Aluminum 2004: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

Alloy 2004 is a member of the 2xxx series of aluminum alloys, which are copper-bearing, heat-treatable alloys formulated primarily for high strength combined with reasonable toughness. The 2xxx series typically trade some natural corrosion resistance for higher mechanical properties; 2004 follows this trend as a medium-to-high strength Al–Cu alloy positioned between older 2xx and 7xx series in capability.

Major alloying elements in 2004 are copper as the primary strengthener, with controlled additions of magnesium and manganese to aid precipitation hardening and grain structure control, and trace elements such as chromium and titanium for recrystallization control. The strengthening mechanism is principally precipitation hardening (age hardening) after solution heat treatment and quenching, although limited work hardening can also modify properties in certain tempers.

Key traits of 2004 include high specific strength, good machinability, and reasonable fatigue resistance for structural applications. Corrosion resistance is moderate and typically inferior to 5xxx and 6xxx series alloys unless protected by cladding or coatings; weldability is challenging compared with non-heat-treatable alloys and requires special filler selections and pre/post-treatment to avoid HAZ softening. Typical industries using 2004 are aerospace for fittings and structural elements, motorsport and high-performance automotive for components where weight and strength are critical, and certain general engineering applications requiring high machinability.

Engineers choose 2004 when a higher strength-to-weight ratio is needed over common commercial alloys while retaining good fatigue and machinability, and when the design can tolerate or mitigate reduced corrosion resistance. It is selected over 7xxx alloys in situations where fracture toughness and manufacturability (machining/forming) are prioritized over the absolute highest peak strength.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High (12–20%)	Excellent	Excellent	Fully annealed, maximum ductility
H14	Medium-Low	Moderate (8–12%)	Good	Fair	Lightly work-hardened for moderate strength
T3	Medium-High	Moderate (6–12%)	Fair	Poor–Fair	Solution heat-treated, cold worked, naturally aged
T4	Medium	Moderate (8–14%)	Good	Poor–Fair	Solution heat-treated and naturally aged
T6	High	Low–Moderate (6–10%)	Limited	Poor	Solution heat-treated and artificially aged for peak strength
T7	Medium	Low–Moderate (6–12%)	Better than T6	Poor	Over-aged for improved SCC resistance and dimensional stability
T651	High	Low–Moderate (6–10%)	Limited	Poor	T6 with stress-relief by stretching to minimize residuals

Temper has a strong influence on the balance between strength and ductility; O and H tempers maximize formability but sacrifice tensile strength. Peak-strength tempers such as T6 produce the highest yield and ultimate strength but reduce elongation and cold formability, and create susceptibility to weld zone softening unless special procedures are followed.

Chemical Composition

Element	% Range	Notes
Si	0.10–0.50	Silicon kept low to minimize brittle intermetallics; improves casting if present
Fe	0.10–0.70	Iron is an impurity that forms intermetallics and reduces ductility
Mn	0.20–1.00	Manganese refines grain structure and improves strength and toughness
Mg	0.10–0.80	Magnesium assists precipitation kinetics and final strength with Cu
Cu	3.0–5.0	Primary strengthening element; increases strength and reduces corrosion resistance
Zn	0.05–0.30	Zinc is kept low to avoid forming 7xxx-type behaviors
Cr	0.05–0.35	Chromium aids recrystallization control and improves stress-corrosion resistance
Ti	0.01–0.20	Titanium used as grain refiner in ingot metallurgy and cast products
Others	0.15 max combined	Includes V, Zr, and residuals; tightly controlled to maintain properties

Copper is the dominant alloying element in 2004 and controls the precipitation hardening response through formation of Al2Cu and related metastable phases during aging. Magnesium and manganese modify the precipitation kinetics and grain structure to improve toughness and reduce the incidence of coarse intermetallic particles. Small additions of Cr and Ti are used to control recrystallization and maintain stable grain sizes during thermomechanical processing.

