Aluminum 7099: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

7099 is a high-strength aluminum alloy belonging to the 7xxx series of Al-Zn-Mg(-Cu) alloys. It was developed for demanding structural applications where high specific strength, good fracture toughness and improved resistance to stress-corrosion cracking are required compared with baseline 7xxx alloys.

Major alloying elements in 7099 are zinc, magnesium and copper, with microalloying additions such as zirconium and small amounts of chromium or titanium to control grain structure and recrystallization. The strengthening mechanism is primarily precipitation hardening (heat-treatable) through the formation of fine η' and η (MgZn2) precipitates after solution treatment and artificial aging; controlled microstructure also provides grain-boundary engineering to mitigate SCC susceptibility.

Key traits of 7099 include very high tensile and yield strength in peak-aged tempers, moderate-to-low intrinsic corrosion resistance typical of high-Zn alloys (but often improved by overaging or post-fabrication treatments), limited direct weldability in peak tempers, and reduced formability relative to 3xxx/5xxx alloys. Typical industries are aerospace, high-performance automotive, defense and select high-strength sporting goods where weight-critical structural components are required. Engineers select 7099 over other alloys when a combination of very high strength, fracture toughness and tailored SCC resistance outweighs trade-offs in formability, conductivity and weld integrity.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed, maximum ductility for forming
T1	Medium	Medium	Good	Poor to fair	Cooled from hot working and naturally aged
T4	Medium-High	Medium	Fair	Poor	Solution treated and naturally aged
T6	Very High	Low-Medium	Limited	Poor	Solution treated and artificially aged to peak strength
T651	Very High	Low-Medium	Limited	Poor	T6 with stress-relief stretching after quench
T73 / T76	Medium-High	Medium	Improved	Better than T6	Overaged tempers to improve SCC and exfoliation resistance
H14 / H24	Medium	Reduced	Limited	Better than T6	Work hardened tempers for sheet applications

Temper has a primary influence on the mechanical balance of strength versus ductility and corrosion resistance. Peak-aged tempers (T6/T651) maximize static strength and fatigue resistance but reduce formability and increase susceptibility to SCC; overaged tempers (T73/T76) trade some strength for improved toughness and environmental performance.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.10	Impurity control; kept low to avoid intermetallics that reduce toughness
Fe	≤ 0.25–0.50	Impurity; promotes intermetallics that can act as fatigue initiation sites
Mn	≤ 0.10	Minor, usually controlled to limit deleterious phases
Mg	~2.0–3.0	Primary alloying element for precipitation strengthening (MgZn2 precipitates)
Cu	~1.2–2.6	Raises strength and contributes to age-hardening sequence; affects corrosion/SCC
Zn	~6.5–8.5	Principal strengthening element producing high peak strengths via Mg-Zn precipitates
Cr	~0.02–0.25	Added in trace amounts to control recrystallization, refine grain structure
Ti	≤ 0.10	Grain refiner when added in controlled amounts
Others	Balance (Al) + trace Zr, Ag, etc.	Zr or other microalloying elements often used for dispersoid control and recrystallization inhibition

Element ranges above are representative of high-strength 7xxx-series practice and are intended as typical composition windows rather than exact specification numbers. Zinc, magnesium and copper synergize to produce the fine precipitate population responsible for high strength; microalloying with Zr/Cr/Ti promotes a stable, recrystallization-resistant subgrain structure that improves toughness and reduces SCC sensitivity.

Mechanical Properties

7099 exhibits a broad tensile-strength envelope that is highly dependent on temper; annealed material shows ductile tensile behavior with significant uniform elongation, while peak-aged tempers reach ultimate tensile strengths comparable to the highest-strength Al alloys used in aerospace. Yield strengths in T6/T651 tempers are high enough to replace some steel components on a weight-for-weight basis, but elongation and bendability are restricted. Hardness tracks closely with tensile/yield state and is useful as a quality and aging-control indicator.

Fatigue performance of 7099 in optimized tempers is strong relative to other Al alloys, benefiting from tight control of inclusions and grain structure; however, fatigue life is sensitive to surface condition, residual stresses and environmental exposure. Thickness effects are pronounced: thicker sections can be more difficult to solution-treat uniformly, may retain gradients in properties through-thickness, and tend to be more susceptible to exfoliation or intergranular corrosion if not properly aged or overaged.

Corrosion-related softening and HAZ effects from welding or localized heating can dramatically reduce local strength and reduce fatigue life; therefore, mechanical performance must always be considered in the context of the finished fabrication process and chosen temper.

