Aluminum 7039: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

Alloy 7039 belongs to the 7xxx series of aluminum alloys, a family characterized by zinc as the principal alloying element and often alloyed with magnesium and trace copper. It is a heat-treatable, precipitation‑hardening alloy designed to deliver high specific strength and reasonable toughness while keeping density advantages inherent to aluminum.

Major alloying elements typically include Zn, Mg and modest Cu additions, with small amounts of Cr, Mn or Ti used for grain control and to limit recrystallization. Strengthening is achieved primarily through solution treatment, quenching and controlled artificial aging to form fine metastable Zn-Mg (and Zn-Mg-Cu where present) precipitates that obstruct dislocation motion.

Key traits of 7039 are high strength-to-weight ratio, good fatigue resistance for a high‑strength alloy, and acceptable corrosion resistance when properly heat treated and surface‑protected. Formability and weldability are moderate: the alloy can be formed in softer tempers and welded with precautions, but overaging and HAZ softening are trade-offs compared with softer 5xxx or 3xxx family alloys.

Typical industries include aerospace forgings and fittings, high‑performance structural components in automotive and motorsport, and specialty marine and defense hardware where a balance of strength, damage tolerance and machinability is required. Engineers select 7039 when higher strength than 6xxx alloys is needed without stepping up to the very high‑cost, ultra‑high‑strength 7075 family or when a particular balance of fatigue and localized toughness is required.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed condition for forming and stress relief
H14	Medium-Low	Low-Moderate	Fair	Good	Strain‑hardened and partially stabilized for moderate strength
T4	Medium	Moderate	Good	Good	Solution heat‑treated and naturally aged to partial strength
T5	Medium-High	Moderate	Fair	Fair	Cooled from elevated temperature and artificially aged
T6	High	Low-Moderate	Limited	Fair-Poor	Solution heat‑treated and artificially aged to peak strength
T62	High (overaged)	Improved	Improved	Better than T6	Artificially aged to a slightly overaged condition to improve SCC resistance
T651	High	Low-Moderate	Limited	Fair-Poor	T6 with stress relief by stretching; common for plate and extrusions

Temper selection strongly controls the balance between strength and formability in 7039. Softer tempers such as O or T4 are used for complex forming and subsequent aging operations, while T6/T651 give maximum static strength at the expense of elongation and formability.

The temper also affects susceptibility to stress corrosion cracking and HAZ softening during welding; designers often choose slightly overaged tempers (T62) or controlled post‑weld aging to trade absolute peak strength for improved durability in aggressive environments.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.50	Typical impurity control; excessive Si reduces toughness
Fe	≤ 0.50	Impurity; forms intermetallics that can affect fatigue initiation
Mn	0.05–0.40	Grain structure control and improved toughness at low levels
Mg	1.0–2.0	Key precipitate former with Zn for strengthening
Cu	0.1–1.2	Increases strength and hardenability; can reduce corrosion resistance
Zn	3.5–5.5	Principal strengthening element in 7xxx series
Cr	0.02–0.25	Controls recrystallization and improves stress corrosion resistance
Ti	0.05–0.20	Grain refining during casting/extrusion
Others (incl. Al balance)	Balance	Trace elements controlled to meet mechanical and corrosion goals

The Zn–Mg ratio and minor Cu additions determine the precipitate chemistry and thereby the peak hardness and aging response. Chromium and manganese are employed to pin grain boundaries and limit excessive grain growth during solution treatment and thermomechanical processing.

Impurity elements like Fe and Si form relatively hard intermetallic particles; their levels are controlled to balance machinability and fatigue performance. Overall, composition ranges above are representative and may vary by supplier and specification.

Mechanical Properties

In tensile behavior, 7039 exhibits a pronounced increase in strength after solution treatment and artificial aging, with a trade‑off in ductility compared with annealed conditions. Peak‑aged tempers (T6/T651) typically show high yield and ultimate tensile strengths with moderate elongation; softer tempers provide the ductility required for forming operations.

Yield strength varies widely with temper and thickness owing to differences in quench effectiveness and cold work. Fatigue performance of 7039 is generally good for a high‑strength aluminum alloy, particularly when shot‑peened or stress‑relieved; however, fatigue crack initiation is sensitive to surface finish and intermetallic particle distribution.

Hardness correlates with temper and aging: annealed alloys are relatively soft and easy to machine/form, while T6/T651 reaches much higher Brinell or Rockwell values. Thickness effects are notable: thick sections can be difficult to quench uniformly which reduces achievable peak strength compared with thin sheet.

Property	O/Annealed	Key Temper (T6/T651)	Notes
Tensile Strength	~230 MPa (typical)	480–540 MPa	Tensile strength varies with thickness and aging schedule
Yield Strength	~130 MPa (typical)	430–500 MPa	Yield increases significantly with precipitation hardening
Elongation	18–25%	6–12%	Elongation drops as strength increases; dependent on processing
Hardness	60–75 HB	140–170 HB	Brinell hardness increases significantly in peak tempers

Values shown are representative ranges and will depend on specific product form, thickness and supplier processing.

