Aluminum 8091: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

8091 is an aluminum-lithium (Al-Li) series alloy developed for aerospace applications where high specific strength and low density are primary drivers. It belongs to the 8xxx series family of Al-Li alloys characterized by lithium as a principal alloying addition; lithium reduces density and increases modulus relative to conventional Al-Mg or Al-Cu alloys.

The dominant alloying elements in 8091 typically include lithium, copper and zirconium, with minor additions or impurities of magnesium, silicon, iron and trace elements such as titanium and chromium. Strengthening derives primarily from age-hardening (precipitation) mechanisms typical of heat-treatable Al-Li alloys, augmented by microstructural control via dispersoids (e.g., Al3Zr) and controlled cold work in selected tempers.

Key traits of 8091 are a high strength-to-weight ratio, reduced density compared with conventional aluminum alloys, good stiffness per unit mass, and a favorable fatigue performance in many tempers. Corrosion resistance and weldability are acceptable but are more sensitive to chemistry and temper than generic 5xxx/6xxx alloys; formability is moderate and best in annealed or solution-treated tempers.

Typical industries using 8091 include aerospace primary and secondary structures, high-performance transport components, and select high-end defense and space structures. Engineers specify 8091 when minimizing mass while retaining high static and fatigue strength is more important than maximizing plain-environment corrosion resistance or absolute thermal stability.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed; best for forming and deep drawing
T3	Medium-High	Medium	Good	Moderate	Solution heat-treated, cold worked, naturally aged
T6	High	Low–Medium	Fair	Moderate	Solution heat-treated and artificially aged for peak strength
T8 / T852	High	Low–Medium	Fair	Moderate	Cold worked then artificially aged; improved fatigue
T351	Medium-High	Medium	Good	Moderate	Solution treated, stress relieved by stretching
H111 / H32	Medium	Medium	Good	Moderate	Commercially strain-hardened tempers, limited hardening

Temper has a primary effect on strength, ductility and formability for 8091 because its strengthening is largely precipitation-based and can be modified by controlled cold work. Annealed tempers maximize ductility and are used for forming operations, while T6-type tempers maximize strength at the expense of elongation and bendability.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.10–0.25	Typical impurity; controlled to limit brittle intermetallics
Fe	≤ 0.10–0.30	Impurity; excessive Fe can form intermetallics reducing toughness
Mn	≤ 0.05–0.30	Minor; can influence recrystallization and fragment size
Mg	0.05–0.40	Minor strengthening contributor in some lots
Cu	0.5–2.5	Major strengthening element improving age-hardening response
Zn	≤ 0.10–0.50	Low-to-moderate; high Zn can increase susceptibility to SCC
Cr	≤ 0.05–0.20	Trace; can affect grain structure and recrystallization
Ti	≤ 0.02–0.10	Grain refiner in cast/ingot production
Li	~0.7–2.5	Primary low-density strengthening element (typical Al-Li range)
Zr	0.05–0.25	Dispersoid former (Al3Zr) to control grain growth and texture
Others	Balance Al + trace elements	Variability by producer; must consult supplier spec sheet

The Li and Cu levels govern the precipitation chemistry and therefore the peak strength achievable in 8091. Zr is deliberately added at low levels to form dispersoids that pin grain boundaries and suppress recrystallization during thermomechanical processing. Minor elements and impurities such as Fe and Si are tightly controlled because they form brittle intermetallics that degrade fracture toughness and fatigue crack initiation resistance.

Mechanical Properties

The tensile behavior of 8091 is strongly temper-dependent. In peak-aged tempers (T6/T8) tensile strengths can be substantially higher than conventional 6xxx alloys on a strength-per-weight basis, with yield strengths elevated by precipitation of Al-Li and Al-Cu phases; ductility is reduced compared with annealed tempers. Elongation to failure is moderate in heat-treated conditions and higher in O or T351 tempers used for forming, influencing allowable forming radii and crashworthiness.

Hardness correlates with age-hardening; peak-aged material exhibits higher Vickers or Brinell hardness values and improved resistance to local indentation. Fatigue performance is a strong suit for many Al-Li alloys including 8091 because lithium increases modulus and certain precipitate distributions reduce crack growth rates; however, fatigue resistance depends on surface condition, temper and corrosion state. Thickness and product form influence mechanical response: thinner gauges tend to achieve more uniform precipitation and higher effective strength after aging, while thick plates may show through-thickness property gradients and require controlled quench/aging schedules.

