Aluminum 8091-O (Annealed)
Temper
AA 8091 · UNS A98091
Softest condition for deep forming, straightening or intermediate machining before solution treatment.
Values for the O condition
Other 8091 tempers and conditions
8091 is listed in 11 conditions; see the grade overview at 8091.
Compare all 11 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
OThis page Annealed | Softest condition for deep forming, straightening or intermediate machining before solution treatment. | 140 | 255 | 14 | 65 HB | 23 % IACS |
As fabricated | Mill product straight from hot working, no controlled thermal treatment — for re-heat-treatment or forging stock. | 190 | 300 | 10 | 85 HB | 25 % IACS |
Solution heat treated and naturally aged | Solution-treated and naturally aged without cold work — used where forming after heat treatment is required. | 250 | 380 | 14 | 105 HB | 17 % IACS |
Solution heat treated, cold worked and naturally aged | Sheet condition with good damage tolerance and formability, aged naturally after stretching/cold work. | 285 | 400 | 12 | 115 HB | 16 % IACS |
T3, stress relieved by stretching | Plate/bar version of T3 with stretched stress relief — low residual stress for stable machining before final ageing. | 295 | 410 | 11 | 118 HB | 16.5 % IACS |
Solution treated, stretched, overaged for toughness/SCC resistance | Overaged damage-tolerant temper for thick sections needing better short-transverse toughness and SCC resistance. | 435 | 495 | 7 | 142 HB | 21 % IACS |
Solution heat treated and artificially aged | Peak-aged condition for forgings and parts where maximum strength matters more than toughness. | 440 | 505 | 5 | 148 HB | 19 % IACS |
T6, stress relieved by stretching | Peak-aged plate with stretched stress relief — the choice for machined-from-solid, dimensionally stable structure. | 455 | 515 | 5 | 150 HB | 19 % IACS |
Solution treated, stretched, under/near-peak artificially aged | Modified ageing of stretched plate trading a little strength for better ductility and short-transverse behaviour. | 460 | 525 | 6 | 150 HB | 19 % IACS |
Solution heat treated, cold worked and artificially aged | Cold work before ageing refines the strengthening dispersion — highest strength sheet condition for Al-Li. | 465 | 525 | 5 | 152 HB | 19 % IACS |
T851Default T8, stress relieved by stretching | The standard supplied condition for 8091 plate/extrusion: near-peak strength with stretched stress relief for stable machining. | 480 | 540 | 5 | 155 HB | 19 % IACS |
What is 8091-O?
8091-O is 8091 in the O temper — softest condition for deep forming, straightening or intermediate machining before solution treatment. 8091-O has a yield strength of 140 MPa (20.3 ksi) and a tensile strength of 255 MPa (37 ksi) — weaker than 70% of aluminum grades. Elongation at break is 14% and the elastic modulus is 80 GPa (11.6 Msi). 8091-O has a density of 2.55 g/cm³ (0.0921 lb/in³), lighter than 99% of aluminum grades. It melts at 570°C (1,058 °F). T851 is the default condition FabDigit quotes; O is available on request or by drawing note.
Advantages
- Stiff — 80 GPa (11.6 Msi), better than 95% of aluminum grades
- Forms and bends easily — 70/100, better than 83% of aluminum grades
- Ductile — tolerates forming and impact — 14%, better than 81% of aluminum grades
Limitations
- High embodied carbon — 18 kg CO₂/kg, worse than 99% of aluminum grades
- Poor heat conductor — 95 W/m·K (54.9 BTU/hr·ft·°F), worse than 98% of aluminum grades
- Low electrical conductivity — 23 % IACS, worse than 97% of aluminum grades
- Limited service temperature — 120°C (248 °F), worse than 79% of aluminum grades
- Difficult to weld — 25/100, worse than 76% of aluminum grades
8091-O properties
Typical room-temperature values for 8091-O; 10 are specific to this condition. Other listed values are 8091 reference data and may not represent this condition. Bars are the position among aluminum grades.
Physical3
8091-O has a density of 2.55 g/cm³ (0.0921 lb/in³), lighter than 99% of aluminum grades. It melts at 570°C (1,058 °F).
Mechanical13
8091-O has a yield strength of 140 MPa (20.3 ksi) and a tensile strength of 255 MPa (37 ksi) — weaker than 70% of aluminum grades. Elongation at break is 14% and the elastic modulus is 80 GPa (11.6 Msi).
Thermal6
8091-O is rated for continuous service to 120°C (248 °F). It conducts heat at 95 W/m·K (54.9 BTU/hr·ft·°F), worse than 98% of aluminum grades. Thermal expansion is 21.4 µm/m·K (11.9 µin/in·°F).
