
Is T651 stronger than T6?
T651 is not stronger than T6. Both carry a 42.0 ksi tensile and 35.0 ksi yield minimum in ASTM B209 and about 45 ksi typical tensile in ASM Vol.2, and their fatigue strength is the same 14 ksi at 5×10^8 cycles. The stretch that makes T651 relieves residual stress; it does not raise the strength numbers. Choose T651 for dimensional stability on machined parts. Strength is identical either way. If you need higher strength, move to a different alloy such as 7075, covered in our 6061 vs 7075 selection guide, which trades weldability for a higher strength-to-weight ratio and cannot be fusion welded.
Which temper should I order for thin-wall parts?
Order T651 plate or T6511 extrusion for any wall under about 3 mm. A thin wall has too little section to hold quench stress in balance, so a T6 wall bows toward the cut by an industry-reported 0.010 to 0.030 in, often past the wall tolerance itself. T651 and T6511 remove most of that stress through the stretch before machining. Leave 0.5 to 1 mm of finish stock, let the part rest, and re-indicate before the finish cut to catch any remaining movement on the tightest feature. For walls over 6 mm, T6 stays stable and saves the stretched-stock premium.
Can I order T6 and receive T651 or T6511?
An order for 6061-T6 can ship as T6511 but not as T651, and the reason is product form. T6511 is the extruded, stress-relieved equivalent that meets the same 6061-T6 property calls, while T651 is plate on a different thickness and form basis. To avoid a substitution surprise, name both the temper and the specification, such as 6061-T651 per ASTM B209 for plate or 6061-T6511 per ASTM B221 for extrusion. Require the mill test report at receipt to confirm what shipped against your drawing, and reject any lot whose temper or lot number does not match before it reaches the machine.
Related resources
Related Capability: CNC machining for tight-tolerance aluminum parts. EPOC holds ±0.05 mm on critical features across T6 and T651 stock.
Related Article: 6061 vs 7075 aluminum comparison. When higher strength justifies the cost and the machining trade-offs.
Related Material Guide: 6061 aluminum grade overview. The base properties, composition, and variants all three tempers share.
Related Standards Reference: flatness and tolerance callouts. How to write the datum and flatness numbers that decide the temper choice.
Related Article: machining 6061 for anodize. Pre-anodize dimensions, surface finish, and masking decisions once the temper is set.
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