Aluminum 2524

What is the history and development context of alloy 2524?

Origin and industrial objectives

Developed in the 1990s by Alcoa (internal code C188), alloy 2524 belongs to the Al-Cu-Mg-Mn family of the 2000 series and was primarily designed for fuselage skins. The objective was to replace 2024-T3 by improving damage tolerance while maintaining comparable strength. It was first introduced on the Boeing 777 program (entry into service: 1995) for fuselage panels, and later extended to other civil and military aircraft. It follows the line of “high purity” alloys (reduced Fe/Si) derived from 2024, alongside 2124/2224/2324, developed between the 1970s and 1990s to enhance toughness, fatigue resistance, formability, and reduce crack initiation sites.

Positioning as a “high damage tolerance” alloy

2524 shows better resistance to crack initiation and fatigue crack growth compared with 2024-T3. This improvement stems from higher purity, optimized thermomechanical treatments, and a finer, more homogeneous microstructure. It was therefore selected for pressurized fuselage areas where fatigue life dictates thickness. The A380 likely used it for parts of its fuselage or wings.

Influence of chemical composition on physical properties

In unprotected atmosphere, the alloy forms an oxide layer and remains sensitive to chloride media, like other Al-Cu alloys. However, its improved purity provides slightly better corrosion resistance than 2024, confirmed by more noble OCP values (e.g., ~–0.59 V_SCE vs –0.72 V for 2024, typ., lab conditions). In aerospace service, sheets are often delivered in Alclad (thin layer of pure Al) to reinforce general and intergranular protection, a nearly systematic practice for 2000-series fuselage alloys.

Role of chemical elements in AA2524

The composition follows the 2024 architecture (Cu + Mg + Mn) with significant reduction of Fe and Si impurities. This purification limits cathodic intermetallic phases detrimental to toughness and corrosion resistance. Typical contents: Cu ~4 %, Mg ~1.3–1.5 %, Mn ~0.5–0.6 %, Fe and Si limited to a few hundredths of a percent.

Physical properties (density, conductivity, expansion)

Density is about 2.79 g/cm³ (typ.), very close to 2024. For comparison, pure aluminum is 2.70 g/cm³. The crystal structure is FCC (face-centered cubic) and the alloy is non-magnetic, like all aluminums. Electrical conductivity reaches ~30–34 % IACS (typ., T3 temper), slightly higher than 2024-T3 (30–32 %) due to higher purity. At 25 °C, the linear expansion coefficient is ~23 µm/m·°C (typ., 33 to 500 °C), with a thermal conductivity ~120 W/m·K (typ., 159 to 500 °C), ensuring efficient heat dissipation.

What are the mechanical properties of 2524 (depending on temper)?

Tensile strength and toughness (T3, the most common temper)

In T3 (solution heat treatment + quench + slight cold work + natural aging), 2524 targets optimal damage tolerance. Typical values: Rm ~445 MPa, Rp_0.2 ~335 MPa, with A ~19 % (typ., longitudinal orientation, thin sheet). These are equivalent to 2024-T3, but with higher ductility (A ~12 %, typ.). This enables reduced thickness margins or longer inspection intervals.

Fracture toughness is ~15–20 % higher than 2024-T3, and crack propagation resistance is superior at high ΔK. The lower crack growth rate (da/dN) delays multisite damage in pressurized skins.

Effect of rarely used tempers (T8/T81/T851)

After solution heat treatment and artificial aging, with or without stress/CAF (T8), 2524 gains strength at the expense of ductility. However, fatigue resistance decreases, limiting its interest for fuselage skins. These tempers are rarely applied to 2524.

How does 2524 resist corrosion in service?

General and pitting corrosion

Compared with 2024-T3, 2524-T3 shows higher electrochemical impedance and less attacked surfaces after chloride immersion (typ., lab tests). Higher impedance indicates better resistance to electrochemical current flow responsible for corrosion. In marine environments, pits still appear if left bare, but their density and average depth are usually lower than in 2024. The use of Alclad and surface treatments (conversion, primer, paint) is essential for durability.

Intergranular, exfoliation corrosion and SCC

ASTM G110 tests rank 2524-T3 at least as good as 2024-T3: it is still sensitive if unprotected. Stress corrosion cracking remains a risk even for T3, the most robust temper against it. Aerospace structures therefore combine stretch-relief and surface protections to control the mechanism.

Positioning of 2524 versus competing alloys

Compared with 2024 (historical reference)

2524 maintains the same metallurgical basis (precipitates S = Al₂CuMg) with equivalent tensile properties and higher ductility (A ~19 % vs ~12 %, typ.). In addition, toughness (K_IC) +15–20 % and reduced crack growth provide better fuselage durability. The stricter quality control, however, leads to a moderate material cost increase.

Compared with 7xxx (e.g., 7075/7050/7055)

7xxx alloys offer Rm 500–600 MPa (T6/T7, typ.) and dominate wing structures for specific stiffness. However, their toughness and SCC resistance often require compromises (T73), reducing strength. For pressurized fuselage, 2524-T3 remains preferable for damage tolerance and fatigue performance.

Compared with Al-Li (e.g., 2198/2196/2050)

Third-generation Al-Li alloys achieve ~3–5 % density reduction and excellent fatigue/toughness properties, sometimes above 2524, but with higher costs and more demanding processing. 2524 therefore remains competitive when cost-performance trade-offs do not justify Li.

Where is 2524 used and for which applications?

The main use of 2524 is fuselage skins: the Boeing 777 introduced 2524-T3 Alclad on large panels in the mid-1990s. The alloy was later reused or adapted for various civil programs (long-haul, business jets, regional aircraft), with partial use on the A380. Typical sheet thicknesses are ~1.6 mm or more depending on location.

Machined internal parts (frames, fittings, reinforcements) benefit from the machinability and toughness of 2524-T351. Outside aerospace, some demanding applications (tanks, lightweight structures, racing) exist but remain marginal compared with the more economical 2024.