Electrical3
8091-O conducts electricity at 23 % IACS, worse than 97% of aluminum grades.
Chemical & Environmental3
Corrosion resistance is fair (45/100), worse than 62% of aluminum grades.
Sustainability4
Producing a kilogram of 8091-O takes about 260 MJ of energy and emits 18 kg of CO₂ — more than 99% of aluminum grades. Typical recycled content is 3%.
Manufacturability7
8091-O's machinability is fair (45/100, worse than 72% of aluminum grades); weldability poor (25/100); formability very good (70/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with 8091-O
Is 8091 easy to machine?
Machines much like 2024-T851 with sharp carbide tools, high speed and flood coolant; the Li-oxide surface film is abrasive and fine chips/dust must be handled as reactive swarf (keep dry, no water accumulation).
Can 8091 be welded?
Poor fusion weldability — Li causes oxide entrapment, porosity and hot cracking; use friction stir welding, mechanical fastening or bonding, and pre-clean/vacuum-protect if arc welding is unavoidable.
Can 8091 be formed, bent or molded?
Form in O/T3/T4; peak-aged T6/T8 tempers have low ductility and strong anisotropy, so bend radii must be generous and orientation relative to rolling direction respected.
What surface finishes work on 8091?
Chromate conversion or chromic/sulfuric anodize plus primer is normal aerospace practice; control alkaline etch time to avoid Li depletion and film non-uniformity, and prime promptly to limit exfoliation risk.
8091 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| Liprimary density-reducing, modulus-raising addition | 2.4 – 2.8 | 2.6 |
| Cu | 1.6 – 2.2 | 1.9 |
| Mg | 0.5 – 1.2 | 0.9 |
| Zrgrain/subgrain control via Al3Zr | 0.04 – 0.16 | 0.12 |
| Siimpurity limit | ≤ 0.3 | — |
| Feimpurity limit | ≤ 0.5 | — |
| Mn | ≤ 0.1 | — |
| Zn | ≤ 0.25 | — |
| Ti | ≤ 0.1 | — |
| Others (each) | ≤ 0.05 | — |
| Others (total) | ≤ 0.15 | — |
| Al | balance | — |
8091-O — frequently asked
What is 8091-O used for?
8091-O is typically used for aerospace fuselage skins, structural aircraft components and weight-critical brackets. In short: low-density aerospace Al-Li; higher modulus than 2024; poor weldability, limited supply.
What is the yield strength of 8091-O?
8091-O has a typical yield strength of 140 MPa (20.3 ksi) and a tensile strength of 255 MPa (37 ksi) — weaker than 70% of aluminum grades. Strength varies by condition: see the 11 listed tempers.
Is 8091-O easy to machine?
Reasonably — machinability is rated fair (45/100, worse than 72% of aluminum grades). Machines much like 2024-T851 with sharp carbide tools, high speed and flood coolant; the Li-oxide surface film is abrasive and fine chips/dust must be handled as reactive swarf (keep dry, no water accumulation).
Can 8091-O be welded?
Not readily — weldability is rated poor (25/100). Poor fusion weldability — Li causes oxide entrapment, porosity and hot cracking; use friction stir welding, mechanical fastening or bonding, and pre-clean/vacuum-protect if arc welding is unavoidable.
What surface finishes work on 8091-O?
Chromate conversion or chromic/sulfuric anodize plus primer is normal aerospace practice; control alkaline etch time to avoid Li depletion and film non-uniformity, and prime promptly to limit exfoliation risk.
Can 8091-O be formed, bent or molded?
Form in O/T3/T4; peak-aged T6/T8 tempers have low ductility and strong anisotropy, so bend radii must be generous and orientation relative to rolling direction respected.
What is the maximum service temperature of 8091-O?
8091-O is rated for continuous use to about 120°C (248 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between 8091-T851 and 8091-O?
T851 is the default condition — the standard supplied condition for 8091 plate/extrusion: near-peak strength with stretched stress relief for stable machining. O: softest condition for deep forming, straightening or intermediate machining before solution treatment. Yield strength is 480 MPa in T851 versus 140 MPa in O.
Sources
- International Alloy Designations and Chemical Composition Limits for Wrought Aluminum (Teal Sheets) — The Aluminum Association (2018)
- ASM Handbook Vol. 2: Properties and Selection — Nonferrous Alloys (Al-Li alloys) — ASM International (1990)
- Aluminum-Lithium Alloys: Processing, Properties and Applications — Elsevier/Butterworth-Heinemann (2014)
- Al-Li alloy 8090/8091 plate and extrusion data — British Alcan / Alcan International (1990)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.