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Aluminum 2024

Aluminum $$

The classic aerospace aluminum — the alloy that flew the DC-3, B-17, and every American military aircraft from 1935 through the 1960s before 7075 displaced it for primary structure. 2024 is an Al-Cu-Mg alloy (~4.4% Cu, 1.5% Mg) with better fatigue resistance than 7075 but worse corrosion. The standard sheet metal for aircraft skins (in alclad form to overcome the corrosion problem). Still the right choice for fatigue-critical aerospace structure where toughness matters more than peak strength. Property data below reflects T3 / T351 temper unless noted.

Service °C
150–175°C continuous (better hot strength than 7075)
Tensile
460–485 MPa (67–70 ksi) — T3/T351 typical
Density
2.78 g/cm³ (0.100 lb/in³)
Cost
$$
$3.20/lb
Trade names: 2024-T32024-T3512024-T42024-T81AlCu4Mg1EN AW-2024Alclad 2024 (corrosion-clad sheet)

The classic aerospace aluminum — the alloy that flew the DC-3, B-17, and every American military aircraft from 1935 through the 1960s before 7075 displaced it for primary structure. 2024 is an Al-Cu-Mg alloy (~4.4% Cu, 1.5% Mg) with better fatigue resistance than 7075 but worse corrosion. The standard sheet metal for aircraft skins (in alclad form to overcome the corrosion problem). Still the right choice for fatigue-critical aerospace structure where toughness matters more than peak strength. Property data below reflects T3 / T351 temper unless noted.

Properties

Mechanical
Mechanical properties for Aluminum 2024
Tensile460–485 MPa (67–70 ksi) — T3/T351 typical
Yield320–345 MPa (47–50 ksi) — T3/T351 typical
Elongation13–18% — better ductility than 7075-T6
Modulus71 GPa (10,300 ksi)
Hardness120 HB (500 kg / 10 mm) / Rockwell B 75
Fatigue strength140 MPa (20 ksi) at 5×10⁸ cycles, rotating beam — better than 7075 at equivalent stress amplitude
Poisson's ratio0.33
Thermal
Thermal properties for Aluminum 2024
Continuous max150–175°C continuous (better hot strength than 7075)
Short-term max~200°C short-term
Min serviceCryogenic-capable; toughness improves at low temperature
Conductivity120 W/m·K — lower than 6061/7075 due to high Cu
CTE22.9 × 10⁻⁶/°C (12.7 × 10⁻⁶/°F)
Specific heat875 J/kg·K
Metal-specific
UNSA92024
ENEN AW-2024 (AlCu4Mg1)
Magneticnon magnetic
Cond.30% IACS
Composition (% wt)
Al 90.7–94.7 (balance) Cu 3.8–4.9 Mg 1.2–1.8 Mn 0.3–0.9 Si ≤0.5 Fe ≤0.5 Zn ≤0.25 Zr ≤0.20 Ti ≤0.15 Cr ≤0.10 other_each ≤0.05 other_total ≤0.15

Variants (6)

2024-T3 (sheet) T3 T3 sheet

Standard sheet temper — the canonical aircraft skin material. Property data above represents T3. Solution + cold work + natural age.

2024-T351 (plate / bar) T351 T351 plate

Stretch-relieved plate. Properties match T3 within spec minimums; the difference is residual stress, not strength. Mandatory for machined plate parts.

2024-T4 T4 T4

Solution treated and naturally aged. Used for parts that need to be formed after solution treatment without the strength cost of T3 cold work.

2024-T81 T81 T81

Artificially aged for maximum strength and improved SCC. Aerospace fittings, brackets, and structural hardware where T3 strength is insufficient and T6 family isn't compatible with chemistry.

Alclad 2024-T3 (corrosion-clad sheet) alclad-T3 T3 sheet

2024-T3 sheet with thin pure-aluminum cladding (typically ~4–5% of sheet thickness per side) bonded during rolling. The cladding is more anodic than the 2024 core and corrodes preferentially — sacrificial corrosion protection. The canonical aerospace skin material for any external surface exposed to atmosphere.

2024-O (annealed) O O

Fully annealed. Soft and formable. Used as a starting condition for severe forming operations, then re-solution treated and aged to T3 / T4 / T81.

