Reading a marking such as 2024-T3
Sheet is usually marked with specification numbers or code markings in rows about 5 in apart. Tubes, bars, rods and extrusions are marked every 3 to 5 ft along the length.
| Part | Example | What it tells you |
|---|---|---|
| Alloy number | 2024 | The commercial code number. It identifies the composition of the alloy (AC 43.13-1B 4-4; Handbook p. 4-30). |
| Letter suffix | -T3 | The temper: how the metal was hardened or softened. The letter and digits are the “basic temper designations and subdivisions.” |
| Alclad | Alclad 2024-T3 | An aluminum alloy with a protective cladding of aluminum to improve corrosion resistance (Handbook p. 4-30). |
AC 43.13-1B CHG 1 par. 4-4 and the Handbook, Chapter 4.
The four basic temper letters
| Letter | Meaning (AC Table 4-4) | Alloys |
|---|---|---|
| F | As fabricated. No mechanical property limits. For castings: as cast. | Wrought and cast |
| O | Annealed, recrystallized: the softest temper of wrought products. | Both groups |
| H | Strain-hardened: hardened by cold work, not by a heat treatment. | Non heat-treatable alloys |
| T | The tempers for heat-treatable alloys, T1 to T10 in Table 4-4: solution heat-treated, aged, cold-worked, or a combination. | Heat-treatable alloys |
Table 4-4 divides the alloys into “non heat-treatable” (O and H) and “heat-treatable” (O, T and F) columns.
H tempers — non heat-treatable alloys (1100, 3003, 5052)
| Temper | Meaning (AC Table 4-4) |
|---|---|
| -H1 | Strain-hardened only, to get the strength wanted, with no extra thermal treatment. |
| -H12, -H14, -H16, -H18 | H1 at one-quarter hard, half-hard, three-quarters hard and full-hard. |
| -H2 | Strain-hardened more than needed, then reduced to the strength wanted by partial annealing. |
| -H22, -H24, -H26, -H28 | H2 at one-quarter, half, three-quarters and full-hard. |
| -H3 | Strain-hardened, then stabilized by low-temperature heating. Strength falls a little and ductility rises. |
| -H32, -H34, -H36, -H38 | H3 at one-quarter, half, three-quarters and full-hard. |
| -H112 | As fabricated, with specified mechanical property limits. |
Second digit after H1, H2, H3: 2 = quarter-hard, 4 = half-hard, 6 = three-quarters hard, 8 = full-hard, as in the table's own rows.
T tempers — heat-treatable alloys (2014, 2024, 6061, 7075)
| Temper | Meaning (AC Table 4-4) |
|---|---|
| -T1 | Cooled from a hot shaping process (extrusion, casting) and naturally aged to a stable condition. |
| -T2 | Annealed (castings only). |
| -T3 | Solution heat-treated, then cold-worked by flattening or straightening. |
| -T36 | Solution heat-treated, then cold-worked by a 6 percent reduction. |
| -T4 | Solution heat-treated. |
| -T42 | Solution heat-treated by the user regardless of prior temper (2014 and 2024 only). |
| -T5 | Artificially aged only (castings only). |
| -T6 | Solution heat-treated, then artificially aged. |
| -T62 | Solution heat-treated and aged by the user regardless of prior temper (2014 and 2024 only). |
| -T8 | Solution heat-treated, cold-worked, then artificially aged. |
| “-T/4” (as printed) | Solution heat-treated, cold-worked by flattening or straightening, then artificially aged. The AC prints the designation as “-T/4”, which looks like a misprint. This definition matches T81 in the Aluminum Association system; that identification is not from the AC. |
| -T86 | Solution heat-treated, cold-worked by a 6 percent reduction, then artificially aged. |
| -T9 | Solution heat-treated, artificially aged, then cold-worked. |
| -T10 | Cooled from a hot shaping process, artificially aged, then cold-worked. |
Stress-relieved (“51”) tempers and the T7 family are in the next table.
T tempers — stress-relieved and T7 variants
| Temper | Meaning (AC Table 4-4) |
|---|---|
| -T351, -T451, -T3510, -T3511, -T4510, -T4511 | Solution heat-treated, then stress-relieved by stretching to a permanent set of 1 to 3 percent, depending on the product. |
| -T651, -T851, -T6510, -T8510, -T6511, -T8511 | Solution heat-treated, stress-relieved by stretching 1 to 3 percent, then artificially aged. |
| -T652 | Solution heat-treated, compressed to a permanent set, then artificially aged. |
| -T7351, -T76 (not in AC Table 4-4) | Not defined in Table 4-4. The Handbook names them for 7075: T7351 has excellent stress-corrosion resistance and better fracture toughness than T6; T76 is often used to improve exfoliation corrosion resistance. The T7 family is the “solution heat-treated and stabilized” group in the Aluminum Association (ANSI H35.1) system; the standard itself was not consulted here. |
The “51” tails (T351, T651) mean stress-relieved by stretching 1 to 3 percent, per the table's own rows.
