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Aluminum Temper Designations Explained

Guide · Updated October 28, 2026

The letters and digits after an aluminum alloy number, like the T6 in 6061-T6, are the temper: how the metal was strain hardened or heat treated. The system is defined by ANSI H35.1, published by The Aluminum Association. Temper changes strength, machinability, chip form and how much a part moves after roughing, so it matters as much as the alloy. Definitions below follow H35.1; strengths are typical values published by The Aluminum Association and ASM, not specification minimums.

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Step by step

Read the alloy: Four digits: 1xxx to 8xxx series. 2xxx, 6xxx and 7xxx are heat treatable; 1xxx, 3xxx and 5xxx are not.
Read the letter: F as fabricated, O annealed, H strain hardened, W solution treated, T thermally treated.
Read the first digit: For H, the process; for T, the heat treatment sequence.
Read extra digits: H second digit is hardness; T digits 51, 52, 511 are stress relief.
Check the cert: Material certs list the temper and properties; trust the cert over the stamp.
Plan machining: Choose stress-relieved tempers (T651, T7351) for flat, stable parts.

Basic temper letters

F: as fabricated, no property control
O: annealed, the softest and most ductile condition
H: strain hardened by cold work (non-heat-treatable alloys)
W: solution heat treated, unstable, ages at room temperature
T: thermally treated to a stable temper other than F, O or H

H tempers

H1x: strain hardened only
H2x: strain hardened then partially annealed
H3x: strain hardened then stabilized
H4x: strain hardened and lacquered or painted

The second digit gives hardness: 2 is quarter hard, 4 half hard, 6 three-quarter hard, 8 full hard, 9 extra hard. So 5052-H32 is strain hardened, stabilized, quarter hard; it bends well and is common for sheet metal.

T tempers

T1: cooled from hot working, naturally aged
T2: cooled from hot working, cold worked, naturally aged
T3: solution treated, cold worked, naturally aged (2024-T3)
T4: solution treated, naturally aged (6061-T4)
T5: cooled from hot working, artificially aged (6063-T5 extrusions)
T6: solution treated, artificially aged (6061-T6, 7075-T6)
T7: solution treated, overaged or stabilized (7075-T73)
T8: solution treated, cold worked, artificially aged
T9: solution treated, artificially aged, cold worked
T10: cooled from hot working, cold worked, artificially aged

Stress relief digits

A 51 after the T digit means stress relieved by stretching: 1.5 to 3% permanent set for plate, 1 to 3% for rod, bar and extrusions. T651 is T6 plus stretching, T7351 is T73 plus stretching. 52 means stress relieved by compressing, and 511 means stretched with minor straightening allowed. Stretched plate moves far less when you hog out pockets.

Typical strengths

6061-O: about 18 ksi tensile, 8 ksi yield
6061-T6: about 45 ksi tensile, 40 ksi yield
2024-T3: about 70 ksi tensile, 50 ksi yield
7075-T6: about 83 ksi tensile, 73 ksi yield
5052-H32: about 33 ksi tensile, 28 ksi yield

Typical values from The Aluminum Association and ASM data. Design and acceptance use the minimums in the governing ASTM or AMS specification.

Machining by temper

O and T4 tempers are gummy: they smear, build up on the edge and form stringy chips. T6 and T651 machine cleanly with sharp, polished, high-rake tools. 7075-T651 machines even better and holds tight tolerances; 7075-T73 is overaged for stress corrosion resistance and is a little softer. Avoid heating a T6 part above about 300 °F for long; you can overage it.

Worked example: choose plate for a flat fixture

A 1.0 in thick fixture plate gets 70% of its material pocketed. 6061-T6 plate without stress relief can bow several thousandths. Ordering 6061-T651 (stretched 1.5 to 3%) removes most residual stress; rough, unclamp, let it relax, then finish both faces. Same alloy and strength, much flatter result.

Common problems

Ordering T6 when the job needs T651, bending T6 too tight and cracking it, welding a T6 part and expecting T6 strength in the heat-affected zone, and reading an alloy stamp as proof of temper.

FAQ

What does T6 mean in aluminum?

Solution heat treated and artificially aged.

What does T651 mean?

T6 plus stress relief by stretching.

What does H32 mean?

Strain hardened and stabilized, quarter hard.

What is O temper?

Fully annealed, the softest condition.

What standard defines aluminum tempers?

ANSI H35.1, published by The Aluminum Association.

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General shop information only — follow your drawing, tool maker's data and your quality system.