6061-T651 Aluminum Plate properties at a glance
Values labeled typical are orientation- and section-dependent references, not design allowables. Use the current governing specification and the supplied mill certificate for acceptance.
| Property | Reference value | How to use it |
|---|---|---|
| Density | 0.0975 lb/in³ (2.70 g/cm³) | Nominal; use the mill cert for controlled calculations |
| Ultimate tensile strength | 45 ksi typical | Representative T651 plate value; minimums vary with thickness and specification |
| Yield strength | 40 ksi typical | Representative T651 plate value; use design allowables where required |
| Elongation | 12% typical | Direction and thickness affect reported values |
| Hardness | 95 HB typical | Reference value, not an acceptance limit unless the order says so |
History of 6061
The alloy began as Alcoa 61S in 1935, created to fill a practical middle ground: a heat-treatable aluminum with useful structural strength, corrosion resistance, weldability, and an ability to take an anodized finish. When the Aluminum Association adopted the four-digit wrought-alloy system in the 1950s, 61S became 6061. It went on to become the most broadly used member of the 6xxx magnesium-silicon family.
T651 is not a different chemistry from 6061-T6. It is a product condition developed around a manufacturing problem: quenching and aging can leave residual stress in thick rolled stock. Stretching the plate after solution heat treatment reduces that locked-in stress before final aging. The result gives machine shops a more stable starting blank while retaining the familiar 6061 strength and fabrication behavior.
- 1935
61S is introduced
Alcoa introduces the medium-strength, heat-treatable alloy that will later carry the 6061 designation.
- 1950s
6061 becomes the standard name
The four-digit wrought-alloy designation system replaces the older company designation.
- Today
T651 becomes the machining default
Stress-relieved plate is widely specified for brackets, fixtures, housings, bases, and general CNC work.
What T651 means
The processing sequence is solution heat treatment, controlled stretching to reduce residual stress, and then artificial aging. The T6 portion describes solution treatment plus artificial aging, while the added 51 identifies the intervening stress-relief stretch. On a plate purchase order, the suffix tells the mill and distributor that dimensional stability matters; it is not merely a marketing variation of T6.
T651 should not be confused with T6511. T651 is associated with rolled plate and some cold-finished bar. T6511 is associated with extrusions and permits minor straightening after stretching. Chemistry may be the same, but the governing product specification, dimensional tolerances, and guaranteed properties can differ.
- Choose T651 for machined plate and blocks.
- Use the exact product form and specification on the drawing.
- Do not use a typical-property table as a substitute for the mill certificate.
Machining, welding, and finishing
6061-T651 is forgiving in the machine shop. It supports high removal rates, produces manageable chips with sharp geometry, and is easier to finish than most high-strength 7xxx plate. Stress relief reduces movement, but it does not eliminate it: rough both sides symmetrically, leave finishing stock, and allow a part to relax before the final pass when flatness is critical.
Unlike 7075, 6061 is readily welded by common processes. The heat-affected zone loses T6/T651 strength, so a welded design must use post-weld properties rather than the parent-metal value. Clear and color anodizing are common, but alloy lot, surface condition, and process control can change the appearance.
How to specify and buy it
A useful purchase description includes alloy, temper, product form, governing specification, nominal thickness, cut dimensions, quantity, and any origin or traceability requirement. If a drawing cites AMS 4027, do not accept a generic “6061 plate” substitution without checking that the material certification reports the requested specification and revision.
For cut blanks, separate mill thickness tolerance from saw-cut length and width tolerance. Ask whether dimensions are nominal or guaranteed and leave machining stock accordingly. Record the heat or lot number from the cert so repeat parts can be investigated if machining response or anodized color changes.
Specifications and purchase callouts
The controlling drawing decides which specification applies. A similar alloy name or typical-property table does not demonstrate compliance.
- ASTM B209
- SAE AMS 4027
- AMS-QQ-A-250/11 (legacy federal callout)
- ASME SB-209 where applicable
Frequently asked questions
Is 6061-T651 the same as 6061-T6?
They share the same alloy chemistry and T6 heat-treatment basis, but T651 adds stress relief by stretching. That extra processing makes T651 the usual choice for rolled plate that will be machined heavily. A drawing should still call out the exact temper it requires.
What is 6061-T651 used for?
Common uses include machined brackets, fixtures, tooling bases, housings, mounting plates, marine hardware, and general structural parts. It is selected when a balanced mix of machinability, corrosion resistance, weldability, availability, and cost matters more than maximum strength.
Will 6061-T651 stay perfectly flat during machining?
No wrought plate is completely free of residual stress. T651 is stretched to reduce it, not eliminate it. Symmetric roughing, sensible workholding, an intermediate relaxation step, and adequate finish stock are still important on thin walls or aggressively pocketed parts.
Which specification should be on the purchase order?
ASTM B209 is the common commercial plate specification, while aerospace drawings often cite AMS 4027 or the legacy AMS-QQ-A-250/11. Use the specification on the controlling drawing and verify it on the mill certification rather than assuming the names are interchangeable.
Primary references
Technical standards change. These sources support the history and reference discussion; obtain the current specification revision for design or acceptance work.
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