Aluminum 2124: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

2124 is an aluminum-copper alloy in the 2xxx series, characterized by copper as the principal alloying addition with magnesium and manganese as important secondary elements. It is a heat-treatable alloy that develops its high strength primarily through solution heat treatment, quenching and artificial aging, with additional strengthening obtained by controlled cold work where specified tempers demand it.

The alloy exhibits high static strength and good fracture toughness for a copper-containing aluminum grade, but it sacrifices some general corrosion resistance and weldability relative to the 5xxx and 6xxx families. Formability in the annealed condition is acceptable, while hard tempers significantly reduce ductility; machinability is generally good for wrought product shapes used in aerospace.

Typical industries that specify 2124 include aerospace primary and secondary structure, high-strength fittings, and some defense components where elevated strength-to-weight and fatigue performance are prime requirements. Engineers select 2124 over other alloys when a combination of high strength, damage tolerance and predictable aging response are required and when cladding or protective measures can mitigate its corrosion sensitivity.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed, maximum ductility for forming
H14	Moderate	Low-Moderate	Fair	Poor	Strain-hardened with limited ductility
T3	Moderate-High	Moderate	Fair	Poor	Solution heat-treated, cold worked, naturally aged
T4	Moderate	Moderate	Fair	Poor	Solution heat-treated and naturally aged
T6	High	Low	Limited	Poor	Solution heat-treated and artificially aged for peak strength
T8	High	Low	Limited	Poor	Solution heat-treated, cold worked, and artificially aged
T351	Moderate-High	Moderate	Fair	Poor	Solution heat-treated, stress-relieved by stretching
T851	High	Low-Moderate	Limited	Poor	Solution heat-treated, stress-relieved (stretched), and artificially aged; common aerospace temper

Tempering strongly governs the trade-offs between strength, ductility and fatigue resistance in 2124. Annealed (O) material is used where forming is primary, whereas T6/T851 tempers are chosen for highest static strength and stable property retention at room temperature.

Cold work levels and subsequent artificial aging (e.g., T8) can be used to tailor yield vs. toughness, but dramatically reduce forming ability and make welding impractical due to HAZ softening and hot cracking susceptibility.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.50	Impurity; controlled to reduce casting defects and maintain ductility
Fe	≤ 0.50	Impurity; excess reduces corrosion resistance and can form intermetallics
Cu	3.8 – 4.9	Principal strengthening element promoting precipitation hardening
Mn	0.20 – 0.50	Controls grain structure and recrystallization; improves toughness
Mg	1.2 – 1.8	Synergistic with Cu for precipitation strengthening and hardenability
Zn	≤ 0.25	Minor; typically low and has little strengthening role here
Cr	≤ 0.10	Grain structure control and helps limit recrystallization
Ti	≤ 0.15	Grain refiner in ingot and cast processing
Others	≤ 0.15 total	Trace elements and residuals; kept low to maintain predictable aging

Copper and magnesium are the key elements enabling 2124’s precipitation hardening; copper forms CuAl2 and associated phases that precipitate during aging. Manganese and trace chromium/titanium refine grain structure and stabilize properties, while silicon and iron are controlled to avoid brittle intermetallics that reduce toughness and corrosion resistance.

Mechanical Properties

Tensile behavior in 2124 is strongly temper-dependent with substantial increases in both yield and ultimate tensile strengths in precipitation-aged tempers. Peak-aged conditions such as T6 or T851 produce high yield-to-ultimate ratios and relatively low uniform elongation; these tempers also raise hardness and reduce notch toughness relative to annealed material. Fatigue strength is generally good for high-strength Al-Cu alloys provided surface quality and residual stress are controlled and corrosion is mitigated by protective treatments or cladding.

Thickness and product form affect mechanical metrics due to through-thickness cooling rates during quench and the potential for differential aging; thin sections can reach peak strength more uniformly than thicker plates. Elongation drops as strength increases and is also reduced in heavily cold-worked states; design for ductility must consider available tempers and forming operations. Hardness correlates closely with tensile properties but localized softening in welds or HAZs can substantially undermine service performance if not accounted for.