Processing

Machinability: good
Chip: Forms manageable medium-length chips. Slightly harder than 6061 in T3/T351 temper. Chip control with standard chip-breaker inserts.
Gumming: Low. 2024 cuts cleaner than 6061 — the copper content makes the matrix harder and less prone to smearing.
Finish: 16 Ra readily; 8 Ra with finishing passes
Tooling: Carbide (uncoated or polished). Speeds 500–1200 SFM, feeds 0.005–0.020 in/rev. AA machinability rating ~70%. Tool life ~80% of 6061. Cutting fluid recommended for finish work and aerospace surface quality.
2024 machines well — better than mild steel, slightly behind 6061 on cycle time. The standard for machined aerospace structural parts where fatigue strength matters more than peak yield. Plate-form parts (T351 with stretch relief) machine without distortion problems that plague T3 plate.
Weldability: poor

2024 is not weldable for structural service by conventional fusion processes. The high copper-magnesium chemistry causes severe hot cracking in the weld zone and rapid precipitate coarsening in the HAZ. Friction stir welding has been qualified for some aerospace applications (Lockheed Martin C-17 wing stringers, certain Airbus structures) but is not generally available. Standard aerospace practice is **mechanical fastening with rivets or bolts** — every modern airliner is a riveted 2024/7075 assembly for this reason.

Heat treatments
Solution + cold work + natural age (T3) (120 HB) — Standard sheet temper. Slight cold work after quench develops dislocation structure that nucleates precipitates during natural aging. The aerospace sheet workhorse.
Solution + stretch + natural age (T351) (120 HB) — Stretch-relieved plate and bar. Mandatory for machined parts with deep pockets or tight flatness tolerances. Standard aerospace plate temper.
Solution + natural age (T4) — Solution treated and naturally aged without cold work. Used when forming is required after solution treatment. Slightly more formable than T3 but property similar.
Solution + cold work + artificial age (T81) (~140 HB) — Artificially aged for maximum strength and improved SCC resistance. Used for parts that must be formed in T3/T4 condition then aged to T81 in service. Aerospace fittings and brackets sometimes specified in T81.
Full anneal (O temper) (47 HB) — Soft annealed for severe forming operations. After forming, re-solution treat and age to recover T3/T4/T81 properties. Rarely stocked at distributors.
Surface treatments
Sulfuric acid anodize (Type II) (5–25 μm) — 2024 anodizes but produces darker, less uniform finish than 6061. Copper migrates into the oxide layer and darkens it. Chromic acid anodize (Type I, MIL-A-8625 Type I) is the aerospace standard for 2024 — produces a thinner, less cosmetically problematic anodize that doesn't degrade fatigue strength as much.
Chromate conversion coating (Alodine / Iridite) (0.05–0.5 μm) — Standard aerospace primer treatment before painting. Alodine 1200 (hexavalent chrome) is the legacy spec; trivalent and chrome-free alternatives are growing for new platforms.
Hard anodize (Type III) (25–125 μm) — Used for wear surfaces. Significant fatigue penalty — not recommended for fatigue-critical aerospace parts without shot peening pretreatment.

Corrosion resistance

general Atmospheric fair Worse than 6061. Develops dark gray oxide and pits in industrial atmospheres. Alclad cladding is the aerospace standard for protected service; bare 2024 must be painted or anodized for outdoor exposure.
saltwater poor Pitting, exfoliation, and SCC susceptibility. Marine 2024 components require heavy protection (alclad + paint + sealant).
acids poor Less acid-resistant than 6061 due to high copper content. HCl and HF attack rapidly.
bases poor Aluminum is attacked by alkalis; 2024 is no improvement.
oxidizing Environments fair Concentrated nitric acid tolerated due to passive film formation.
reducing Environments fair
Corrosion is THE 2024 design consideration. The aerospace solution is **alclad 2024** — a thin (typically 4–5% of sheet thickness per side) pure-aluminum cladding metallurgically bonded during rolling. The cladding is more anodic than the 2024 core and corrodes preferentially, providing sacrificial protection. For machined parts and forgings (no alclad option), anodize + paint + sealant is the protective stack.
⚠ Galvanic risks with
Carbon steel (severe)Stainless steel (severe)Copper and copper alloys (severe)Graphite (severe — common composite-airframe issue)Titanium (mild)

Regulatory

○ FDA grade
○ NSF 51
○ NSF 61
○ USP Class VI
✓ RoHS
✓ REACH
○ EU 10/2011

Not specified for food, water, or medical contact applications. Copper content (3.8–4.9%) excludes 2024 from regulated contact markets. Used industrially for aerospace and structural service. RoHS and REACH compliant (no restricted substances in the alloy itself).