Physical Properties

Property	Value	Notes
Density	~2.79 g/cm³	Slightly higher than pure Al due to Zn content
Melting Range	~480–640 °C	Solidus–liquidus ranges depend on alloying; use conservative machining temps
Thermal Conductivity	~140 W/m·K	Lower than pure Al but still favorable for heat dissipation
Electrical Conductivity	~30–40 %IACS	Reduced from pure Al due to alloying; varies with temper
Specific Heat	~875 J/kg·K	Typical for aluminum alloys near room temperature
Thermal Expansion	~23–24 µm/m·K (20–100 °C)	Similar to other Al‑Zn‑Mg alloys; consider in joined assemblies

7039 retains aluminum’s high thermal conductivity relative to steels, which is beneficial for heat‑dissipating components. Its density advantage continues to provide gains in specific stiffness and specific strength for weight‑critical designs.

Electrical conductivity is reduced compared with pure aluminum and some 6xxx alloys; the alloy is not chosen where maximum electrical conductivity is primary. Thermal expansion is in the typical aluminum range and must be accommodated when joining to dissimilar materials.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.5–6.0 mm	Good through thickness for thin gauges	T4, T5, T6	Used for formed panels and skins; quench sensitivity manageable
Plate	6–150+ mm	Reduced achievable strength in thick sections	T651, T62	Thick plate may require specialized quench/aging to maximize properties
Extrusion	Complex profiles up to several meters	Good directional strength	T6, T651	Extrusion die design and quench rate influence final properties
Tube	OD up to several hundred mm	Strength varies with wall thickness	T6, T651	Common for structural tubing and high‑strength frames
Bar/Rod	Diameters up to 200 mm	Machinable high‑strength stock	T6, T61	Used for machined components and fittings

Manufacturing route and product form significantly affect mechanical performance. Extrusions and thin sheets can be quenched rapidly and reach near‑peak properties after aging, while plate and thick sections often suffer from quench gradients that require modified heat‑treat cycles or overaging to improve uniformity.

Designers must coordinate material supplier capabilities (e.g., quench tanks, overstretching, homogenization) with application requirements, as processing choices determine the balance of properties delivered in final parts.

Equivalent Grades

Standard	Grade	Region	Notes
AA	7039	USA	Designation in aluminum association systems; principal standard reference
EN AW	7039	Europe	Often referenced as EN AW‑7039; check specific EN chemical and mechanical spec
JIS	—	Japan	No direct one‑to‑one JIS equivalent; nearest properties may align with high‑strength 7xxx families
GB/T	—	China	Chinese standards may list similar 7xxx formulations; verify composition and temper specifications

There is not always an exact one‑to‑one cross‑reference for 7039 across standards because regional specifications control impurity limits, trace additions and permitted tempers. When substituting alloys, engineers must compare guaranteed tensile/yield values, toughness, quench sensitivity and corrosion prescriptions rather than relying solely on grade numbers.

Suppliers may offer proprietary variants sold under similar 7039 labeling; procurement should always request full chemical and mechanical certificates and, for critical applications, test coupons or full batch mechanical verification.

Corrosion Resistance

7039 offers moderate atmospheric corrosion resistance comparable to other Zn‑Mg‑based 7xxx alloys when properly overaged or coated. In neutral atmospheres its performance is acceptable, but susceptibility to localized corrosion such as pitting and exfoliation increases with higher Zn and Cu levels and with peak‑aged tempers.

In marine or chloride‑rich environments, 7039 requires protective measures—such as anodizing, chromate conversion coatings, or organic paint systems—to achieve long service life. Overaged tempers (T62 or T7xx variants) and appropriate design detailing (drainage, sealed joints) significantly reduce risk of intergranular attack.

Stress corrosion cracking (SCC) is a known concern for high‑strength 7xxx alloys: T6 condition maximizes strength but also increases SCC susceptibility under tensile stress in corrosive environments. Choosing slightly overaged tempers and controlling residual stress through stretching or post‑weld heat treatments mitigates SCC risk and improves long‑term durability compared with peak‑aged conditions.

Fabrication Properties

Weldability

Welding 7039 requires careful control: fusion welding (TIG/MIG) is feasible but the heat‑affected zone (HAZ) can experience softening and reduced toughness. Recommended practice includes using fillers matched to a slightly overaged 7xxx filler alloy or, where corrosion and strength permit, 5356/5183 fillers to improve ductility and corrosion resistance at the joint.

Pre‑ and post‑weld thermal treatments or mechanical stress relief (stretching) are commonly used to restore property balance after welding. Hot‑cracking risk is moderate to high in peak tempers, so joint design and weld parameters must minimize restraint and avoid rapid solidification conditions that promote cracking.

