6061-T6 vs T651 vs T6511: Which Temper Do You Actually Need?
By Sabine Wren
6061-T6, T651 and T6511 are the same alloy, solution heat treated and artificially aged to the same T6 condition. The suffixes tell you two things: whether the material was stress relieved by stretching, and what product form it is. T651 is rolled plate and cold-finished bar. T6511 is extruded bar, rod and shapes. The practical consequences are that stress-relieved tempers hold their shape when machined, and that extruded product carries a lower guaranteed ultimate tensile strength than plate or cold-finished bar at the identical yield. This post explains which one to specify and when the difference actually bites.
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.
- →Yield is 35,000 psi minimum across all of them. The ultimate minimum is not the same: 42,000 psi for rolled plate and cold-finished bar, 38,000 psi for extruded product.
- →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 | Any, base condition | Not specified | Varies by form | 35,000 psi |
| 6061-T651 | Rolled plate, 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: heat treated, aged, stretched to relieve stress, then straightened a little.
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. Buy plate or cold-finished bar.
- 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.
- Where it does not matter: a yield-driven structural part in a stocky section. Yield is 35,000 psi across all three and the parts will behave identically.
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 is T6 that has additionally been stress relieved by stretching. Both are solution heat treated and artificially aged to the same strength, but T651 has had residual quench stress relieved, so it holds dimension far better when you machine material off one side. 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 to extruded bar, rod and shapes with minor straightening permitted after stretching. Both are stress relieved and both carry a 35,000 psi yield minimum, but extruded product has a 38,000 psi ultimate minimum against 42,000 psi for plate and cold-finished bar.
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 checking. They are different product forms with different guaranteed ultimate minimums, 38,000 psi versus 42,000 psi. If the print is yield-driven the substitution is usually harmless, but on a certified job the temper on the cert has to match the print or receiving will reject it.
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.