The Same Alloy, Two Processing Routes
6061 is a precipitation-hardened Al-Mg-Si alloy with silicon at 0.40-0.80% and magnesium at 0.80-1.20%, plus copper at 0.15-0.40% and chromium at 0.04-0.35% under ASTM B209/B209M. Tempers describe how that analysis has been thermally and mechanically treated, and the definitions come from ANSI H35.1/H35.1(M).
Both T6 and T651 begin with the same three-stage treatment. The alloy is solution heat treated at roughly 530-550 °C, quenched, usually in water, and then artificially aged at about 175 °C for several hours to precipitate the Mg2Si phase that provides strength. The only difference between the two tempers is the mechanical step inserted after quenching and before aging.
Where T651 Diverges from T6
Quenching a thick section leaves a residual stress pattern, tensile near the surface and compressive in the core. When metal is removed during machining, that balance shifts and the part moves. T651 addresses this by stretching the plate by roughly 1 to 3% between quenching and aging. The stretch yields the material slightly and equalises the internal stresses, so a subsequent machining operation removes metal from a far more stable stock.
| Item | 6061-T6 | 6061-T651 |
|---|---|---|
| Solution heat treatment | About 530-550 °C | Same |
| Quench | Water or polymer | Same |
| Stress relief | None | Stretched 1-3% |
| Artificial aging | About 175 °C, several hours | Same |
| Tensile strength, typical | About 310 MPa | About 310 MPa |
| 0.2% proof stress, typical | About 276 MPa | About 276 MPa |
| Typical Brinell hardness | About 95 HB | About 95 HB |
| Residual stress | Higher | Substantially lower |
| Machining stability | Prone to movement on heavy cuts | Stable, holds flatness |
| Availability by form | Sheet, plate, extrusions, bar | Mainly plate and bar |
Because properties are the same, a certificate quoting either temper will show near-identical tensile and hardness results. The practical difference appears only when geometry is machined from thick stock.
Practical Consequences in the Workshop
Dimensional control: a part with tight flatness or bore-position tolerance should be quoted from T651, since T6 plate can bow or twist as layers are removed.
Machining strategy: both tempers machine well, but T6 usually needs more roughing allowance and an intermediate stress relief pause to reach the same result.
Welding: behaviour is equivalent, since the difference in residual stress is small compared with the heat of welding, and both lose strength in the heat-affected zone.
Anodising and finishing: identical responses, because the chemistry and precipitation state are the same.
Lead time and cost: T651 typically carries a slightly higher price and longer lead time since plate must be stretched on specialised equipment.
Thickness limits: the stretch process is applied to plate and bar rather than thin sheet, so very thin gauges are normally supplied as T6 only.
Typical Applications of Each Temper
T651: precision machined plates, mould and die bases, fixture and tooling plates, vacuum chucks, semiconductor and automation components, large flat frames.
T6: general structural members, brackets, welded assemblies, formed sheet components, marine fittings, automotive parts and anodised trim.
Where a drawing simply states 6061-T6 without conditions, procurement should confirm whether dimensional stability is critical; if it is, the specification is best changed to T651 before the order is placed rather than corrected after a part has moved out of tolerance.
Verification and Documentation
Acceptance testing for either temper follows ASTM B209/B209M: chemical analysis per heat, tensile and proof stress per production lot, and hardness checks where agreed. Dimensional inspection should include thickness tolerance and flatness, since flatness is the property most affected by the choice between the two tempers.
Composition limits verified against ASTM B209/B209M for alloy 6061.
Tensile, proof stress and elongation reported per lot.
Hardness survey available for temper verification.
Flatness and thickness measured against the standard's tolerances and any tighter purchase specification.
EN 10204 3.1 mill test certificate with heat number and full test results.
Frequently Asked Questions
Q: Are 6061-T6 and 6061-T651 different alloys?
A: No. They are the same alloy with the same chemistry; the difference is the stress-relieving stretch applied during processing of T651.
Q: Which temper is stronger?
A: Neither. Typical values for both are about 310 MPa tensile and 276 MPa proof stress, with Brinell hardness near 95.
Q: Why choose T651 if properties are identical?
A: For dimensional stability. Lower residual stress keeps heavy machined parts flat and within tolerance.
Q: Can T6 be substituted for T651?
A: Only after assessing risk. On lightly machined or formed parts it is usually acceptable; on precision parts the substitution frequently leads to distortion.
Q: Is T651 available in sheet?
A: It is primarily a plate and bar product, because the stretching operation needs a section thick enough to grip and elongate.
Q: Does the temper affect anodising results?
A: No. Both tempers share the same chemistry and precipitation state, so the anodic film forms identically.
