Aluminum 771: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

Alloy 771 is positioned in the 7xxx-series family of aluminum alloys, which are primarily aluminum-zinc-magnesium(-copper) systems engineered for high strength through precipitation hardening. Its nominal chemistry emphasizes Zn as the principal alloying addition supplemented by Mg and Cu to promote age-hardening precipitates, with trace additions of Cr, Zr or Ti used to refine grain structure and control recrystallization.

The strengthening mechanism for 771 is heat-treatable precipitation hardening: solution treatment dissolves solute elements, rapid quenching traps a supersaturated solid solution, and subsequent artificial aging produces fine, dispersed η (MgZn2) and related precipitates to raise yield and tensile strength. Key traits include high strength-to-weight ratio, moderate-to-poor intrinsic corrosion resistance unless overaged or clad, limited weldability in peak tempers, and reduced room-temperature formability compared with 5xxx and 6xxx alloys.

Typical industries that use 771 are aerospace for high-stress fittings and structural forgings, high-performance automotive for structural components and suspension parts, marine for high-strength fittings where protective coatings are applied, and specialty sports equipment where stiffness and light weight are critical. Engineers choose 771 over other alloys when the design calls for a combination of elevated static strength and fatigue resistance with a premium on mass reduction, accepting the trade-offs in fabrication and corrosion management.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed, maximum ductility for forming
T4	Moderate	Moderate	Good	Reduced	Naturally aged after quench; intermediate strength
T6	High	Low–Moderate	Poor–Fair	Poor	Solution-treated + artificial aging to peak strength
T73	Moderate–High	Improved	Fair	Poor	Overaged for better SCC and corrosion resistance
T651	High (stabilized)	Low–Moderate	Poor–Fair	Poor	Stress-relieved by stretching after T6 to reduce residual stresses
H12 / H14	Moderate	Low–Moderate	Limited	Good	Strain-hardened tempers for sheet with incremental increases in strength

Temper selection strongly changes the alloy’s mechanical envelope and fabrication behavior. Peak-tempers such as T6 deliver maximum static strength and fatigue resistance but substantially reduce elongation and bendability, making machining and forming more demanding and increasing susceptibility to stress-corrosion cracking.

Overaged tempers (T73 or stabilized tempers like T651) trade some peak strength for improved corrosion resistance and fracture toughness; these are used when environmental durability or resistance to SCC is more valuable than absolute yield strength.

Chemical Composition

Element	% Range	Notes
Al	Balance	Primary metal; remainder after alloying additions
Zn	5.5–7.5	Principal strengthening element forming MgZn2 precipitates
Mg	1.6–3.0	Combines with Zn to promote age-hardening; affects ductility
Cu	1.0–2.2	Adds strength and improves creep resistance; can reduce corrosion resistance
Cr	0.05–0.25	Grain structure control and recrystallization inhibitor
Ti	0.01–0.15	Grain refiner in castings and ingots
Fe	≤0.5	Impurity that forms intermetallics; controlled to limit brittleness
Si	≤0.5	Impurity from processing; limited to avoid brittle phases
Mn	≤0.3	Minor contribution to strength and corrosion behavior
Zr / Others	0.01–0.25	Optional microalloying elements for grain control and thermal stability

The Zn–Mg–Cu balance governs the precipitation sequence and the size/distribution of strengthening phases in 771. Zinc and magnesium control peak strength through η′/η precipitates, while copper refines precipitate structure and raises strength at the expense of increased sensitivity to localized corrosion. Trace elements such as Cr and Zr act as recrystallization inhibitors and nucleation control agents, which improve stability during thermomechanical processing and help maintain fine-grained microstructures for enhanced toughness.

Mechanical Properties

As a heat-treatable 7xxx-series alloy, 771 exhibits a wide envelope of mechanical behavior depending on temper and thickness. In annealed (O) condition it offers good ductility and formability with relatively low yield and tensile strength, making it suitable for heavy forming and stretch operations. In peak-aged conditions (T6/T651) tensile strength and yield are dramatically higher with typical reduction in elongation and bendability; HAZ or welded regions will be softened unless post-weld heat treatment is applied.

Fatigue resistance of 771 in peak tempers is generally excellent when microstructure is tightly controlled and when surface condition is preserved; however, fatigue performance is very sensitive to corrosion pits and machining marks which act as crack initiation sites. Thickness influences achievable properties: thicker sections are more difficult to bring to full solution treatment and quench uniformly, which can reduce effective strength and increase scatter in properties for plate and forgings compared with thin sheet and extrusions.