Property	O/Annealed	Key Temper (e.g., T6/T651)	Notes
Tensile Strength	~220–300 MPa (typical)	~540–620 MPa (typical)	Peak-aged values are alloy- and temper-dependent; ranges represent typical engineering values
Yield Strength	~90–150 MPa	~470–560 MPa	Yield-to-ultimate ratio varies with temper and processing history
Elongation	~15–25%	~6–12%	Ductility decreases as strength increases; design for limited forming in high-strength tempers
Hardness	~40–80 HB	~150–185 HB	Brinell or Vickers hardness correlates well with tensile strength for process control

Physical Properties

Property	Value	Notes
Density	~2.78–2.81 g/cm³	Typical for high-strength Al-Zn-Mg-Cu alloys; enables high specific strength
Melting Range	Solidus ≈ 475–500 °C; Liquidus ≈ 635–655 °C	Alloying lowers the solidus vs pure Al; ranges depend on exact chemistry
Thermal Conductivity	~120–160 W/m·K (room temp, approximate)	Lower than pure Al; conductivity decreases with heavier alloying
Electrical Conductivity	~30–50 %IACS (typical)	Significantly reduced vs pure Al; temper and precipitation state affect values
Specific Heat	~0.85–0.92 J/g·K	Close to other Al alloys; useful for thermal design
Thermal Expansion	~23–24 µm/m·K (20–100 °C)	Typical aluminum expansion; consider in assemblies with low-expansion materials

Physical properties above are approximate engineering values intended for preliminary thermal and mass calculations. Thermal conductivity and electrical conductivity are reduced relative to pure aluminum because of alloying and precipitation; these changes affect heat dissipation and electromagnetic behavior in high-performance components.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.4–6.0 mm	Good strength-to-weight in T6/T651; full-thickness T73 for improved SCC	O, Hx, T6, T651, T73	Widely used for panels and formed skins where strength and stiffness are needed
Plate	6–100+ mm	Through-thickness property gradients possible; heavier sections often require special solution treatments	T6, T651, T76	Plate processing needs larger furnaces and quench control to avoid soft cores
Extrusion	Profiles up to several hundred mm	High tensile in peak tempers after age; extrusion directionality affects properties	T6, T651, T73	Extruded structural members benefit from recrystallization control additives
Tube	Diameter and wall thickness vary	Similar behavior to extrusions; hoop vs axial properties differ	T6, T651	Tubular components require post-extrusion aging to reach target properties
Bar/Rod	Diameters from small to large	Properties depend on feedstock and cooling	O, T6, T651	Used for machined high-strength parts and fastener blanks

Processing route strongly influences final properties: rolling and extrusion involve substantial deformation and recrystallization behavior that must be controlled by microalloying (Zr, Cr) to retain a favorable subgrain structure. Plate and thick-section parts require more aggressive solution treatment and careful quenching to avoid centerline softening, while thin sheets are easier to age uniformly and can be formed in softer tempers prior to a final aging step.

Equivalent Grades

Standard	Grade	Region	Notes
AA	7099	USA	Designation used in some supplier catalogs and aerospace specifications
EN AW	No direct universal equivalent	Europe	No single EN number universally maps to 7099; similar alloys include EN AW-7075 / EN AW-7050 family variants
JIS	—	Japan	Direct JIS equivalent is not common; material may be sourced to proprietary aerospace specs
GB/T	—	China	Chinese standards may list high-strength Zn-Mg-Cu alloys but direct equivalents require composition/tempering matching

Direct equivalents for 7099 are limited because the alloy is often proprietary or produced to aerospace supplier specifications that control microalloying and thermomechanical processing. When substituting, engineers should compare full chemical and mechanical property tables rather than rely solely on nominal designation.

Corrosion Resistance

In atmospheric conditions 7099 performs better than some peak-aged high-Zn alloys when appropriate overaging or protective coatings are applied, but it is generally less corrosion-resistant than 5xxx and 3xxx series alloys. Surface treatments such as chromate conversion, anodizing and protective paints are commonly used to provide serviceable life in exposed environments and to mitigate local pitting.

Marine behavior is a critical consideration; seawater exposure promotes pitting and intergranular attack in high-Zn, high-Cu alloys unless mitigated by overaged tempers (T73/T76), cladding or sacrificial protection. Use in splash zones or prolonged immersion needs careful alloy/temper selection, surface preparation and cathodic protection where required.

Stress corrosion cracking (SCC) is a known risk for high-strength 7xxx alloys in peak-aged conditions, particularly under sustained tensile stress in corrosive environments. Alloy variants such as 7099 are engineered with microalloy additions and recommended tempering (overaging) to reduce SCC propensity, but designers should account for galvanic interactions when 7099 is mated to more noble materials such as stainless steel or titanium, and should minimize crevices and tensile residual stresses.

Fabrication Properties

Weldability

Welding of 7099 is challenging in high-strength tempers because precipitation-hardened zones in the heat-affected zone (HAZ) and fusion zone are prone to significant softening and loss of mechanical properties. TIG and MIG welding are possible for localized repairs or joints but typically require post-weld heat treatment (PWHT) or mechanical design to avoid high-stress concentrations. Recommended filler alloys are usually from lower-strength 7xxx variants or specially formulated fillers that balance strength and crack resistance; however, lap or friction welding methods and mechanical fastening are often preferred for structural assemblies to avoid HAZ degradation.

