An aluminum callout has two halves: the alloy and the temper. 6061-O and 6061-T6 are the same chemistry, but one bends easily and the other has about five times the yield strength. The temper letters and digits (from the Aluminum Association system in ANSI H35.1) say how the metal was processed. Here is what each one means and which alloys use which.
The basic tempers
Basic aluminum temper letters F, O, H, W and T
Temper
Name
What it means
F
As fabricated
Left as it came off the rolling mill, extrusion press or casting line. No property limits are guaranteed.
O
Annealed
Fully softened by heating: the lowest strength and the best formability the alloy offers.
H
Strain hardened
Strengthened by cold work (rolling or drawing). Used for the non-heat-treatable 1xxx, 3xxx and 5xxx alloys.
W
Solution heat treated
Quenched from the solution treatment and still aging at room temperature, so properties keep changing. Written with a time, e.g. W 1/2 hr.
T
Thermally treated
Heat treated, with or without cold work, to a stable condition. Used for the heat-treatable 2xxx, 6xxx and 7xxx alloys.
H tempers: strain hardened
The first digit says what was done after cold work; the second says how hard. 5052-H32 is strain hardened, stabilized, quarter hard.
H temper first digit: H1x, H2x, H3x and H4x
Temper
Name
What it means
H1x
Strain hardened only
Cold worked to the target strength with no heat treatment afterward.
H2x
Strain hardened, then partially annealed
Cold worked past the target, then heated to bring strength back down. Slightly more ductile than the H1x temper of the same strength.
H3x
Strain hardened, then stabilized
Given a low-temperature heat treatment after cold work so the properties do not drift. Mainly for magnesium-bearing 5xxx alloys, which otherwise age-soften at room temperature.
H4x
Strain hardened, then lacquered or painted
Cold worked, then partly softened by the heat of a paint or coating cure.
H temper second digit: degree of strain hardening
Second digit
Hardness
Hx2
Quarter hard: about a quarter of the way from annealed to full hard
Hx4
Half hard: midway between annealed and full hard
Hx6
Three-quarter hard
Hx8
Full hard: the strength of about 75 percent cold reduction after an anneal
Hx9
Extra hard: stronger than full hard
Hx (1, 3, 5, 7)
In between the even digits on either side
Three-digit H tempers: H111, H112, H116 and H321
Temper
Name
What it means
H111
Slightly strain hardened
Annealed, then picked up a little cold work from straightening or leveling, less than an H11. Common on 5xxx extrusions.
H112
Hot worked, properties tested
Some strain hardening from hot working, with property limits the mill must meet. Common on plate and extrusions.
H116
Marine, corrosion tested
For 5xxx alloys with 3 percent or more magnesium (5083, 5086, 5456). Made and tested for resistance to exfoliation and intergranular corrosion.
H321
Marine, stabilized and corrosion tested
Same purpose as H116, reached through a stabilizing heat treatment. Also for 5083 and 5086 plate.
T tempers: heat treated
Solution treating dissolves the alloying elements; quenching traps them; aging (at room temperature, "natural", or in an oven, "artificial") precipitates them as the particles that give the strength. The T digit records that sequence and whether cold work was added.
Aluminum T tempers T1 through T10
Temper
Name
What it means
T1
Cooled from hot working, naturally aged
Quenched straight off the press or mill (no separate solution treatment) and left to age at room temperature.
T2
Cooled from hot working, cold worked, naturally aged
As T1, with cold work added for strength before room-temperature aging.
T3
Solution treated, cold worked, naturally aged
Solution heat treated, cold worked (often by stretching or leveling), then aged at room temperature.
T4
Solution treated, naturally aged
Solution heat treated and left to age at room temperature, with no deliberate cold work. Softer and more formable than T6.
T5
Cooled from hot working, artificially aged
Quenched off the extrusion press, then aged in an oven. The standard route for architectural 6063 extrusions.
T6
Solution treated, artificially aged
Solution heat treated, quenched, then oven aged to peak strength. The most common heat-treated temper.
T7
Solution treated, overaged
Aged past peak strength on purpose to gain stress-corrosion resistance or dimensional stability.
T8
Solution treated, cold worked, artificially aged
Cold work between solution treatment and oven aging, which raises strength in alloys that respond to it (2024, 2219).
