Aluminum 2618: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

Alloy 2618 is a member of the 2xxx series of aluminum alloys, which are primarily aluminum-copper based. The 2xxx classification indicates a heat-treatable, high-strength aluminum family where copper is the chief strengthening element augmented by magnesium and small additions of other elements such as iron, nickel, and chromium.

The alloy’s major alloying elements are copper and magnesium, with purposeful microalloying by nickel, iron, manganese and trace titanium/chromium. Strengthening is principally achieved by solution heat treatment followed by quenching and artificial aging, producing fine Al2Cu (θ′) and related precipitates; nickel additions modify precipitate stability for improved elevated-temperature performance.

Key traits of 2618 include high static and elevated-temperature strength, moderate ductility, and relatively poor intrinsic corrosion resistance compared with 5xxx/6xxx families. Weldability is limited and requires special practice; formability is moderate in annealed conditions but reduced after age-hardening. Typical industries for 2618 are aerospace, high-performance automotive (esp. engine components), and other applications requiring high strength at elevated temperatures or superior fatigue resistance.

Engineers choose 2618 when a combination of high strength, retained properties at elevated temperature, and fatigue performance outweighs the alloy’s reduced corrosion resistance and more difficult weldability. The alloy is often selected over lower-strength aluminum grades when component mass reduction, dimensional stability at temperature, and cyclic-load performance are design drivers.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed, easiest to form and machine
H12	Low-Medium	Medium-Low	Fair	Fair	Strain-hardened limited strengthening
H14	Medium	Low-Medium	Limited	Fair	Light strain hardening for moderate strength
T4	Medium-High	Medium	Fair	Poor	Solution treated and naturally aged
T6	High	Low-Medium	Poor	Poor	Solution heat treated and artificially aged for peak strength
T61 / T651	High	Low-Medium	Poor	Poor	Stabilized tempers with controlled residual stress/aging
T62 / T64	High	Low-Medium	Poor	Poor	Alternative aging profiles for tailored creep/strength

Temper has a primary effect on the balance between strength and ductility: annealed (O) material provides maximum formability and machinability but low strength, while T6/T61 families maximize strength at the expense of elongation and cold-forming capability. Stabilized T61/T651 tempers reduce residual stresses and distortion in machined parts, which is important for aerospace forgings and heavy sections where dimensional stability is critical.

Chemical Composition

Element	% Range	Notes
Si	0.10–0.50	Controlled low silicon to limit casting defects; minor strength influence
Fe	0.20–1.20	Impurity element; higher levels form intermetallics that reduce ductility
Mn	0.30–1.30	Improves strength via dispersoids and refines grain structure
Mg	1.00–1.70	Works with Cu to promote age-hardening precipitates and increase strength
Cu	2.30–3.30	Principal strengthening element forming Al2Cu precipitates during aging
Zn	≤0.25	Low zinc; not a principal alloying element in 2618
Cr	0.05–0.35	Microalloying for grain control and to inhibit recrystallization
Ti	0.05–0.30	Grain refiner used during casting/solidification
Others	Balance Al; trace Ni ~0.60–1.30	Nickel additions (often 0.6–1.3%) are common in 2618 variants to improve elevated-temperature strength; other residuals vary

The alloy chemistry is tuned to support precipitation hardening primarily through Al–Cu phases with Mg accelerating precipitation kinetics and modifying precipitate chemistry. Nickel and chromium act as microalloying additions to stabilize dispersoids and extend strength retention at elevated temperatures, while manganese and titanium help control grain structure and intermetallic morphology, improving toughness and fatigue life.

Mechanical Properties

In service, 2618 exhibits high tensile strength and reasonable yield strength when given a T6/T61 style heat treatment, with tensile-to-yield ratios typically in the 1.2–1.4 range. Elongation is lower in peak-age tempers, often in the single-digit to low double-digit percent range, which influences forming and joining strategies. Fatigue strength is a strong suit of 2618 relative to many other aluminum alloys, particularly when microstructure and surface finish are controlled.

Hardness correlates closely with temper; annealed material is soft and machinable while peak-aged tempers reach substantially higher Brinell/Vickers hardness values consistent with the development of fine precipitates. Thickness and section size influence achievable properties due to cooling rate during quench and subsequent aging; thick sections may show lower peak strength and longer aging times to achieve target properties.

Corrosion and environmental factors interact with mechanical performance: stress concentration and surface defects can degrade fatigue life and accelerate crack initiation in chloride environments. Appropriate surface treatments, coatings, and design for corrosion allowance are often required to exploit the mechanical advantages of 2618 reliably.