Mechanical Properties

In tensile behavior, 2004 exhibits a strong dependence on temper: annealed conditions provide good elongation and moderate strength suitable for forming, while T6-type treatments produce much higher ultimate tensile strengths and corresponding increases in yield strength. Yield strength in heat-treated 2004 rises substantially due to fine precipitate distributions, and the material typically shows a relatively flat strain-hardening response once precipitates are established.

Elongation varies from high ductility in O temper to modest ductility in peak-aged tempers, influencing forming limits and fatigue crack-initiation resistance. Hardness correlates with aging state: annealed material has low hardness, while T6 can reach high hardness levels typical of aerospace-grade Al–Cu alloys, which benefits wear resistance but can impede cold forming.

Fatigue performance of 2004 is generally good for its strength class when careful attention is paid to surface finish and corrosion protection; corrosion pits can dramatically reduce fatigue life. Thickness effects are notable: thicker sections often have coarser microstructures after solidification and require tailored heat treatment cycles to achieve uniform properties through the section.

Property	O/Annealed	Key Temper (e.g., T6)	Notes
Tensile Strength	180–280 MPa	350–480 MPa	T6 peak-aged values depend on exact Cu/Mg balance and aging temperature/time
Yield Strength	80–150 MPa	250–400 MPa	Substantial gain from solutionizing and artificial aging
Elongation	12–20%	6–10%	Trade-off between strength and ductility; gauge-dependent
Hardness	40–70 HB	110–150 HB	Brinell values approximate typical ranges for sections and tempers

Physical Properties

Property	Value	Notes
Density	~2.78 g/cm³	Slightly higher than pure Al due to Cu content
Melting Range	~500–640 °C	Solidus-liquidus range depends on composition and alloying elements
Thermal Conductivity	~110–130 W/m·K	Lower than pure Al; copper content reduces conductivity
Electrical Conductivity	~28–38 % IACS	Reduced compared with pure Al and 1xxx series
Specific Heat	~0.88 J/g·K (880 J/kg·K)	Typical for Al alloys at room temperature
Thermal Expansion	~23–24 µm/m·K (20–100 °C)	Coefficient slightly lower than some 5xxx alloys

The addition of copper reduces both thermal and electrical conductivity compared to pure aluminum grades, but 2004 still maintains sufficiently high thermal conductivity for many heat-sinking or thermal-management applications. Density is higher than low-alloy Al grades but still provides a favorable strength-to-weight ratio when compared to many ferrous alloys.

Thermal expansion is typical of aluminum alloys and must be accounted for in multi-material assemblies to avoid thermal stress concentrations. The melting range informs heat-treatment windows and brazing/welding considerations; solution treatments are carried out below the solidus to prevent incipient melting.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.2–6.0 mm	Good uniformity in thin gauges after T4/T6	O, H14, T3, T4, T6	Widely used for formed and machined components
Plate	6–150 mm	Requires longer solution cycles; risk of soft cores	O, T4, T6	Thick sections need tailored treatments to avoid through-thickness gradients
Extrusion	Profiles up to large cross-sections	Moderately good; aging response depends on section	O, T4, T6	Extrusion die design essential for homogeneous flow; grain control important
Tube	1–20 mm wall	Similar to extrusion behavior; cold-worked variants possible	O, T4, T6	Used for structural tubing and machined fittings
Bar/Rod	Diameters up to 200 mm	High machinability; patterns influenced by billet history	O, T6	Produced by extrusion or direct chill casting; used for forgings and machined parts

Sheets and thin products are the most common forms for 2004, allowing effective solution treatment and rapid quenching to lock in supersaturated solid solutions. Thick plates and large extrusions require longer soak times and controlled quench media to avoid softness at the core; this complicates heat treatment and can limit achievable properties in very thick sections.