Property	O/Annealed	Key Temper (T851/T6)	Notes
Tensile Strength (UTS)	~200–300 MPa	~470–520 MPa	Values depend on section thickness and processing; aero spec materials often on higher end
Yield Strength (0.2% offset)	~80–220 MPa	~350–470 MPa	Yield increases strongly with aging; T851 typically provides higher yield than T6 for some processing conditions
Elongation (in 50 mm)	~18–26%	~6–12%	Elongation falls with increased aging and cold work
Hardness (Brinell HB)	~30–60 HB	~120–160 HB	Hardness corresponds to tensile strength and aging condition; surface treatments alter service hardness

Physical Properties

Property	Value	Notes
Density	2.78 g/cm³	Typical of high-strength aluminum alloys, beneficial for strength-to-weight
Melting Range	~500–640 °C	Solidus/liquidus range depends on copper and other minor elements
Thermal Conductivity	~120 W/m·K	Lower than pure Al but still good for many thermal applications
Electrical Conductivity	~30–40 %IACS	Reduced by alloying; not selected where high conductivity is required
Specific Heat	~0.90 J/g·K (900 J/kg·K)	Typical for aluminum alloys at ambient temperatures
Thermal Expansion	~23–24 µm/m·K	Coefficient of thermal expansion similar to other Al alloys

Density and thermal properties make 2124 attractive where low mass and reasonable thermal conduction are required alongside mechanical capacity. Electrical conductivity is substantially reduced by copper additions, so 2124 is not a substitute for electrical-grade aluminum when conductivity is a design driver.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.5 – 6 mm	Good uniformity in thin gauges	O, T3, T6, T851	Common aerospace skin and panel thicknesses; cladding possible
Plate	6 – 200 mm	Through-thickness property gradients possible	O, T6, T851	Thick plate requires controlled quench to avoid soft core
Extrusion	Profiles up to large cross-sections	Dependent on section cooling	T6 (ageable) after solution treat	Less common than sheet/plate; good for complex stiffeners
Tube	OD variable, wall thickness variable	Mechanical limits set by wall thickness	T3, T6	Used in fittings and structural tubing in aerospace
Bar/Rod	Diameters up to 200 mm	Homogeneous in smaller diameters	O, T6	Used for machined fittings and forgings

Differences in processing lead to distinct property consequences: sheets and thin extrusions quench quickly and achieve more uniform aging, while thick plates require special quench practices and may need post-process aging cycles. Product selection should consider required post-forming heat treatments, potential for cladding and whether machining or welding will be performed on the component.

Equivalent Grades

Standard	Grade	Region	Notes
AA	2124	USA	Primary designation under the Aluminum Association
EN AW	2124	Europe	Often referenced as EN AW-2124 in European standards with similar chemistry
JIS	A2618/A?	Japan	No exact direct JIS equivalent; similar high-strength Al-Cu grades exist
GB/T	AlCu4Mg1 / 2124 analog	China	Chinese standards may list compositions under descriptive names rather than identical alloy numbers

2124 is primarily codified in AA/ASTM and recognized in EN nomenclature as AW-2124; however, some regions implement closely related alloys with slightly different impurity limits and tempers. Minor differences in allowable trace elements or lot acceptance procedures can lead to slightly different mechanical outcomes, so cross-referencing of mechanical specifications and lot certification is essential when substituting between regions.

Corrosion Resistance

In atmospheric environments 2124 is less resistant than 5xxx and 6xxx alloys because copper-containing precipitates can promote localized corrosion initiation. When exposed to humid or mildly corrosive atmospheres, 2124 benefits from protective cladding or chromate conversion coatings commonly applied in aerospace practice. Cladding (alclad) is frequently used for exterior skin applications to provide a sacrificial pure aluminum barrier and improve resistance to pitting and exfoliation.

In marine environments the alloy is susceptible to pitting and intergranular corrosion unless protected by coatings, cladding, or cathodic protection; continuous exposure to salt spray accelerates attack on copper-rich precipitate regions. Stress corrosion cracking (SCC) can occur in chloride-bearing environments and under tensile stresses, particularly in peak-aged conditions and when residual tensile stresses from forming or machining remain.

Galvanic interactions are significant when 2124 contacts more noble metals such as stainless steel or copper; aluminum will act anodically and corrode preferentially if not electrically isolated or protected. Compared with 6xxx (Al-Mg-Si) alloys, 2124 trades some corrosion resilience for higher strength, and compared with 5xxx alloys it is significantly less tolerant to marine chloride exposure.

Fabrication Properties

Weldability

Welding of 2124 is challenging and generally discouraged for applications that require full structural properties because of hot-cracking susceptibility and HAZ softening. When welding is necessary, processes such as TIG or MIG with matched Al-Cu filler alloys (for example 2319 or specially formulated 4047/5356 variants) are commonly specified to reduce cracking and improve ductility in the weld metal. Post-weld heat treatment cannot fully restore original peak-aged properties in welded zones, and designers typically avoid welding critical, highly stressed members.

