Aluminum 2618

What is the 2618 aluminum alloy (Hiduminium RR58) and how was it developed?

Origins and industrial objectives

Alloy 2618 (UNS A92618) is an Al–Cu–Mg designed in the 1950s for aerospace zones exposed to high temperature. It was initially developed by High Duty Alloys (linked to Rolls-Royce) as Hiduminium RR58 for forged jet-engine parts. The aim was improved creep resistance around 100–150 °C via Fe/Ni additions, since conventional Al–Cu–Mg (e.g., 2024) tend to soften in this range. This chemistry delivered a robust compromise between hot strength, machinability, and industrial availability.

Concorde: flagship structural use

Under the French designation AU2GN, RR58 was chosen as the primary material for the Concorde in the 1960s. Most of its airframe used 2618, enabling operation around 120 °C on the skins during prolonged supersonic cruise. The alloy was supplied as clad sheet, plate, forgings, and extruded profiles, covering structural and maintenance needs.

What designations and standards apply to 2618?

Designations, variants, and standards

2618A is a wrought, heat-treatable alloy (solution treatment + aging). Internationally one finds 2618 and 2618A (Europe, EN 573-3), the DTD 5014A specification (legacy UK), plus AMS and AIR 9048 by product form and use. Variant 2618A permits very low Zn and Mn (≤ 0.20 % each) with little effect on bulk properties (see comparison here).

Metallurgical role of alloying elements

Cu and Mg provide precipitation hardening via the S/S′ family (Al₂CuMg). Fe and Ni form dispersoids (e.g., Al₉FeNi) that stabilize the microstructure at temperature and improve creep resistance. Excess coarse intermetallics can embrittle; controlled Fe/Ni in 2618 is therefore intentional. Mn is limited in the original composition, likely to avoid perturbing Al–Fe–Ni precipitation.

What mechanical properties does 2618 offer by temper?

Tensile strength and ductility (20 °C)

In T6/T61 (typical aging: 185 °C for ~20 h), typical values are: UTS ~ 400–440 MPa, 0.2% YS ~ 320–370 MPa, and elongation ~ 5–10 %. More concretely, longitudinal properties of 2618A plate (12.5–40 mm) reach UTS = 430 MPa, YS = 370 MPa, A = 8 %. Static strength sits slightly below 2014-T6 and well below 7075-T6 at room temperature. However, stability at 120–150 °C gives an advantage in hot service.

Fatigue, creep, and hardness (T61/T851)

The rotating-bending endurance limit (R = −1; RR Moore) is about 124 MPa (≈ 18,000 psi). Temperature sensitivity is lower than other Al–Cu–Mg alloys, with notable creep capability thanks to Al₉FeNi dispersoids—hence use in thermomechanically cycled parts (pistons, rotors).

Brinell hardness in T61 is ~ 115–120 HB. For T851 plate, typical values are UTS ~ 400 MPa, YS ~ 320 MPa, A ~ 5 %.

What physical and thermal properties characterize 2618?

Density, modulus, and melting

Density is about 2.75 g/cm³. Elastic modulus is ~ 73 GPa. Solidus is ~ 549 °C and liquidus ~ 638 °C.

Conductivities and temperature effects

At 25 °C, thermal conductivity is ~ 146 W/m·K, and electrical conductivity ~ 37 % IACS. The coefficient of linear expansion is ~ 22 × 10⁻⁶ / °C, demanding managed clearances for hot parts (pistons, cowlings). At equal design, expansion exceeds that of a 4032 alloy richer in silicon, which is tailored for such parts.

As with FCC aluminum alloys, below 0 °C strength tends to increase while toughness remains good. Above ~200 °C strength drops sharply, bounding continuous structural service to about 150–175 °C. Between −50 °C and +150 °C, the strength/ductility balance stays favorable for cyclic duty.

What about corrosion resistance and surface treatments?

Cu content makes corrosion more problematic than in 5xxx/6xxx; resistance is poor to fair in standard atmospheres. In marine/humid environments, apply coatings, paints, or cladding to mitigate pitting and intergranular attack. In aerospace, surface protection is nearly systematic.

On Concorde, RR58 sheet was clad with commercially pure Al modified with Zr to improve corrosion resistance and surface creep behavior (ref.).

Bare 2618 is sensitive to galvanic couples in the presence of electrolyte; appropriate fasteners and isolation are required in critical areas. Sub-optimal heat treatment can promote continuous CuAl₂ at grain boundaries, increasing intergranular susceptibility. Slight over-aging can improve resistance without excessively degrading strength.

How to use 2618A (forming, welding, machining)?

Forming and welding 2618

Designed for forging, 2618 shows good ductility in annealed or T4, with common operations around ~ 400–450 °C. Rolling and cold forming are feasible in these tempers prior to final hardening, providing good die fill with few cracks.

Fusion welding is discouraged (hot cracking, Cu/Fe/Ni segregation). Prefer mechanical fastening or bonding; if welding is unavoidable, use tightly controlled processes (TIG with moderate preheat, suitable filler, or FSW) followed by post-weld heat treatment. Joint strength remains lower than the aged base material.

Machining behavior

2618 offers good machinability, especially in T6/T651: short chips and consistent finishes. Without “chip-breaker” elements (Pb, eutectic Si), built-up edge may occur if parameters are unsuitable. Conversely, high material removal rates are achievable with proper thermal and lubrication control.

Which applications illustrate 2618 use?

Aerospace

Primary domain: clad sheets for skins, machined plates (spars, ribs), forgings and extrusions near hot zones. Concorde remains emblematic, with a largely RR58 airframe and service around ~ 120 °C. Historical uses include compressor components in early turbojets and engine-adjacent parts.

Automotive and motorsport

Forged pistons are a major use thanks to 2618’s resilience in extreme conditions (> 300 °C at the crown, cyclic loads). Designs must incorporate appropriate clearances and coatings to manage thermal expansion and maximize reliability under boost.