6061-T6 vs T651 vs T6511: Which Temper Do You Actually Need?
By NOX METALS
6061-T6, T651 and T6511 are the same alloy, but the suffixes tell you whether the material was stress relieved by stretching and which product form it is. T651 is used for rolled plate and cold-finished bar. T6511 is used for extruded bar, rod and shapes. The practical consequences are that stress-relieved tempers are more dimensionally stable during machining and that guaranteed properties depend on product form, dimension, test direction, and specification. This post explains which one to specify and when the difference actually matters.
In Short
- →All three are 6061 solution heat treated and artificially aged. The suffixes tell you whether it was stress relieved and what product form it is.
- →T6 is the base condition, no stress relief specified. T651 is plate and cold-finished bar, stress relieved by stretching. T6511 is extrusions, stretched then lightly straightened.
- →For the cited longitudinal ranges, yield is 35,000 psi minimum, but ultimate strength depends on form and dimension: cold-finished bar and ASTM B209 T651 plate through 4 inches are 42,000 psi; T651 plate at 4.001–6 inches is 40,000 psi; referenced extrusions are 38,000 psi.
- →If you are machining, specify a stress-relieved temper. Unrelieved material moves when you cut material off one side, and on a long or thin part that is the difference between a good part and scrap.
- →You rarely choose the temper freely. The product form chooses it: plate comes as T651, extruded bar comes as T6511.
- →Read the ultimate minimum on your print before substituting extruded bar for a cold-finished or plate callout.
The Three Tempers Side by Side
One table settles most of the confusion.
| Temper | Product form | Stress relieved | Min ultimate | Min yield |
|---|---|---|---|---|
| 6061-T6 | Base condition; multiple forms | Not specified | Varies by form and dimension | Varies by form and dimension |
| 6061-T651 | Rolled plate, ASTM B209 | Yes, by stretching | 42,000 psi through 4 in; 40,000 psi at 4.001–6 in | 35,000 psi through 6 in |
| 6061-T651 | Cold-finished bar | Yes, by stretching | 42,000 psi | 35,000 psi |
| 6061-T6510 | Extrusions | Yes, no straightening after | 38,000 psi | 35,000 psi |
| 6061-T6511 | Extrusions | Yes, minor straightening after | 38,000 psi | 35,000 psi |
What the Digits Mean
The temper code is a readable string once you know the grammar, and it is worth being able to read it off a print without looking anything up.
- T = thermally treated to a stable temper other than F, O or H.
- T6 = solution heat treated, then artificially aged. This is the step that produces the strength.
- 51 = stress relieved by stretching. Appended to T6 it gives T651.
- 510 = stretched, and not straightened afterwards. Applies to extruded rod, bar and shapes.
- 511 = stretched, with minor straightening permitted afterwards to meet dimensional tolerances.
- So T6511 reads as: solution heat treated, stretched to relieve stress, artificially aged, with minor straightening permitted after stretching.
Why Stress Relief Decides Whether Your Part Holds Size
This is the reason the suffix exists and the reason it belongs on your purchase order.
- Quenching after solution heat treatment traps residual stress in the material. While the piece is whole and symmetric, that stress is balanced and invisible.
- Machining unbalances it. Cut a deep pocket in one face, or take a flat off one side of a bar, and the remaining stress pulls the part out of shape.
- The effect scales with how much you remove and how slender the part is. A small bracket barely notices. A 24 inch long plate machined to a thin web bows.
- Stretching the material past yield after quenching relieves that stress at the mill, before you ever touch it.
- The same logic drives T7451 in thick 7050 plate. Stress relief is a machinability specification, not a strength specification.
You Usually Do Not Get to Choose
A point that saves a lot of fruitless searching: the temper mostly follows from the product form you need.
| You need | You will be buying | Temper you will get |
|---|---|---|
| A rectangular blank, any size | Rolled plate cut to size | 6061-T651 |
| A long round section | Extruded round bar | 6061-T6511 |
| A long rectangular section | Extruded flat bar | 6061-T6511 |
| Tight-tolerance small diameter bar | Cold-finished bar | 6061-T6 or T651 |
| A hex or square section | Extruded bar | 6061-T6511 |
When the Difference Actually Matters
Four situations where getting this wrong has a cost, and one where it does not.
- Your print calls a minimum ultimate of 42,000 psi. Extruded T6511 bar does not meet that on paper. Cold-finished bar or ASTM B209 T651 plate through 4 inches may meet it, but verify the form, thickness, orientation, and governing specification before substituting.
- Your part is long, thin, or heavily pocketed. Specify a stress-relieved temper explicitly rather than accepting plain T6.
- You are on a certified job. The certification states the temper, and a receiving inspector will compare it to the print. A substitution that is metallurgically fine can still be rejected on paperwork.
- You are welding. All of these lose strength in the heat affected zone regardless of starting temper, so the temper argument matters less than the joint design does.
- For a yield-driven structural part in a stocky section, all three share the cited 35,000 psi longitudinal minimum yield. That alone does not make them interchangeable; the controlling specification, orientation, fatigue and toughness requirements, and engineering approval still decide whether a substitution is acceptable.
Treat the suffix as two pieces of information: was it stress relieved, and what form is it. Specify a stress-relieved temper for anything you are going to machine seriously, let the product form pick between T651 and T6511, and check the ultimate minimum on your print before assuming extruded bar can stand in for plate or cold-finished bar.
Frequently Asked Questions
What is the difference between 6061-T6 and 6061-T651?
T651 receives the same solution-treatment and artificial-aging basis as T6, with a controlled stretch after solution treatment to reduce residual quench stress before aging. For a given product form and dimension range, the specified strengths can match, while the stress relief improves machining stability. T651 is the normal temper for rolled plate.
What is the difference between T651 and T6511?
Product form. T651 applies to rolled plate and cold-finished bar; T6511 applies to extruded bar, rod, and shapes with minor straightening permitted after stretching. Guaranteed properties then depend on the product specification and dimension: the referenced extrusion is 38,000 psi ultimate, cold-finished bar is 42,000 psi, and ASTM B209 T651 plate is 42,000 psi through 4 inches and 40,000 psi at 4.001–6 inches.
Is T6511 stronger than T6?
No. They have the same specified strength for a given product form. T6511 adds stress relief, which is about dimensional stability during machining rather than about strength.
Can I substitute 6061-T6511 bar for a 6061-T651 callout?
Not without engineering and specification review. They are different product forms with different guaranteed properties, tolerances, and test orientations. A shared cited longitudinal yield minimum is not enough to approve a substitution; the drawing authority must verify form, governing specification, dimension range, orientation, fatigue and toughness requirements, and certification.
Which 6061 temper is best for machining?
Any stress-relieved temper: T651 for plate and cold-finished bar, T6511 for extrusions. Plain T6 without stress relief is the one to avoid for parts with heavy or asymmetric material removal, because the part will move as you cut.
Right temper, right form
Tell us the part, we will price the right stock.
6061-T651 plate cut to any rectangle, or 6061-T6511 bar cut to length. Mill certs with heat number on every order.