Property	O/Annealed	Key Temper (T6/T8)	Notes
Tensile Strength	200–320 MPa (typical)	450–550 MPa (typical peak)	Values vary with chemistry, processing, and thickness
Yield Strength	110–220 MPa (typical)	360–460 MPa (typical peak)	Yield-to-tensile ratio influenced by precipitate state
Elongation	20–30%	6–15%	Annealed states provide highest ductility for forming
Hardness	40–70 HB	100–140 HB	Hardness rise corresponds to precipitation and cold work

Physical Properties

Property	Value	Notes
Density	~2.60–2.65 g/cm³	Lower than conventional Al (2.70 g/cm³) due to Li content
Melting Range	~500–640 °C (solidus–liquidus approximated)	Alloying alters solidus; follow supplier TTT data for casting
Thermal Conductivity	~120–150 W/m·K	Lower than high-conductivity 1xxx series but adequate for many structures
Electrical Conductivity	~30–45% IACS	Reduced compared with pure Al due to alloying additions
Specific Heat	~880–920 J/kg·K	Similar order of magnitude to common Al alloys
Thermal Expansion	~21–24 µm/m·K (20–100 °C)	Slightly lower than many Al-Mg alloys due to Li content

The lower density of 8091 is one of its main advantages for mass-critical structures; this yields improved specific strength and stiffness. Thermal and electrical conductivities are reduced relative to pure aluminum because alloying scatters electrons and phonons; designers must account for these reductions in thermal management or electrical applications. Thermal expansion is somewhat reduced by lithium, improving dimensional stability in temperature-cycling applications.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.5–6 mm	Uniform through gauge when properly processed	O, T3, T6, T8, T351	Preferred for formed aerospace skins and secondary panels
Plate	6–100+ mm	Potential through-thickness gradients; thicker plate requires careful quench	T6, T8, T351	Used for forgings, structural webs and high-load members
Extrusion	Profiles up to several hundred mm	Can retain high strength if precipitation controlled	T6, T8, O	Complex cross-sections possible but limited by alloy flow stress
Tube	Various diameters/wall	Similar temper behavior to sheet/extrusion	T6, T351	Used in structural tubing and landing gear components in some cases
Bar/Rod	Diameters up to 200 mm	Higher section thickness reduces quench effectiveness	T6, T8, O	Used for machined fittings and fasteners where high specific strength needed

Sheets and thin gauge products are often processed to maximize through-thickness quench effectiveness and uniform precipitation, yielding higher and more consistent strength. Thick plate and bar demand tailored thermal processing and often hot-forming followed by solution treatment and staged aging to minimize property gradients and retain toughness. Extrusions must balance alloy flow characteristics with final heat-treatment schedules to achieve designed mechanical properties.

Equivalent Grades

Standard	Grade	Region	Notes
AA	8091	USA	Recognized Al-Li aerospace alloy in some supplier catalogs
EN AW	—	Europe	No direct EN AW numerical equivalence; classified within Al-Li family
JIS	—	Japan	No simple direct JIS equivalent; material usually supplier-specified
GB/T	—	China	Local equivalents not standardized; material often imported or specified by composition

There is frequently no direct one-to-one cross-reference for advanced Al-Li alloys like 8091 across global standards. Differences in compositional limits, processing, and proprietary heat-treatment practices mean that “equivalent” grades should be validated by mechanical testing and chemistry checks rather than by nominal alloy number alone. When substituting, verify temper response, quench sensitivity and fracture/fatigue behavior under the intended manufacturing sequence.

Corrosion Resistance

In atmospheric environments 8091 generally performs acceptably when properly alloyed and heat treated, but its corrosion behavior is more complex than that of typical 5xxx/6xxx series alloys. The presence of copper and lithium can increase susceptibility to localized corrosion and intergranular attack if impurity levels or manufacturing induced precipitate networks are not tightly controlled. Surface finish, cladding, and protective coatings are commonly used on 8091 components intended for long exposure to aggressive atmospheres.

In marine and high-salinity environments the copper content can promote localized pitting in some tempers, so design allowances and corrosion-protection systems are important when 8091 is used for near-shore or marine structures. Stress corrosion cracking (SCC) susceptibility is temper and chemistry sensitive; overaged conditions and appropriate temper design can reduce SCC risk, while certain peak-aged states may be more vulnerable under sustained tensile stress in corrosive media.

Galvanic interactions follow standard aluminum practice: 8091 should be isolated from cathodic materials such as stainless steel, copper, or graphite composites when electrical continuity and moisture exposure exist. Compared with 5xxx and 6xxx alloys, 8091 offers competitive fatigue/corrosion performance when properly processed but generally does not match the innate chloride corrosion resistance of higher-magnesium 5xxx alloys.

Fabrication Properties

Weldability
8091 can be welded using fusion and solid-state methods, but weldability depends on chemistry and temper. Tungsten inert gas (TIG) and metal inert gas (MIG) are common; filler alloys specifically designed for Al-Li or low-Li Al-Cu systems (consult supplier recommendations, e.g., Al-Cu based fillers) are preferred to avoid brittle weld metal. Hot-cracking and porosity risks exist if joint design, heat input and filler alloy are not optimized; HAZ softening can occur in peak-aged parent metal and post-weld aging or mechanical repair may be required.

