Guides / Alloys

The Letters After the Alloy Number Are Not Optional

September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked

An aluminum drawing that says “6061” is only half a specification. The alloy number tells a shop the chemistry. The temper tells it how the metal was worked and heat treated, and that decides the strength, how it machines and whether the finished part holds its shape.

What the codes mean

The temper designation follows the alloy with a dash. The first character is the basic treatment.

Code	What was done	Typical use
O	Annealed, the softest state	Material that has to be formed or bent
F	As fabricated, no controlled treatment	Rough stock, rarely specified for a machined part
H	Strain hardened, used on non heat treatable alloys	5052-H32, 5083-H111
T4	Solution treated then naturally aged	2024-T4, parts that will be formed then age
T6	Solution treated then artificially aged to peak strength	The default for machined 6061 and 7075
T651	T6 plus a controlled stretch to relieve internal stress	Plate, and anything where flatness matters
T73	Overaged below peak strength	7075 parts that need stress corrosion resistance

T6 against T651, the one that catches people out

T651 is T6 with an extra step. After solution treatment the plate is stretched a controlled amount, usually one to three percent, which evens out the locked-in stress left over from rolling and quenching.

That matters the moment a machinist starts removing material. Metal with internal stress is in balance with itself. Cut a deep pocket on one side and that balance breaks, so the part bows. On a small bracket nobody notices. On a 300 mm plate with a pocket down the middle, a part machined from plain rolled plate can come off the table visibly curved while the same part from stress relieved plate stays flat.

Practical rule: if the part is cut from plate, if it is large, or if you are removing more than about half the starting volume, write T651. If flatness is the whole point of the part, see whether cast tooling plate is the better answer entirely.

Why 7075-T73 exists

7075-T6 is the strongest common aluminum, and it is also the one most prone to stress corrosion cracking, where a part under sustained tensile load cracks in a corrosive environment over months rather than failing on day one.

T73 is deliberately overaged. It gives up roughly ten to fifteen percent of the yield strength of T6 in exchange for far better resistance to that failure mode. If a 7075 part is permanently loaded, lives outdoors or is used in aerospace, T73 or T7351 is usually the correct call and the drawing should say so.

Machining behaviour by temper

O and T4 are soft and gummy. They smear, build up on the cutting edge and give a poor surface finish. Parts are rarely machined in these tempers unless forming comes later.

T6 and T651 machine well. Chips break, surface finish is predictable and the material is stiff enough to resist cutting forces. This is where you want to be.

T73 behaves much like T6 on the machine. The difference is in service, not in the cut.

The temper also changes how the part anodizes. A consistent temper across a batch is part of getting a consistent colour, because the heat treatment affects the alloy’s microstructure and the oxide grows on top of that.

Writing it on the drawing

Put the full designation in the material block, not in a note somewhere: 6061-T651 PER ASTM B209. That single line tells the shop the chemistry, the treatment, the stress relief state and the standard the certificate should reference.

If you genuinely do not care, say so explicitly: 6061-T6 OR T651. A shop that sees a choice will quote the one it has on the shelf, and you avoid paying for a mill order over a difference that does not affect your part.

What you should avoid is leaving it blank. A blank temper means the shop guesses, and you will not find out what it guessed until the parts arrive.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

• Aluminium alloy temper designations
• ASTM B209, aluminum sheet and plate specification
• Precipitation hardening

Frequently asked questions

Keep reading

• Giving Up 10 Percent of Strength to Stop a Crack That Starts Alone
• Why Thick Plate Is the Reason 7050 Exists