Machinability

Machining behavior of 7099 is generally good for high-strength aluminum alloys: it machines more easily than high-strength steels and may achieve high material-removal rates, but tool geometry and tooling material must account for the alloy's work-hardening tendency and abrasive precipitates. Carbide tooling with positive rake geometry, high feed and moderate speeds provides the best balance; coolant and chip evacuation are recommended to avoid built-up edge formation. Machinability index is typically lower than 6xxx-series alloys but acceptable for complex, precision components when using modern tooling.

Formability

Cold formability is limited in peak-aged tempers; minimum bend radii are larger than for 5xxx or 3xxx alloys and springback is significant due to high yield strength. Best practice is to form in softer tempers (O or T4/H strains) and perform a final artificial aging (T6) after forming when feasible. Stretch forming, incremental forming and superplastic techniques may be used for complex shapes, and temper selection (e.g., H1x series) can improve formability for limited deformation tasks.

Heat Treatment Behavior

As a heat-treatable alloy, 7099 follows the classic solutionize–quench–age sequence. Solution treatment is typically performed near the upper end of the solid solution range (roughly 470–480 °C, alloy dependent) to dissolve soluble phases, followed by rapid quenching to retain a supersaturated solid solution. Artificial aging is done at intermediate temperatures (commonly 120–180 °C) for controlled times to precipitate fine η' particles and reach peak strength (T6).

Overaging (T7x variants) is used to coarsen precipitates and reduce the electrochemical potential differences at grain boundaries, improving resistance to SCC and exfoliation at the expense of some ultimate strength. The T651 designation indicates stress relief by stretching after quench to control residual stresses and distortion; this is common in aerospace applications. Proper heat-treatment control, quench rate, and subsequent aging recipes are critical for achieving intended mechanical and environmental properties.

Non-heat-treatable behaviors are not applicable to 7099 in the classical sense because precipitation hardening is the primary strengthening mechanism; however, local annealing (e.g., for forming) and work-hardening sequences can be used in production to reach intermediate property sets prior to final aging.

High-Temperature Performance

7099 loses strength progressively as temperature increases above ambient because precipitate stability is temperature-sensitive; sustained service temperatures above approximately 100–120 °C will reduce yield and ultimate strength and may accelerate precipitate coarsening. Short-term exposure to higher temperatures can anneal or overage the microstructure, changing mechanical and corrosion characteristics.

Oxidation of aluminum alloys at typical service temperatures is minimal compared with steels, but surface oxide properties and protective coatings must be considered for thermal cycling environments. HAZ in welded regions can experience localized softening and loss of toughness at elevated temperatures, so design for thermal excursions should restrict localized heating and account for changes in residual stress and microstructure.

Applications

Industry	Example Component	Why 7099 Is Used
Aerospace	Fuselage stiffeners, wing fittings, structural forgings	High specific strength and improved SCC resistance in selected tempers
Automotive	High-performance chassis components, structural crash members	Weight reduction with comparable strength to lower-grade steels
Marine	Structural members, outboard brackets (with protective treatment)	High strength-to-weight where corrosion control measures exist
Defense	Small-arms components, vehicle structural parts	High strength and toughness for demanding service loads
Sports / Recreation	High-end bicycle frames, racing components	Excellent stiffness-to-weight and fatigue performance

7099 is selected for components where very high strength and fracture resistance are prioritized and where manufacturing controls (heat treatment, protective finishes) can be implemented reliably. Its role is often as an enabling material for weight-critical, high-load designs.

Selection Insights

7099 should be selected when structural weight savings and high static and fatigue strength are primary design drivers and when the supply chain can control temper and surface protection. It is most appropriate where the design allows limited forming after final aging or incorporates post-forming aging to achieve required strength.

Compared with commercially pure aluminum (e.g., 1100), 7099 trades much higher strength and lower ductility and conductivity for an order-of-magnitude improvement in load-bearing capability; use 1100 only for excellent formability and conductivity when strength is not critical. Compared with work-hardened alloys (e.g., 3003 / 5052), 7099 provides substantially higher strength at the expense of formability and simpler corrosion resistance; choose 5052/3003 when forming and marine corrosion resistance are paramount. Compared with common heat-treatable alloys (e.g., 6061 / 6063), 7099 offers substantially higher peak strength and better fracture toughness in peak tempers, making it preferable where strength-to-weight is critical, though 6061/6063 remain easier to weld and form and are often lower cost.

Closing Summary

7099 remains relevant in modern engineering where the combination of very high specific strength, controlled fracture toughness and engineered SCC resistance enables designs that cannot be accomplished with lower-strength aluminum alloys, provided that fabrication, finishing and inspection are aligned with the alloy’s temper-sensitive behavior.