Bars and rods intended for high-precision machining are frequently supplied in T6 or T651 forms to provide dimensional stability and high hardness for tooling operations. Tubes and extrusions are used where profile stiffness and localized machining are needed, and temper selection balances formability during fabrication against the final required strength.

Equivalent Grades

Standard	Grade	Region	Notes
AA	2004	USA	Designation within the Aluminum Association family; composition-based
EN AW	—	Europe	Direct EN AW-2004 equivalents are rare; 2024 or 2014 often used as functional substitutes
JIS	—	Japan	No widely adopted JIS direct equivalent; similar uses filled by 2014/2024-class alloys
GB/T	—	China	Local alloys may exist but 2004 is not universally standardized in all regions

Direct cross-standard equivalents for 2004 are uncommon because national standards tend to favor more widely adopted alloys such as 2014 and 2024 in the Al–Cu family. Where exact chemistry and process control are required, engineers typically specify the Aluminum Association AA2004 composition and temper. When standards require an EN, JIS, or GB/T number, 2014 or 2024 are commonly referenced as the nearest functionally similar alternatives with the caveat that mechanical properties and corrosion behavior will differ.

Corrosion Resistance

Atmospheric corrosion resistance of 2004 is moderate but inferior to 5xxx and 6xxx series alloys; unprotected exposure to aggressive industrial or marine environments can lead to pitting and intergranular corrosion, especially in heat-treated conditions where Cu-rich precipitates localize attack. Cladding with pure aluminum or application of robust organic/inorganic coatings is common practice to protect structural components in corrosive service.

In marine environments, 2004 should be used with caution unless adequately protected; immersion or splash zones accelerate localized corrosion and reduce fatigue life. For seawater exposure, 5xxx series alloys and anodic coatings often outperform 2004. In many aerospace and marine applications where Al–Cu alloys are necessary for strength, sacrificial coatings, anodizing, or cathodic protection are employed to extend service life.

Stress corrosion cracking (SCC) is a concern for Al–Cu alloys when tensile stresses and corrosive species combine, particularly in peak-aged conditions. Overaging (T7) can improve SCC resistance at the expense of peak strength by coarsening precipitates and reducing local galvanic couples. In galvanic couples, 2004 is anodic to stainless steel and cathodic to pure aluminum depending on local conditions; careful material pairing and isolation are required to avoid accelerated corrosion.

Compared with other alloy families, 2004 sacrifices corrosion resistance for strength relative to 5xxx and 6xxx series, yet it offers superior strength and machinability compared to 1xxx and 3xxx series alloys. Selection must therefore consider surface treatment and maintenance cycles when choosing 2004 for long-term outdoor or marine use.

Fabrication Properties

Weldability

Welding of 2004 is challenging because Al–Cu alloys are prone to hot cracking and significant HAZ softening from dissolution of strengthening precipitates. Fusion welding (MIG/TIG) should be approached cautiously; filler alloys such as 4043 or specially formulated Al–Cu fillers can be used depending on required mechanical and corrosion performance, but the joint strength will often be lower than base metal and HAZ soft zones are common. For critical structures, bonding, mechanical fasteners, or friction stir welding (FSW) are preferred to preserve mechanical properties and avoid significant loss in the heat-affected region.

Machinability

2004 typically has good machinability compared with many other high-strength aluminum alloys due to its ability to form short, controllable chips and its high strength that supports stable cutting. Carbide tooling with rigid setups and positive rake geometries are recommended, with moderate to high cutting speeds and ample coolant to avoid built-up edge. Surface finishes after machining can be excellent, and post-machining stress relief or aging can be applied to restore or stabilize properties where necessary.

Formability

Cold formability of 2004 depends strongly on temper: O and H tempers are suitable for complex forming operations with relatively small bend radii, whereas T6 and other peak-aged tempers have limited cold formability and lower allowable bend strain. Minimum bend radii should be determined experimentally, but as a rule of thumb, thin-sheet O-tempers can be bent to 1–2× thickness without cracking, whereas T6 may require radii of 3–6× thickness or preheating/annealing to achieve similar results.

