Aluminum 2024 - AU4G1 - 3.1354 - EN AW-2024 - Al-Cu4Mg1

Inside Alloy 2024

Three key points to remember:

• Composition and origin: Alloy 2024 contains mainly copper (~4%) and magnesium (~1.5%), whose combination produces a very effective precipitation hardening after heat treatment.
• Properties and applications: it offers high mechanical strength (yield strength ~330 MPa, UTS ~470 MPa in T3 condition) and good fatigue resistance. It is widely used in automotive and aerospace parts when lightweight and performance are required.
• Treatments and evolutions: its properties vary depending on the metallurgical condition (T3, T4, T351, etc.), based on precipitation hardening. Appropriate treatments improve strength (e.g., cold working for T3) or formability (annealed O). Newer variants (high-toughness 2524 alloys, Al-Li alloys) aim to further improve durability and damage resistance.

Let’s dive deeper into the details.

A chemical composition designed for precipitation hardening

Alloy 2024 is mainly composed of aluminum (~90–94%) alloyed with about 3.8–4.9% copper, 1.2–1.8% magnesium, and 0.3–0.9% manganese. Each element plays a specific role:

• Copper significantly increases strength by forming hardening precipitates (phases Al₂Cu and Al₂CuMg) in the matrix;
• Magnesium works synergistically with copper to form the S-phase (Al₂CuMg), which increases hardness after aging;
• Manganese (~0.5%) contributes to strength via dispersoid compounds (e.g., Al₂₀Cu₂Mn₃ or T-phase) that stabilize the microstructure and limit grain growth.

In short, this Al-Cu-Mg-Mn alloy has a heat-treatable structure, with aluminum as a lightweight base and alloying elements strengthening (Cu, Mg) or refining (Mn) its structure.

Origin of the alloy: birth of duralumin 2.0

Alloy 2024 belongs to the lineage of duralumin, the first age-hardenable Al-Cu alloys discovered in the early 20th century. In 1906, Alfred Wilm demonstrated natural aging hardening on Al-Cu-Mg alloys, leading to the birth of Duralumin (today’s 2017 alloy), famously used in Zeppelin airships.

In 1931, Alcoa introduced alloy 24S (later designated 2024) to surpass 2017. This super-duralumin, with three times more magnesium, offered improved strength. Its poor corrosion resistance was mitigated by the simultaneous development of Alclad cladding. The adoption of 24ST Alclad in 1930s aircraft (e.g., the Douglas DC-3) allowed weight savings, enabling higher payloads and lower fuel consumption.

Mechanical properties of 2024-T3, its most common condition

Mechanical properties of alloy 2024 vary significantly with the metallurgical condition. In the solution heat treated and cold-worked T3 state, it typically exhibits an ultimate tensile strength (UTS) of about 470 MPa and a yield strength around 320 MPa, with an elongation at break of ~18% (depending on thickness). In the fully annealed 2024-O state, the alloy is much softer (UTS ~120 MPa) but highly ductile.

The main advantage of 2024: outstanding fatigue resistance

In T3, the fatigue endurance limit (10^7 cycles) is about 140 MPa (unnotched samples), higher than most 6xxx alloys. This explains its long-standing use for aircraft fuselage and wing skins, where 2024-T3, thanks to its toughness, delays both crack initiation and propagation, ensuring good damage tolerance.

Its toughness (resistance to crack propagation) is noteworthy: it can dissipate part of the energy through plastic deformation before failure. Thus, 2024-T3 “ages” better than stronger but more brittle alloys, which justified its widespread use in civil aircraft for decades.

Heat treatments and delivery conditions

Reminder: principle of precipitation hardening

Alloy 2024 is heat-treatable and owes its strength to precipitation hardening. The alloy is first solution heat treated (~495 °C) to dissolve Cu and Mg in the matrix, then quenched (typically in cold water) to retain them in solid solution. A controlled aging step then allows the formation of precipitates. These precipitates form naturally at room temperature (T1 to T4 conditions) over several days. This process can increase yield strength by a factor of 3–4 compared to the annealed O state. Artificially aged conditions (T6, T8) are mostly used for thicker products, providing higher strength, dimensional stability, and resistance to stress-corrosion cracking.

The most common delivery states for 2024

• 2024-T3: solution heat treated, cold worked (a few % strain), and naturally aged. Offers the best compromise between static strength, toughness, and fatigue resistance. Less formable than T4 due to cold working.
• 2024-T4: solution heat treated and naturally aged without cold working. Slightly lower UTS (~450 MPa) but better formability.
• 2024-T351: solution heat treated, stretched (1–3%) to relieve stresses, then naturally aged. Very similar to T3 in strength but with improved dimensional stability and better resistance to stress-corrosion cracking.
• 2024-O (annealed): minimum strength but maximum ductility (yield strength ~60–100 MPa). Suitable for forming operations, followed by heat treatment to restore mechanical properties (T4/T3).

Copper, the culprit for 2024’s corrosion sensitivity

Copper-rich 2000-series alloys are less corrosion resistant than Mg- or Mg-Si-based alloys. 2024 is no exception: the addition of ~4% Cu, while beneficial for strength, makes it prone to corrosion (pitting, intergranular) in humid environments.

Copper-rich precipitates create micro-galvanic potential differences between the aluminum matrix and precipitates, promoting pitting in the presence of electrolytes. 2024 is also sensitive to stress-corrosion cracking (SCC): under sustained stress in corrosive environments, cracks may propagate rapidly.

Protection solutions against corrosion

Two major protection methods are used: Alclad and anodizing.

Alclad: a thin cladding layer of pure aluminum (~5% thickness on each side) is bonded to the 2024 sheet. This layer acts as a sacrificial barrier, oxidizing instead of the core alloy. Despite its softness (which may affect crack growth if scratched deeply), Alclad remains an effective and economical solution for rolled products.

Anodizing: an electrochemical process forming a thick aluminum oxide layer (Al₂O₃) on the surface. 2024 responds well to sulfuric or chromic anodizing, providing improved resistance to corrosion and wear, though sometimes with a slight reduction in fatigue resistance.

An alloy easy to machine, difficult to form, and poor to weld by fusion

Machinability: excellent. Its relatively hard, brittle matrix with Cu/Mg precipitates promotes clean chip breaking and reduces tool sticking, allowing high cutting speeds and efficient production of complex parts.

Cold forming: less suitable than 5052/6061 alloys. In T3/T351, its limited ductility restricts bending and deep drawing without cracking. Workaround: use the O or W states (before aging), form, then heat treat.

Hot forming: around 200 °C, strength decreases and ductility increases, allowing more severe forming. Must be followed by quenching and aging to restore properties.

Fusion welding (MIG/TIG): not recommended. 2024 is very sensitive to hot cracking due to its wide solidification range (Cu lowers eutectic melting point to ~548 °C). Welded joints are weak (≈50% of base metal strength) and prone to cracks. Mechanical fastening (rivets, bolts) is preferred for structural applications.

Ecology: recycling of aluminum

Aluminum is highly recyclable, and alloy 2024 is no exception. At end of life, 2024 parts can be remelted and reused, provided proper sorting is done (due to its Cu content and risks of Mg/Si contamination).

Re-melting consumes only about 5% of the energy needed for primary extraction. Thus, 2024 fits into a model of circular economy, with materials reused multiple times with minimal loss. Today, nearly 75% of all aluminum ever produced is still in use.