Notes & applications

Overview

2024 is the original aerospace aluminum — developed by Alcoa in 1931, used in the DC-3, every WWII Allied aircraft, the B-29, and every American military and commercial airliner through the 1960s before 7075 began displacing it for primary structure. Half a century later, 2024 is still the dominant material for aircraft sheet metal skins and fatigue-critical aerospace structure.

The 2024 selection logic is straightforward:

  • Better fatigue than 7075 — 2024 has higher fatigue strength at equivalent stress amplitude, making it the right alloy for cyclically loaded structure (lower wing skins, fuselage panels)
  • Worse peak strength than 7075 — UTS ~67 ksi vs 7075’s ~80 ksi; for static-strength-dominated parts, 7075 is the better choice
  • Worse corrosion than 6061 — the alclad cladding layer is the aerospace solution; bare 2024 requires significant protection
  • Not weldable — fundamentally same problem as 7075. Mechanical fastening is the structural joining method.

Modern aircraft tend toward 2024 for fuselage skins and lower wing surfaces (fatigue dominates), 7075 for upper wing surfaces and landing gear (static strength dominates), and 6061 for non-flight- critical secondary structure.

Machining notes

2024 machines well — better than mild steel, slightly behind 6061. Practical recipe:

  • Carbide tooling (uncoated or polished); avoid TiN coatings
  • Speed: 500–1200 SFM
  • Feed: 0.005–0.020 in/rev
  • Cutting fluid recommended for finish work and aerospace surface quality
  • Tool life ~80% of 6061 at equivalent feeds

T351 plate is essentially mandatory for machined parts. Plain T3 plate carries residual stress from quenching that releases during deep-pocket machining and warps the part. The cost premium for T351 stretch relief is small; rejection rates without it are high.

For aerospace machined parts where fatigue matters, finish surfaces to 32 Ra or better. 2024 is fatigue-sensitive to machined surface defects.

Temper selection guidance

  • T3 — Sheet metal for fuselage and skin (typically alclad). The aerospace workhorse temper.
  • T351 — Plate that will be heavily machined. Mandatory for deep-pocketed parts and tight-flatness machining.
  • T4 — Forming operations where T3 cold work would compromise formability. Used for stretch-formed and roll-formed shapes.
  • T81 — Higher strength + better SCC than T3 for fittings and brackets requiring more capacity. Less ductile.
  • O — Severe forming only. Re-solution treat and age after.

Welding and joining

Don’t fusion-weld 2024 for structural service. TIG, MIG, and stick welding produce severely weakened, crack-prone joints. The copper-magnesium chemistry hot-cracks during solidification.

Friction stir welding (FSW) has been qualified for select aerospace applications (Lockheed Martin C-17 cargo floor stringers, some Airbus skin panels). FSW preserves ~60–70% of parent strength without the precipitate damage of fusion welding. Not commercially available outside qualified aerospace programs.

In practice, 2024 aerospace structures are riveted assemblies. Every modern airliner is a riveted 2024/7075 skin/spar/frame construction. This is by design, not by accident — the failure mode of properly designed riveted aluminum aircraft is well-understood (fatigue crack growth from rivet holes, monitored by inspection), while welded aluminum aircraft structure has historically had much higher failure rates.

Corrosion considerations and alclad

This is 2024’s central weakness, and the alclad solution is one of the cleverest pieces of materials engineering in aerospace history.

Bare 2024 in atmospheric service:

  • Pits within months in industrial atmospheres
  • Develops intergranular corrosion at grain boundaries (copper precipitates create local galvanic cells)
  • Exfoliates under sustained tensile stress in marine atmospheres
  • Has SCC susceptibility in T3/T4 tempers

Alclad 2024 solves this with a thin pure-aluminum cladding layer (typically 4–5% of sheet thickness per side) metallurgically bonded to the 2024 core during rolling. The cladding is more anodic than the 2024 core — when corrosion attacks the surface, the cladding corrodes preferentially, protecting the structural 2024 underneath. This is sacrificial cathodic protection built into the sheet.

Alclad 2024-T3 sheet has been the aerospace skin material for ~80 years for exactly this reason. The cladding adds <10% to sheet weight and dramatically extends corrosion life.