T9
Solution treated, artificially aged, cold worked
Oven aged first, then cold worked for extra strength. Seen on some screw-machine rod (6262-T9).
T10
Cooled from hot working, cold worked, artificially aged
As T5, with cold work added before oven aging.
Common tempers and suffixes on drawings
Digits after the first T digit describe stress relief or a special aging practice: 51 is stretched, 510 and 511 are stretched extrusions (511 allows minor straightening afterward), 52 is compressed.
Common aluminum tempers and suffixes explained
Temper
Name
What it means
T4
Solution treated, naturally aged
Forming temper. 6061-T4 bends tighter than T6 and can be oven aged to T6 after forming.
T5
Press quenched, oven aged
Architectural and trim extrusions, mostly 6063. Lower strength than T6.
T6
Peak aged
Sheet, drawn tube and drawn rod in 6061, 7075 and others. Not stress relieved, so thick sections can move when machined.
T651
T6, stress relieved by stretching
Plate and rolled or cold-finished bar stretched a small controlled amount (on the order of 1 to 3 percent) after quench, with no straightening afterward. The machining temper for plate.
T6511
T6, stretched, minor straightening allowed
The extrusion version of T651: stretched to relieve stress, then allowed some straightening to meet straightness tolerances.
T73
Overaged for stress-corrosion resistance
A 7xxx temper aged well past peak. Gives up some strength versus T6 for much better resistance to stress-corrosion cracking.
T7351
T73, stress relieved by stretching
T73 plate or bar that was stretched after quench. Common on aerospace 7075 bar and plate drawings.
T76
Overaged for exfoliation resistance
Aged less than T73: better exfoliation-corrosion resistance than T6 with less strength penalty than T73. T7651 is the stretched version.
H14
Half hard, strain hardened only
The usual temper for 3003 and 1100 sheet.
H32
Quarter hard, stabilized
The usual temper for 5052 sheet: formable and dimensionally stable.
Which alloys use which tempers
Typical tempers and product forms by aluminum alloy
Typical stocked and specified tempers, not every temper registered for each alloy. Property limits for a given alloy, temper and thickness are in the product spec (ASTM B209, B211, B221).
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Calling out temper on a drawing
Always give the temper. "6061" alone lets the shop buy any temper, and O-temper 6061 has a fraction of the strength of T6.
Machined plate and bar: T651 or T6511. The stretched tempers move far less when pockets are cut. Plain T6 plate is not a stock item.
Formed parts: bend soft, then age. Forming 6061 in T4 (or O) and aging to T6 afterward avoids cracking on tight radii. For sheet parts that do not need heat-treat strength, 5052-H32 bends tight as delivered.
Finishing does not change temper.Anodizing and chem film are room-temperature or near it; welding and hot forming are not. Re-aging after welding is a heat-treating job, and a full solution treatment can distort the part.
What is the difference between 6061-T6 and 6061-T651?
Same alloy, same heat treatment. T651 adds a controlled stretch after quenching that relieves most of the residual stress, so the material moves far less when it is machined. Plate and rolled bar are sold as T651; sheet and drawn tube are T6.
What does H32 mean in 5052-H32?
H3 means strain hardened and then stabilized with a low-temperature heat treatment so the properties do not drift at room temperature. The second digit, 2, means quarter hard. It is the standard temper for 5052 sheet because it forms well and stays put.
Can 5052 or 3003 be heat treated to T6?
No. The 1xxx, 3xxx and 5xxx alloys are not heat-treatable; they get their strength from cold work, which is why their tempers are H tempers. Only the 2xxx, 6xxx and 7xxx wrought alloys (and some casting alloys) take T tempers.
Why do aerospace drawings call out 7075-T73 instead of T6?
7075-T6 is prone to stress-corrosion cracking in thick sections loaded across the grain. T73 is aged past peak strength to fix that, giving up some strength. T76 is a middle ground aimed at exfoliation corrosion.
Does welding change the temper?
Yes. The weld heat locally anneals heat-treated tempers, so the heat-affected zone of welded 6061-T6 ends up far weaker than the base metal unless the whole weldment is solution treated and aged again. Strain-hardened tempers soften near the weld too. Design welded aluminum joints to the as-welded strength.
