What the O Temper Means for 7075 and 2024
O is the fully annealed condition. In 7075-O and 2024-O the alloying elements are no longer held as the fine precipitates that give the aged tempers their strength, so both materials arrive soft, ductile and easy to form. They are used as the starting stock for severe forming operations and for parts that are subsequently solution heat treated and aged, rather than as finished structural material. Neither should be confused with the aged conditions of the same alloy numbers.
Choosing between the two annealed alloys comes down to what happens after forming. 7075-O leads to a much stronger finished part once it is aged, while 2024-O is easier to shape and produces a finished product with a different balance of fatigue and damage-tolerance behaviour.
7075-O Aluminum: Properties and Behaviour
7075 is a zinc-bearing 7000 series alloy with copper and magnesium additions. In the annealed condition it offers a tensile strength of around 280 MPa and a yield strength of approximately 140 MPa, with elongation generally in the 9 to 10 percent range. Ductility improves in thin sections: material 1.59 mm thick can reach elongation of about 17 percent, while thicker sections such as 12.7 mm product typically reach around 16 percent.
Tensile strength: about 280 MPa
Yield strength: about 140 MPa
Elongation: 9 - 10 percent typical, up to 17 percent in thin gauge
Shear modulus: 26.9 GPa
Shear strength: about 152 MPa
Density: 2.81 g/cm3
Thermally and electrically the alloy behaves conventionally for aluminium. Electrical resistivity is around 0.00000380 ohm.cm, specific heat capacity is about 0.96 J/g.degC and thermal conductivity is approximately 173 W/m.K. Annealing is carried out at about 413 degC, while the solution heat-treatment range used before ageing lies between 466 and 482 degC.
2024-O Aluminum: Properties and Behaviour
2024 is a copper-based aluminium alloy, alloyed mainly with copper, magnesium and manganese. In the annealed condition the yield strength is approximately 75.8 MPa and the ultimate tensile strength around 186 MPa, which is noticeably lower than 7075-O. Ductility is high: elongation at break is around 20 percent for 1.6 mm material and up to 22 percent for 12.7 mm product, which is what makes the alloy attractive for stretch forming and deep drawing.
Tensile strength: about 186 MPa
Yield strength: about 75.8 MPa
Elongation: about 20 to 22 percent depending on thickness
Machinability rating: about 30 percent of free-cutting stock
Fatigue strength: about 89.6 MPa
Shear modulus: about 28 GPa, shear strength about 124 MPa
2024 has a melting range of roughly 502 to 638 degC and relatively high thermal conductivity of approximately 193 W/m.K. Its annealing temperature is about 413 degC, the same as for 7075, and solution heat treatment before ageing is carried out at approximately 495 degC.
Side-by-Side Comparison of 7075-O and 2024-O
| Property | 7075-O | 2024-O |
|---|---|---|
| Alloy family | 7000 series, zinc-based | 2000 series, copper-based |
| Tensile strength | about 280 MPa | about 186 MPa |
| Yield strength | about 140 MPa | about 75.8 MPa |
| Elongation | 9 - 10 percent, up to 17 percent thin gauge | about 20 - 22 percent |
| Shear modulus | 26.9 GPa | about 28 GPa |
| Shear strength | about 152 MPa | about 124 MPa |
| Thermal conductivity | about 173 W/m.K | about 193 W/m.K |
| Annealing temperature | about 413 degC | about 413 degC |
| Solution heat-treatment temperature | 466 - 482 degC | about 495 degC |
The practical reading of this table is straightforward. 7075-O is roughly fifty percent stronger than 2024-O in the annealed state and delivers far higher strength after ageing, whereas 2024-O is considerably more ductile, which translates directly into better forming behaviour for complex shapes.
Forming, Machining and Joining Considerations
Both alloys are intended for forming in the annealed condition, and the difference in elongation explains the usual choice. Parts with tight bend radii, deep drawn pockets or complex curved skins are normally formed from 2024-O, while 7075-O is used where the forming demand is moderate and maximum final strength is the priority. Springback must be compensated in both cases, and tooling should be kept smooth because the annealed surfaces mark easily.
Machining the O tempers is less pleasant than machining aged material: the alloys are gummy, produce long chips and tend to smear. 2024-O is rated at roughly 30 percent machinability and is generally regarded as the better of the two, but rough machining of either alloy should use sharp, polished tooling, generous positive rake and effective lubrication.
Neither alloy is recommended for fusion welding. Assemblies in the O tempers are joined by riveting, bolted fastening or adhesive bonding, or are formed first and then heat treated and aged, with fastening operations completed afterwards.
Aerospace and Industrial Applications
2024-O: wing skins and wing beams, fuselage skins and load-bearing panels, stretch-formed and roll-formed aircraft parts, and general structural sheet where high fatigue performance is required.
7075-O: heavily loaded structural components including wing and fuselage frames, landing gear parts, high-stress fittings and military airframe hardware.
Both: parts that are formed in the soft condition and then aged to a higher-strength temper before entering service.
Frequently Asked Questions
Q: Which alloy is stronger in the O temper, 7075-O or 2024-O?
7075-O. Its tensile strength is about 280 MPa against roughly 186 MPa for 2024-O, and its yield strength is about 140 MPa against approximately 75.8 MPa. The gap widens further once both alloys are aged.
Q: Why use an annealed temper instead of the aged condition?
Because the O condition is far more formable. Complex shapes are produced from soft material and then solution heat treated and aged to develop the required strength, which would not be possible if the alloy were supplied already hardened.
Q: Can 7075-O or 2024-O be welded?
Neither alloy is suitable for fusion welding. The copper-bearing and zinc-bearing compositions cause cracking and loss of properties in the weld zone, so assemblies are riveted, bolted or bonded instead.
Q: What heat-treatment temperatures apply to these alloys?
Both are annealed at approximately 413 degC. Solution heat treatment before ageing is carried out at 466 to 482 degC for 7075 and at approximately 495 degC for 2024.
Q: How does thickness affect elongation?
Thin material usually stretches further. In 7075-O, 1.59 mm product reaches about 17 percent elongation, while 12.7 mm product is typically around 16 percent. For 2024-O the figures are about 20 percent at 1.6 mm and up to 22 percent at 12.7 mm.
Q: Which of the two is easier to machine?
2024-O, with a machinability rating of roughly 30 percent of free-cutting stock. Both annealed alloys cut gummily, so sharp tooling, high positive rake angles and plenty of lubricant are needed to control chip formation.
