Start With the Property Question, Not the Temper
Both tempers of 6061 are supplied to the same technical levels under ASTM B209/B209M: typical tensile strength near 310 MPa, proof stress near 276 MPa and Brinell hardness around 95. Temper designations follow ANSI H35.1/H35.1(M). If the application is limited by strength or hardness, the T6 or T651 decision does not change the answer. The question that does change the answer is how much the finished part will move when metal is removed from it.
6061-T651 differs from T6 only because the plate is stretched by roughly 1 to 3% after quenching and before aging, which releases most of the quenching stress. That single step is what makes a precision part hold its flatness.
A Decision Framework for Buyers and Designers
| Selection factor | Choose T6 | Choose T651 |
|---|---|---|
| Metal removal from stock | Under about 20% of section thickness | More than about 20%, or deep pockets and long slots |
| Flatness after machining | Not critical, or generous tolerance | Critical, with tight tolerance over long spans |
| Plan dimensions | Small to medium parts | Large plates, frames and bases |
| Bore and hole position tolerance | Loose positional tolerance | Positional tolerance near the limit of the machine |
| Forming requirement | Bending, rolling or stretch forming after delivery | No forming, machining only |
| Thickness available | Sheet from about 0.5 mm upward | Plate and bar from several millimetres upward |
| Cost and lead time | Lower cost, shorter lead time, wider stock | Higher cost, longer lead time |
Where two of the T651 columns apply, the safer commercial decision is almost always T651. The incremental material cost is small compared with the cost of scrapping or reworking a distorted part.
Machining and Process Planning Consequences
Roughing strategy: T6 plate is often machined in two stages with a pause to let stress equilibrium settle, while T651 can normally be machined in a single setup.
Fixturing: T6 parts may need stronger or more frequent clamping and even sacrificial tabs, adding cycle time.
Heat generation: both tempers machine cleanly, but excessive heat in T6 increases the risk of movement as stresses redistribute.
Inspection: T651 reduces the need for intermediate dimensional checks after each operation.
Post-machining finishing: anodising and painting behave identically in both tempers because the chemistry is unchanged.
Welding is a separate consideration and behaves similarly in both tempers: the heat-affected zone loses strength because the aging precipitates coarsen, so a welded and machined part needs either acceptance of reduced local properties or a full solution treat and age cycle after welding.
Applications That Typically Drive Each Choice
T651: mould bases and cavity plates, jigs and fixtures, robot and automation frames, vacuum tables, large inspection plates, semiconductor tooling.
T6: formed sheet brackets, enclosures, welded structures, marine fittings, automotive components, anodised decorative parts, general fabrication.
Either temper: non-critical spacers, prototype plates, covers and panels where flatness and stock removal are modest.
Ordering Details and Verification on Receipt
Purchase orders for 6061 plate should state alloy, temper, thickness and tolerances, width and length, edge condition, surface requirement and the governing standard, together with the documentation package expected on delivery. Where the end use involves heat treatment, welding or anodising, those operations should be declared so that the supplier can confirm whether the chosen temper and thickness remain suitable.
Order to ASTM B209/B209M unless a national standard such as GB/T 3880.2 or EN 485-2 is specified.
Quote thickness tolerance class explicitly; thin plate tolerances differ from heavy plate.
State flatness requirements, since this is the property most affected by the temper choice.
Request EN 10204 3.1 mill test certificates with heat number, chemistry and mechanical results.
Agree protective packaging, edge protection and marking, and confirm whether PVC film may remain on the surface during machining.
On arrival, verify the heat number against the certificate, check thickness at several points across the plate, and inspect for flatness deviation, surface scratches and edge damage. Where the material will be heavily machined, a small test cut on an offcut is a cheap way to confirm that the plate behaves as expected before production metal is committed.
Certificate data reviewed against ASTM B209/B209M limits for alloy 6061.
Dimensional check of thickness, width, length, squareness and flatness.
Hardness spot check to confirm temper condition.
Visual inspection of surface quality and edge condition.
Storage on flat timber or steel supports, clear of moisture, to preserve flatness before machining.
Frequently Asked Questions
Q: How much stock can be removed from 6061-T6 before distortion becomes a problem?
A: It depends on geometry, but removing more than about 20% of the section thickness from T6 plate commonly causes movement; T651 is preferred above that level.
Q: Is T651 worth the extra cost?
A: For precision parts, yes. The cost difference is usually far smaller than the cost of scrapping or reworking a distorted component.
Q: Does T651 have better corrosion resistance than T6?
A: No. Corrosion behaviour is determined by chemistry and surface treatment, which are identical for both tempers.
Q: Can I specify T6 and machine it like T651 if I take light cuts?
A: Sometimes, but the result is machine and geometry dependent. Light cuts reduce but do not eliminate the stress release that causes movement.
Q: What tolerances should be quoted on the order?
A: Thickness tolerance class, width and length tolerance, squareness and flatness. Flatness is the most important one to state when the part will be machined from plate.
Q: Which documents should accompany the shipment?
A: An EN 10204 3.1 mill test certificate with heat number and mechanical results, plus dimensional inspection records and marking that identifies alloy, temper and heat.