Property	O/Annealed	Key Temper (e.g., T6/T61)	Notes
Tensile Strength	~180–260 MPa	~420–480 MPa	Peak-aged values depend on aging profile and section thickness
Yield Strength	~100–150 MPa	~320–380 MPa	Yield varies with temper and prior deformation
Elongation	~20–30%	~6–12%	Elongation drops significantly after age hardening
Hardness	~50–80 HB	~120–150 HB	Hardness correlates with precipitate density and distribution

Physical Properties

Property	Value	Notes
Density	~2.78 g/cm³	Slightly higher than pure Al due to Cu and other alloying elements
Melting Range	~500–635 °C	Solidus–liquidus range depends on local chemistry and intermetallics
Thermal Conductivity	~120–140 W/m·K	Lower than pure Al; copper lowers conductivity relative to 1xxx series
Electrical Conductivity	~20–40 %IACS	Reduced by alloying; values depend on temper and processing
Specific Heat	~880 J/kg·K	Typical for Al alloys; varies slightly with temperature
Thermal Expansion	~23–24 µm/m·K	Comparable to other Al alloys; design for differential expansion required

The relatively high thermal conductivity compared with steels makes 2618 useful where heat dissipation is important, although it is inferior to high-conductivity aluminum grades. The alloy’s density and thermal expansion are typical for aluminum but must be accounted for when mating with dissimilar materials or designing for tight thermal tolerances. The melting/solidus range informs forging and heat-treatment windows and dictates safe processing temperatures.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.5–6 mm	Thin sheets reach near-peak through appropriate tempers	O, T4, T6	Used where modest forming and high strength-to-weight needed
Plate	6–100+ mm	Thick sections may under-age and require longer aging	T6, T61, T651	Heavy plates used for structural parts and forgings
Extrusion	Complex profiles up to large cross-sections	Extruded properties vary with cooling and aging	O, T6 (post-aging)	Extrusion benefits from grain control and tempering after forming
Tube	Thin- to thick-wall tube	Strength depends on forming method and subsequent heat treatment	O, T6	Used in structural and high-load tubing applications
Bar/Rod	Diameters up to large sizes	Bars maintain good machinability in O, high strength when aged	O, T6, T61	Common for turned and milled aerospace components

Processing route (casting, extrusion, rolling, forging) significantly influences microstructure, precipitate distribution, and residual stress state. Thicker sections require careful quench and aging schedules to minimize internal soft zones and ensure uniform mechanical performance; for critical aerospace components, straightening and stress-relief (T651) operations are standard to control distortion.

Equivalent Grades

Standard	Grade	Region	Notes
AA	2618	USA	Primary American Aluminum Association designation
EN AW	AlCu2.5Mg (approx)	Europe	Approximate chemical analogue, not a strict one-to-one match
JIS	A2618 (approx)	Japan	Local designations vary; check national standard for exact spec
GB/T	2A61	China	Commonly cited domestic equivalent in Chinese standards

Direct one-to-one equivalents are approximate because regional specifications control impurity limits, allowable microalloying and mechanical test methods. When substituting, always cross-check mechanical property requirements and heat-treatment protocols rather than relying solely on nominal chemical equivalence. Trace elements and microalloying (particularly Ni content) in 2618 variants can make significant differences in elevated-temperature and fatigue behavior across standards.

Corrosion Resistance

Atmospherically, 2618 is less corrosion resistant than the 5xxx (Mg) and 6xxx (Mg+Si) series because of the relatively high copper content; copper-rich precipitates and intermetallic phases act as local cathodic sites that drive galvanic corrosion. In neutral to mildly corrosive environments with appropriate coatings or anodizing, acceptable service life can be achieved, but unprotected exposure to aggressive atmospheres is generally avoided.

In marine or chloride-laden environments, 2618 is susceptible to pitting and intergranular attack if not properly protected; chloride-induced localized corrosion is a common failure mode. Stress corrosion cracking (SCC) propensity is higher than for many Al-Mg alloys, particularly under tensile stress and corrosive exposure; design must minimize triaxial stresses and consider protective coatings, cathodic protection, or sacrificial anodes.

Galvanic interactions with more noble metals (e.g., stainless steel, copper) can accelerate localized corrosion of 2618, so dielectric isolation or compatible fasteners are recommended. Compared with 1xxx/3xxx families, 2618 trades corrosion resistance for strength and elevated-temperature capability, therefore corrosion mitigation strategies (coatings, inhibitors, environment control) are frequently required in long-term applications.

Fabrication Properties

Weldability

Welding of 2618 is challenging due to high copper content and age-hardening behavior that produce softening in the heat-affected zone and susceptibility to hot cracking. Fusion welding (TIG/MIG) is feasible for non-critical joints with strict control of preheat, filler metal selection and post-weld heat treatment; fillers based on Al-Cu-Mg alloys or Al-Cu-Ni systems are commonly recommended to match strength and reduce cracking risk. For critical aerospace parts, welding is often avoided in favor of mechanical fastening or adhesive bonding because post-weld heat treatment to restore properties is difficult for large assemblies.