Machinability

Machinability of 7039 is favorable relative to very high‑strength 2xxx or tool steels but is more challenging than 6xxx family alloys due to its higher strength and harder precipitates. Carbide tooling with positive rake, rigid setups and conservative feed/speed schedules deliver best results; chip control is aided by coolant and proper tool geometry.

Surface finishes and burr formation are influenced by temper: softer T4/O conditions machine more easily but require subsequent heat treatment if peak strength is needed. For production machining, consider pre‑hardening (if applicable) or near‑net forging to minimize material removal.

Formability

Cold formability is limited in peak‑aged conditions; for forming operations use O or T4 tempers to achieve tighter bend radii and complex profiles. Typical minimum internal bend radii for sheet in softer tempers can be as low as 1–2× thickness for simple bends, but designers should validate with trial forming due to variable strain hardening behavior.

Work hardening occurs if forming is done in softer tempers and can be leveraged to increase local strength after natural or artificial aging. For severe forming, warm forming or subsequent solution‑treat/age cycles may be required to achieve both geometry and final mechanical property targets.

Heat Treatment Behavior

As a heat‑treatable alloy, 7039 follows the classic solution‑treat, quench and age pathway for precipitation strengthening. Solution treatment is typically performed at elevated temperatures sufficient to dissolve Zn‑Mg precipitate precursors, followed by rapid quenching to retain a supersaturated solid solution. Quench rate is critical: inadequate quench produces coarse precipitates that lower final strength.

Artificial aging (T6) develops peak strength through controlled temperature/time cycles that promote fine, dispersed precipitates. Overaging treatments (T62 or T7 variants) intentionally coarsen precipitates to improve stress corrosion resistance and HAZ stability at the cost of some peak strength. For components sensitive to residual stress, a T651 temper or post‑stretch is applied to relieve residual stresses after quenching.

Thin sections achieve target properties more readily due to rapid quench; thick sections often require specialized quench media, interrupted quenching, or modified aging schedules to balance strength and toughness. For welded assemblies, post‑weld heat treatment is limited by distortion concerns, so designers often design to achieve acceptable properties with minimal thermal cycles.

High-Temperature Performance

7039 is designed for ambient to moderately elevated temperatures; above ~100–150 °C, precipitation‑hardened strength begins to decline as precipitates coarsen and solute redistribution occurs. Long‑term exposure at elevated temperatures accelerates overaging and reduces both yield strength and fatigue performance compared with room temperature properties.

Oxidation at typical service temperatures is minimal compared with steels, but prolonged high temperature can affect surface condition and promote dezincification of Zn‑rich precipitates at grain boundaries. HAZ behavior in welded regions is particularly sensitive to thermal excursions; local overaging can reduce strength and increase susceptibility to localized corrosion.

For high‑temperature structural applications, designers should validate life under thermal cycling and consider alternative alloys or protective design measures; 7039 is best deployed below the limits where precipitate stability is compromised by extended elevated temperature exposure.

Applications

Industry	Example Component	Why 7039 Is Used
Automotive	Structural braces and suspension links	High specific strength and good machinability for safety‑critical parts
Marine	Structural fittings and brackets	Balance of strength and corrosion resistance when coated/anodized
Aerospace	Fittings, forgings, machined fittings	High strength‑to‑weight and fatigue performance for primary and secondary structures
Defense	Armoring brackets, launcher components	High static strength and damage tolerance with controlled processing
Electronics	Structural frames and thermal spreaders	Good thermal conductivity combined with high strength for compact designs

7039 is selected where a high‑strength aluminum is needed but where 7075’s higher cost or greater SCC sensitivity is a disadvantage. It occupies a niche for machined, forged and extruded parts that need a combination of good fatigue life, machinability and adequate corrosion resistance.

Selection Insights

For an engineer choosing material, 7039 offers a clear trade‑off: versus commercially pure aluminum (1100), 7039 sacrifices some electrical and thermal conductivity and much of the exceptional formability, while gaining several times the yield and tensile strength. This makes 7039 appropriate when structural performance outweighs conductivity needs.

Compared with common work‑hardened alloys such as 3003 or 5052, 7039 delivers substantially higher static strength and better machinability for high‑performance components, though its corrosion resistance—especially in marine chloride environments—requires more deliberate surface protection. If formability or weldability under ambient conditions is primary, the 3xxx/5xxx family remains preferable.

Compared with more ubiquitous heat‑treatable alloys like 6061 or 6063, 7039 typically offers higher peak strength and improved fatigue performance, making it preferable when weight reduction and higher working stress are needed. However, 6061/6063 may be chosen when joining, anodizing color consistency, or cost/availability are more critical than maximum strength.

Closing Summary

Alloy 7039 remains a viable engineering choice where higher specific strength and good fatigue resistance are required alongside acceptable corrosion behaviour when properly protected. Its heat‑treatable nature allows designers to tailor property balances through temper selection and controlled processing, making it useful across aerospace, automotive, marine and defense sectors where weight, strength and machinability must be carefully balanced.