6061-T6 vs 6061-T651
T6 and T651 are the same 6061 alloy, solution heat treated and artificially aged to the same minimum properties. The 51 means the material was stretched a controlled 1 to 3% permanent set after quenching and before aging. That stretch relieves the residual stress locked in by the quench, so a T651 plate does not bow or twist when you machine away one face.
Buy T651 for plate that will be machined asymmetrically, especially thick plate with deep pockets or thin floors. For sheet, extrusions, rod and parts that are lightly machined, T6 behaves the same and is what the mill sells. Do not pay a premium for T651 where it is not stocked; most plate 6.35 mm (0.25 in) and up already is T651.
| Property | 6061-T6 | 6061-T651 |
|---|---|---|
| Alloy | 6061 | 6061 |
| Heat treatment | Solution treat, quench, artificial age | Solution treat, quench, stretch 1-3%, artificial age |
| Tensile (min, plate) | 290 MPa (42 ksi) | 290 MPa (42 ksi) |
| Yield (min, plate) | 240 MPa (35 ksi) | 240 MPa (35 ksi) |
| Typical tensile / yield | 310 / 276 MPa (45 / 40 ksi) | 310 / 276 MPa (45 / 40 ksi) |
| Residual stress | Present from the quench | Relieved by stretching |
| Machining distortion | Can bow when one side is removed | Stays flat |
| Product forms | Sheet, extrusions, rod, tube, thin plate | Plate (most thicknesses), some bar |
| Density | 2.70 g/cm³ | 2.70 g/cm³ |
| Cost | Baseline | Same or slightly more; usually the stocked plate temper |
Typical values for the stated conditions. Confirm against the mill certificate and your spec.
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Decision matrix
A check marks the side with the edge on that dimension.
| Dimension | 6061-T6 | 6061-T651 |
|---|---|---|
| Machining | Good; thick parts may move after roughing | ✓ Good; predictable flatness on deep pockets |
| Welding | Good; HAZ softens | Good; identical behavior |
| Forming | Fair; T6 temper limits bends | Fair; identical |
| Strength | Same minimums | Same minimums |
| Corrosion resistance | Good | Good; identical |
| Finishing | Anodizes well | Anodizes identically |
| Material cost | Baseline | Same or a small premium on thin plate |
| Availability | Sheet, extrusions, rod, tube | Plate; specify it and the supplier will match |
Choose 6061-T6 when
- Sheet, extrusions, tube and rod, where T651 does not exist
- Parts that are lightly machined or machined symmetrically
- Formed or welded parts where residual stress is reset anyway
Choose 6061-T651 when
- Plate that will be machined on one side or pocketed deeply
- Thin-floored or thin-walled parts cut from thick plate
- Fixture plates, tooling plates and anything with a flatness callout after machining
- Long, slender parts that would banana when a face is removed
Frequently asked questions
- What does T651 mean?
- T6 plus a stress relief by stretching. After quenching, the plate is pulled to a permanent set of 1 to 3%, then artificially aged. The stretch evens out the quench stresses that otherwise release as distortion during machining. Mechanical minimums are the same as T6.
- Is 6061-T651 stronger than T6?
- No. Both meet the same minimums for a given product form, and typical values are identical. The only difference is residual stress, which affects how the material behaves when machined, not how strong it is.
- Should I specify T651 on my drawing?
- For machined plate parts, yes: write "6061-T651" or "6061-T6/T651" so the shop can buy plate, which is stocked as T651 anyway. For sheet, extrusion or rod parts, write T6, because the stretched temper is not produced in those forms.