Machinability

2618 in the annealed condition machines reasonably well with conventional carbide tooling; peak-aged tempers are harder and more abrasive because of precipitates. Typical practice uses rigid tooling, positive rake, and coolant to control cutting temperatures; cutting speeds should be conservative relative to free-cutting aluminum alloys, and tool coatings that resist built-up edge (BUE) are helpful. Chip formation tends to be continuous and ductile; aggressive feeds and sharp tools reduce work hardening ahead of the cut.

Formability

Forming is best performed in the O (annealed) condition where bend radii can be tight and springback predictable; typical minimum bend radii are roughly 1–2× material thickness depending on tooling and wall thickness. Cold-forming after age-hardening is limited due to reduced ductility and high residual stresses; when forming is required for final geometry, a solution-treat-and-form or form-in-annealed-followed-by-aging strategy is recommended. For complex shapes, warm-forming or superplastic approaches are generally not used—other alloy families are preferred for extreme formability needs.

Heat Treatment Behavior

As a heat-treatable 2xxx series alloy, 2618 responds to solution treatment, quenching and controlled artificial aging to develop high strength. Solution treatment is typically performed in the range of about 510–535 °C to dissolve Al2Cu phase, followed by rapid quench to retain a supersaturated solid solution. Artificial aging profiles commonly use intermediate temperatures (e.g., 160–190 °C) for several hours to precipitate fine θ′ and related phases that maximize strength while balancing toughness.

T temper transitions depend on specific processing: T4 indicates solutionized and naturally aged, T6 is solutionized and artificially aged to peak hardness, and T61/T651 denote stabilization and stress-relief steps to limit residual stresses or pre-deformation effects. Overaging produces coarser precipitates that reduce strength but can improve toughness and corrosion resistance; controlled overaging is sometimes used to improve SCC resistance or reduce quench sensitivity.

High-Temperature Performance

2618 exhibits superior retained strength at elevated temperatures relative to common 6xxx series alloys because of nickel and copper additions that stabilize precipitates. Useful static strength retention can extend to approximately 150–250 °C depending on temper and Ni content; above this range, precipitate coarsening and softening accelerate and long-term creep becomes a design concern. Oxidation is not a primary failure mode for aluminum at these temperatures in air, but loss of mechanical properties and potential surface scaling may occur in aggressive atmospheres.

Heat-affected zones near welds lose strength due to dissolution and coarsening of strengthening precipitates, and recovery/softening can occur at relatively low post-weld temperatures. For service above ~200–250 °C, designers should validate short- and long-term creep behavior and consider alloys specifically tailored for high-temperature stability if continuous operation at elevated temperatures is required.

Applications

Industry	Example Component	Why 2618 Is Used
Automotive	High-performance pistons, connecting rods	High static and elevated-temperature strength; fatigue resistance
Marine	Structural brackets and fittings (protected)	High strength-to-weight where coatings mitigate corrosion
Aerospace	Fittings, bushings, landing gear components	High strength, fatigue resistance, dimensional stability after aging
Electronics	Heat spreaders and structural supports	Good thermal conductivity with higher mechanical strength

Although 2618 is not a general-purpose sheet alloy, its combination of high strength and relatively good thermal properties makes it attractive for components where weight, strength at temperature, and fatigue life are primary design drivers. Protective surface treatments and careful joining strategies are commonly applied to realize long-term performance in service environments.

Selection Insights

Select 2618 when the design requires high static strength and retained mechanical properties at elevated temperatures and fatigue resistance outweighs the need for intrinsic corrosion resistance or weldability. Use annealed 2618 for forming and machining, and apply controlled aging or stabilization when dimensional stability and peak strength are required.

Compared with commercially pure aluminum (e.g., 1100), 2618 trades electrical and thermal conductivity and superior formability for much higher strength and fatigue performance. Compared with work-hardened alloys such as 3003 or 5052, 2618 provides substantially higher strength but typically worse corrosion resistance and more difficult joining; therefore choose 2618 for structural, high-load parts rather than for general sheet metal parts. Compared with common heat-treatable alloys like 6061/6063, 2618 often offers better elevated-temperature strength and fatigue performance; however, 6061 provides better corrosion resistance and weldability—use 2618 when high-temperature mechanical performance is the deciding factor.

Closing Summary

Alloy 2618 remains a specialized high-strength aluminum choice where heat-treatable, copper-based strengthening and elevated-temperature performance are required despite sacrifices in corrosion resistance and weldability. With careful processing, temper selection and surface protection, 2618 delivers a compelling combination of strength, fatigue resistance and thermal properties for demanding aerospace, automotive and high-performance structural applications.