For machined 2024 parts (no alclad option):

  • Anodize (chromic acid Type I is the aerospace standard — less fatigue penalty than sulfuric Type II)
  • Chromate conversion + primer + paint
  • Sealant in mating surfaces
  • Avoid bare 2024 exposed to atmosphere

For 2024 forgings and machined structure, the standard aerospace protective stack is: chromate conversion → epoxy primer → polyurethane topcoat. Sealant in faying surfaces to prevent crevice corrosion.

Welding alternatives — riveting practice

Aerospace 2024 structure is riveted, not welded. The standard rivet alloys for 2024 sheet assembly:

  • 2017-T4 / 2017A — older standard, cold-driven
  • 2024-T31 (ice-box rivet) — refrigerated to suppress aging, cold-driven, then ages to T3 strength
  • 5056 / 5052 — for skin to non-flight-critical components, cold-driven without refrigeration
  • Hi-Lok / Hi-Tigue / Cherry Max — proprietary blind fasteners for structural assembly access from one side

Cold expansion of rivet/fastener holes (Fatigue Technology Split-Sleeve, etc.) introduces residual compressive stress around the hole and doubles or triples fatigue life. Aerospace 2024/7075 structures specify cold-expanded holes for fatigue- critical locations.

Applications by industry

  • Commercial aviation — fuselage skins (alclad sheet), lower wing skins (alclad sheet), structural plates, machined fittings, ribs. The fatigue-critical airframe alloy.
  • Military aviation — same applications + munitions structural components, missile bodies (older platforms).
  • General aviation — most general aviation aircraft (Cessna, Piper, Beechcraft) are predominantly 2024 sheet construction.
  • Defense — armored vehicle non-structural panels, projectile bands, fuze housings.
  • Truck and heavy equipment — premium aluminum truck wheels (2024 has the fatigue resistance that 6061 lacks at wheel cycle rates).
  • Specialty — bicycle component machining, gun receivers, high- end consumer products.

Failure modes worth designing around

Corrosion is the #1 2024 in-service issue. Bare 2024 fails by pitting, intergranular attack, and exfoliation long before mechanical overload. The aerospace solution is alclad sheet for surfaces, chromate + paint for forgings and machined parts. Never leave 2024 bare in atmosphere.

Fatigue at fastener holes is the dominant aerospace 2024 mechanical failure mode. Stress concentration at riveted holes, combined with cyclic loading, initiates fatigue cracks. Mitigation:

  • Cold-expanded holes (Fatigue Technology Split-Sleeve, J-Tech)
  • Interference-fit fasteners
  • Bushed fastener holes for ultra-fatigue-critical locations
  • Aerospace fatigue analysis assumes presence of small initial cracks (damage tolerance) and tracks growth via scheduled inspection

Stress corrosion cracking in T3/T4 tempers under sustained tensile stress in chloride environments. The T81 temper improves SCC behavior; for SCC-critical applications, use T81 or move to T7-series aluminum alloys.

Intergranular corrosion at grain boundaries — heat treatment sensitization (slow cooling after solution treatment) causes copper-rich precipitates at grain boundaries that create local galvanic cells. Proper heat treatment with rapid quenching prevents this. Improperly heat-treated 2024 can show intergranular attack in environments that wouldn’t bother properly treated material.

Galvanic corrosion with carbon-fiber composites is the modern aerospace concern. Composite-airframe aircraft (787, A350) have 2024 fittings adjacent to carbon-fiber structure; the carbon is cathodic to the 2024, and electrolyte presence (condensation, spills) drives accelerated 2024 corrosion. Sealants and isolation plies are the design solution.

Hard anodize fatigue penalty is more severe on 2024 than 6061 due to the higher operating stresses in 2024 parts. For fatigue- loaded 2024 components, avoid hard anodize or shot peen first to introduce compressive surface residual stress.

Sources & standards

Standards: ASTM B209 (sheet and plate)ASTM B210 (drawn seamless tubes)ASTM B211 (rolled or cold-finished bar, rod, wire)ASTM B221 (extruded shapes)AMS 4035 (sheet, annealed)AMS 4037 (sheet, T3, alclad)AMS 4041 (sheet/plate, T351)AMS 4045 (plate, T351, alclad)AMS 4120 (bar, T351)AMS-QQ-A-200/3 (extruded shapes)AMS-QQ-A-250/4 (sheet and plate)EN AW-2024 / EN AW-AlCu4Mg1DIN EN 573-3

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