Machinability
Machinability of 8091 is generally fair and comparable to other high-strength Al alloys; it machines more cleanly than some high-strength steels but demands rigid setups due to low modulus relative to steels. Carbide tooling and sharp geometry promote good surface finish and chip control; recommended cutting speeds are higher than for ferrous alloys but must be optimized to avoid built-up edge and thermal softening. Chip morphology tends to be short to semi-continuous with appropriate tool geometry and coolant.

Formability
Formability is best in annealed or lightly pre-aged tempers and degrades as strength increases in T6/T8 conditions. Minimum bend radii depend on temper and thickness but designers often start with 2–3T (where T = thickness) for moderately severe bends in annealed sheet and increase radius for heat-treated material. Cold working can be used to incrementally shape parts prior to final aging cycles to minimize springback and cracking.

Heat Treatment Behavior

8091 is heat-treatable; designers and fabricators must control solution treatment, quenching and aging to develop target properties. Typical solution treatment involves heating to a range where Cu- and Li-bearing phases dissolve (consult supplier data; commonly in the 520–560 °C range), followed by rapid quench to retain solutes. Artificial aging at moderate temperatures (e.g., 150–190 °C) precipitates strengthening phases to reach T6 or T8 conditions; aging time and temperature govern peak versus overaged trade-offs between strength and toughness.

Transition between tempers is predictable but quench sensitivity is a critical processing variable for thicker sections where centerline cooling is slower. Overaging can improve toughness and SCC resistance but reduces peak strength. For non-heat-treatable processing steps (where applicable), work hardening and annealing remain the primary tools to tailor mechanical response.

High-Temperature Performance

8091 experiences significant strength loss with increasing temperature; designers should limit continuous service temperatures to well below aging or precipitate dissolution thresholds. Practical upper service limits are often in the 120–150 °C range for load-bearing structural applications; exposure to higher temperatures accelerates overaging and softening. Oxidation is modest at typical service temperatures, but elevated temperature exposure during fabrication (welding, brazing or heat straightening) can produce localized property changes in the HAZ and adjacent heat-affected material.

Fatigue and fracture behavior at elevated temperatures degrades faster than at ambient due to accelerated creep-like relaxation of precipitate structures in long-duration exposures. Where thermal cycling is significant, repeated excursions through aging or overaging ranges demand conservative design margins and qualification testing.

Applications

Industry	Example Component	Why 8091 Is Used
Aerospace	Fuselage and wing skin panels, shear clips	High specific strength and lower density for weight-sensitive primary/secondary structures
Marine	Lightweight structural components	Favorable stiffness-to-weight and reduced mass for craft efficiency (with corrosion protection)
Aerospace Defense	Fittings, bulkheads, stiffeners	Good fatigue performance and tailored temper response for cyclic loads
Electronics / Thermal Management	Structural supports and housings	Lower density and acceptable thermal conductivity where mass savings matter

8091 is chosen where high specific strength and stiffness, combined with acceptable fatigue and manufacturability, deliver system-level mass savings. It is less often used where the primary drivers are low cost, very high corrosion resistance in aggressive chloride environments, or prolonged high-temperature exposure. Qualified material specifications, processing routes and protective surface treatments are routine prerequisites for flight hardware.

Selection Insights

8091 is appropriate when minimizing mass and maximizing strength-per-weight are higher priorities than absolute material cost or easy field reparability. Choose 8091 for aerospace primary or secondary structures, or other high-performance frames, when life-cycle weight savings justify specialized handling and qualification.

Compared with commercially pure aluminum (1100), 8091 trades higher strength and lower density for reduced electrical/thermal conductivity and formability. Compared with work-hardened alloys such as 3003 or 5052, 8091 achieves far higher specific strength but generally requires heat treatment and more controlled processing for corrosion and SCC resistance. Compared with common heat-treatable alloys like 6061, 8091 provides lower density and higher specific stiffness; 6061 may still be preferred for general-purpose parts due to cost, broader availability and simpler welding behavior.

When selecting 8091, weigh factors including supply-chain availability, need for specialized filler metals and postweld aging, and environmental exposure; if easy field welding or maximum corrosion resistance in harsh marine environments is required, consider alternative alloys or protective system designs.

Closing Summary

8091 remains a relevant Al-Li alloy for modern engineering where reducing mass while preserving high static and fatigue strength is critical. Its performance depends strongly on careful chemistry control, heat treatment and fabrication practices, and when those are managed it offers a compelling combination of low density, high specific stiffness and fatigue resistance for aerospace and high-performance structural applications.