Common aircraft alloy and temper pairs
Tempers are given where the AC's bend radius table (Table 4-6) or the Handbook names them. For 1100, 3003 and the heat-treatable alloys not shown, the sources name the alloy without a temper.
| Alloy-temper | Group | What the sources use it for | Source |
|---|---|---|---|
| 1100 | Non heat-treatable | Fuel and oil tanks, cowlings, wingtip repair; where strength is not important. Weldable. | Handbook p. 4-30 |
| 3003 | Non heat-treatable | Like 1100 but stronger and harder. | Handbook p. 4-30 |
| 5052-O, -H32, -H34, -H36, -H38 | Non heat-treatable | Fuel, hydraulic and oil lines; good workability, corrosion resistance and fatigue strength. | AC Table 4-6 (tempers); Handbook p. 4-31 |
| 2024-O, -T3, -T6 (bare or Alclad) | Heat-treatable | Structures, airfoil and fuselage skins, extrusions, fittings. | AC Table 4-6; Handbook p. 4-30 |
| 2024-T4 | Heat-treatable | DD rivets: too hard to drive once aged, so they are kept refrigerated (or re-heat-treated) until driven. | Handbook p. 4-32 |
| 2117-T4 | Heat-treatable | AD rivets, the most used repair rivet; driven as received. | Handbook p. 4-32; AC 4-57d |
| 2017-T4 | Heat-treatable | Rivets (limited use now). Above 3/16 in, keep refrigerated until driven. | Handbook pp. 4-30, 4-32; AC 4-57d |
| 6061-O, -T4, -T6 | Heat-treatable | Weldable, easily extruded; hydraulic and pneumatic tubing. | AC Table 4-6; Handbook p. 4-31 |
| 7075-O, -T6 | Heat-treatable | Stronger than 2024 but lower fracture toughness; tension use where fatigue is not critical. T6 avoided in corrosive environments. | AC Table 4-6; Handbook p. 4-31 |
| 7075-T7351, -T76 | Heat-treatable | Better stress-corrosion (T7351) and exfoliation (T76) resistance than T6. | Handbook p. 4-31 |
Rivet alloys: AD = 2117-T4, DD = 2024-T4 (Handbook p. 4-32). For the bend radii for these tempers see the bend radius chart.
Where the temper matters in the shop
Hardness testing. On aluminum it should be limited to telling annealed from heat-treated material of the same alloy (AC 43.13-1B 4-16). It does not identify the alloy.
Forming. Temper sets the bend radius. Immediately after quenching, 2024 “may be formed over appreciably smaller radii” (Table 4-6 footnote), which is the idea behind forming a part soft and letting it age. Use the bend radius chart for the numbers.
Dimpling. 2024-T3 can be dimpled hot or cold but may crack near the dimple after cold dimpling because of hard spots; hot dimpling prevents that. 7075-T6 is always hot dimpled (Handbook p. 4-43).
Rivets. 2117 rivets can be driven as received. 2017 rivets above 3/16 in and all 2024 rivets must be kept packed in dry ice or refrigerated in the “quenched” condition until driven, or be reheat-treated just before driving (AC 4-57d).
Same alloy where possible. The Handbook says to choose rivets of the same alloy number as the material: 1100 and 3003 rivets in 1100 and 3003, and 2117 and 2017-T rivets in 2017 and 2024 (p. 4-32).
Worked example — reading two markings
A sheet is marked 2024-T351 and an extrusion is marked 6061-T651.
- 2024-T351: alloy 2024 (heat-treatable). T3 = solution heat-treated, then cold-worked by flattening or straightening. The 51 on the end = stress-relieved by stretching 1 to 3 percent.
- 6061-T651: alloy 6061 (heat-treatable). T6 = solution heat-treated, then artificially aged. The 51 = stress-relieved by stretching 1 to 3 percent.
- Both T351 and T651 are listed in AC Table 4-4. Neither is the same as the plain T3 or T6 the bend radius table lists, so do not treat them as interchangeable without data.
Alloy first, then the heat treatment, then the stress relief. Substitution is a decision against approved data, not a shop-floor swap.
Questions people actually ask
What does T3 mean on aluminum?
Solution heat-treated and then cold-worked by a flattening or straightening operation (AC 43.13-1B Table 4-4).
What is the difference between T6 and T651?
Both are solution heat-treated and artificially aged. T651 also has stress relief by stretching of 1 to 3 percent.
What does O temper mean?
Annealed: the softest temper of wrought products.
What is the difference between H32 and H34?
Both are strain-hardened and then stabilized. H32 ends at one-quarter hard and H34 at half hard.
Is 2024-T3 the same as Alclad 2024-T3?
No. Alclad has a protective cladding of aluminum to improve corrosion resistance (Handbook p. 4-30). Use the one the drawing calls for.
Limits — when NOT to use this chart
This chart explains the designations in AC 43.13-1B Table 4-4 and names alloy-temper pairs from the AC and the Handbook. It is not a material specification.
- The drawing or SRM names the material. A different temper of the same alloy is not a substitute without approved data.
- Properties are not on this page. Strength, allowables and corrosion behaviour come from MMPDS or the manufacturer, not from the temper letter.
- Temper marks can differ. The AC prints “-T/4”, which is not a real designation, and Table 4-4 does not cover T7 tempers.
- Castings and other products are only touched on here. Table 4-4 mentions them with F, T1, T2 and T5.
- Not for composites or other metals. Steel, titanium and magnesium have their own designation systems.
Sources
- FAA AC 43.13-1B CHG 1, Chapter 4, par. 4-2 to 4-4 and Table 4-4 (p. 4-4) — “Basic temper designations and subdivisions from aluminum alloys”: every definition in the H and T tables. Checked against the page image; the “-T/4” misprint is the AC's own.
- FAA AC 43.13-1B CHG 1, Chapter 4, Table 4-6 and par. 4-16, 4-57d — the alloy and temper list, the note on forming 2024 after quenching, hardness testing of aluminum, rivet storage.
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 4, pp. 4-30, 4-31, 4-32 and 4-43 — alloy uses (1100, 3003, 2014, 2017, 2024, 5052, 6061, 7075), the 7075 tempers, rivet alloys, dimpling by temper. The Handbook refers the reader to AC 43.13-1 Chapter 4 for the full discussion.
- ANSI H35.1 / Aluminum Association temper designation system — the standard behind Table 4-4. Named for reference; not read for this chart. The T7 note above is from general knowledge of that system and should be checked against the standard.