Machinability

2124 is considered reasonably machinable in comparison to many high-strength aluminum alloys; it machines well with high-speed steel or carbide tooling when proper speeds and feeds are maintained. Chips tend to be continuous and ductile; coolant and rigid setups improve surface finish and dimensional control. Tool life can be longer than some copper-free alloys because of the alloy’s tendency to form favorable chip morphology, but the high strength increases cutting forces and must be accommodated.

Formability

Forming is best performed in annealed (O) or lightly worked tempers; hard tempers such as T6/T851 exhibit low ductility and are difficult to form without cracking. Recommended minimum bend radii and draw depths must be approached conservatively for aged tempers, and warm-forming or solution-treatment followed by age can be used to achieve complex shapes. Springback in high-strength tempers is significant and must be compensated for in tool design.

Heat Treatment Behavior

Solution treatment for 2124 is conducted at temperatures typically in the range of 495–505 °C to dissolve copper- and magnesium-bearing phases into a supersaturated solid solution. Rapid quenching, usually water quenching from solution temperature, is necessary to retain solute in supersaturation and enable subsequent precipitation during artificial aging. Artificial aging schedules for T6-like conditions commonly use temperatures around 160–190 °C for several hours to achieve a balance of strength and toughness with reproducible precipitation sequences.

T851 and similar aerospace tempers add a controlled stretching (stress relief) step after quench and before aging to reduce residual stresses and improve dimensional stability. Overaging can be used intentionally to improve stress-corrosion resistance at the cost of peak strength, and controlled re-aging or repair aging can be applied to parts after limited thermal excursions. Non-heat-treatable behavior is not applicable for 2124, as its primary strengthening is precipitation-based rather than purely work-hardening.

High-Temperature Performance

2124 loses strength progressively with increasing temperature, with noticeable reductions above 100–150 °C and significant softening as approach to typical artificial aging temperatures occurs. Long-term exposure at elevated temperatures promotes overaging and coarsening of strengthening precipitates, reducing yield and fatigue resistance. Oxidation is minimal for aluminum at these temperatures in dry air, but mechanical property loss is the primary limitation for high-temperature service.

Heat-affected zones produced by welding experience localized softening and microstructural changes that reduce load-bearing capacity, particularly at temperatures where precipitate coarsening accelerates. For cyclic or creep-sensitive applications, designers typically limit service temperatures to below the onset of significant precipitate coarsening and specify protective coatings to limit environmental interactions.

Applications

Industry	Example Component	Why 2124 Is Used
Aerospace	Wing fittings, splice plates, highly loaded structural brackets	High strength-to-weight and good fatigue/fracture resistance
Marine	High-strength structural members (protected or clad)	Strength and toughness where corrosion is controlled
Defense	Armor components, missile structures, structural forgings	High static strength combined with relatively low mass
Electronics	Structural mounts, chassis components	Stiffness and thermal conduction with high strength

2124 is most often found where high specific strength and predictable aging behavior are required and where protective measures mitigate corrosion risk. It remains a material of choice for aerospace structural elements where weight savings and damage tolerance provide system-level benefits.

Selection Insights

Use 2124 when the primary design drivers are high yield and tensile strength coupled with good fatigue and fracture properties, and when protective coatings, cladding or controlled environments mitigate corrosion risk. Specify T851 or similar stretched-and-aged tempers for aerospace-grade dimensional stability and elevated yield strength; choose O or less-aged tempers for forming operations prior to final heat treatment.

Compared with commercially pure aluminum (1100), 2124 trades electrical and thermal conductivity and superior formability for a much higher strength and better fatigue performance. Compared with common work-hardened alloys such as 3003 or 5052, 2124 provides substantially higher strength but lower general corrosion resistance and poorer weldability. Compared with widely used heat-treatable alloys like 6061 or 6063, 2124 often offers higher achievable yield and superior fracture toughness for demanding structural applications, but at the cost of corrosion tolerance and easier weld/repairability; choose 2124 when strength and fatigue efficiency outweigh those trade-offs.

Closing Summary

2124 remains a relevant high-strength aluminum alloy in modern engineering where elevated specific strength and reliable precipitation-hardening response are required, particularly in aerospace and defense sectors. Its selection must be accompanied by appropriate corrosion protection, temper control and fabrication planning to fully leverage its mechanical advantages.