Heat Treatment Behavior

As a heat-treatable Al–Cu alloy, 2004 responds well to conventional solution heat treatment followed by quenching and artificial aging to develop peak strength. Typical solution temperatures are in the range of approximately 495–510 °C with times scaled to section thickness to achieve full solute homogenization without incipient melting. Rapid quenching into water or controlled polymer quenches is necessary to retain supersaturation for subsequent aging.

Artificial aging for T6 temper is commonly carried out at 160–190 °C for times from 6 to 24 hours depending on section and desired property balance; the aging treatment precipitates metastable phases such as θ' (Al2Cu) responsible for strength. Natural aging (T3/T4) produces moderate hardness over days at room temperature but not the peak levels of artificial aging. Overaging (T7) at higher temperatures or longer times coarsens precipitates, reducing strength but improving stress-corrosion resistance and dimensional stability.

Non-heat-treatable strengthening routes (work hardening) are limited in 2004 because much of the strength comes from precipitates; however, controlled cold work before or after solution treatment can tailor properties in some tempers. Full annealing restores ductility and removes prior strain hardening to enable forming operations.

High-Temperature Performance

2004 experiences significant loss of strength at elevated temperatures because precipitate phases dissolve or coarsen, reducing precipitation hardening effectiveness. Above ~150 °C sustained service will accelerate overaging and lead to measurable reductions in yield and ultimate strengths; for continuous structural service a conservative upper use temperature is typically limited to 100–120 °C to maintain most mechanical properties.

Oxidation in air is minimal due to aluminum's protective oxide, but prolonged exposure at elevated temperatures can promote scale formation and accelerate precipitate coarsening. In welded or heat-affected zones, the combination of thermal cycles and elevated service temperatures can exacerbate softening and reduce fatigue life. For high-temperature structural applications, nickel or steel alloys and specialized high-temperature aluminum alloys are generally preferred.

Applications

Industry	Example Component	Why 2004 Is Used
Automotive	Structural brackets, performance subframes	High strength-to-weight and good machinability for precision parts
Marine	Fittings, non-immersion structural elements	Strength advantage when protected/coated; used in less aggressive zones
Aerospace	Fittings, landing gear components (non-critical)	High specific strength and fatigue resistance after aging
Electronics	Heat sinks, structural mounts	Good thermal conductivity combined with machinability

2004 is deployed where its higher strength and machinability justify additional corrosion protection or where components are sheltered from the harshest environments. Its balance of mechanical properties and ease of machining make it attractive for precision components manufactured in moderate volumes.

Selection Insights

Choose 2004 when design priorities emphasize high strength combined with excellent machinability and when corrosion can be mitigated by coatings, cladding, or controlled environments. It is especially suitable for machined structural parts and where heat-treatable strengthening is required to meet load-bearing requirements.

Compared with commercially pure aluminum (1100), 2004 offers far greater strength but reduced electrical conductivity and lower general formability. Compared with work-hardened alloys such as 3003 or 5052, 2004 provides higher tensile and yield strengths at the expense of corrosion resistance and weldability. Compared with common heat-treatable alloys such as 6061 or 6063, 2004 often yields greater peak strength for certain tempers and better machinability, making it preferable when higher strength and specific fatigue performance matter more than aluminum–magnesium–silicon’s superior corrosion resistance or corrosion-weldability balance.

Practical selection rule: use 2004 for high-strength machined or aged parts where protective coatings are feasible; use 5xxx/6xxx alloys for exposed marine/architectural applications where corrosion resistance and weldability dominate decision factors.

Closing Summary

Alloy 2004 remains a relevant engineering choice where its precipitation-hardening response delivers a favorable strength-to-weight ratio and excellent machinability, provided designers address its reduced corrosion resistance and welding limitations with suitable surface protection and assembly methods.