Property	O/Annealed	Key Temper (e.g., T6/T651)	Notes
Tensile Strength	240–320 MPa	540–660 MPa	Peak-aged strengths typical of high-strength Al-Zn-Mg-Cu alloys
Yield Strength	120–210 MPa	470–600 MPa	Substantial increase after heat treatment; dependent on section thickness
Elongation	12–20%	6–12%	Ductility drops in peak tempers; older tempers (T73) recover some ductility
Hardness	60–90 HB	150–210 HB	Hardness correlates with precipitation state and temper stability

Physical Properties

Property	Value	Notes
Density	~2.78 g/cm³	Typical of high-strength aluminum alloys; contributes to high specific strength
Melting Range	~480–635 °C	Solidus–liquidus region depends on alloying; melting behavior is broadened by solutes
Thermal Conductivity	120–150 W/(m·K)	Lower than pure Al due to alloying; sufficient for many heat dissipation uses
Electrical Conductivity	~28–40 % IACS	Reduced relative to pure aluminum because of solute scattering
Specific Heat	~0.9 J/(g·K)	Approximately 900 J/(kg·K); useful for thermal design calculations
Thermal Expansion	~23–24 µm/(m·K)	Typical linear expansion close to other aluminum alloys

The density and thermal properties make 771 attractive when high strength with modest thermal conduction is required, such as lightweight structural components that may also dissipate heat. Electrical conductivity is sacrificed relative to pure aluminum and 1xxx-series alloys, so 771 is rarely chosen for primary electrical conductors; instead it is chosen where mechanical performance per unit mass is the dominant criterion.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.3–6.0 mm	Can be produced in O, T4, T6	O, T4, T6, T73	Thin gauge sheet achieves uniform aging and high strength after T6
Plate	6–200 mm	Strength may decline with thickness due to quench limitations	O, T6, T651	Thick plate requires controlled quench methods; used for forgings and structural members
Extrusion	Cross-sections to 200 mm	Good directional strength; properties depend on cooling	O, T4, T6	Extruded profiles allow complex sections with high static stiffness
Tube	0.5–25 mm wall	Strength similar to sheet when heat-treated	O, T6	Seamless or welded; wall thickness affects heat-treatment response
Bar/Rod	Diameter 5–200 mm	Autofriction/wear characteristics vary with temper	O, T6	Forged or rolled bars used for high-load fittings and fasteners

Processing route strongly affects microstructure and resultant properties; cast or wrought product forms differ markedly in as-delivered grain size and inclusion population. Sheets and thin extrusions are easier to bring to full solution and quench, giving more consistent T6 properties, while plate and heavy forgings often require special quench fixtures, interrupted quenching, or modified tempers to limit residual stresses and maintain toughness.

Equivalent Grades

Standard	Grade	Region	Notes
AA	771	USA	Designation used in some manufacturer catalogs; aligns with high-strength Al-Zn-Mg-Cu family
EN AW	—	Europe	No exact 1:1 match in EN list; comparable to EN AW-7075/7010 families with adjusted composition
JIS	—	Japan	Similar high-strength Al-Zn-Mg-Cu alloys exist, but direct equivalent requires composition crosswalk
GB/T	—	China	Local equivalents exist in Al–Zn–Mg series; specification differences in impurity limits and tempers

Direct cross-references between national standards are not always exact for a proprietary or less-common designation such as 771. Small differences in allowable impurity content, trace microalloying additions (e.g., Zr vs. Ti), and prescribed tempers can result in measurable differences in SCC susceptibility and fracture toughness. Engineers must compare full chemical and temper specifications rather than relying on a grade label alone when substituting materials across regions.

Corrosion Resistance

In atmospheric environments 771 shows fair resistance when appropriately painted, anodized, or overaged, but its inherent susceptibility to localized corrosion and pitting is higher than aluminum-manganese (3xxx) or aluminum-magnesium (5xxx) families. The presence of copper and high zinc increases the electrochemical activity of the alloy and concentrates galvanic potentials, making protective coatings or cladding common in many applications.

In marine or chloride-rich environments, 771 requires special consideration: localized attack and stress-corrosion cracking (SCC) are primary failure modes, especially in peak-aged tempers. Overaged tempers (T73) and protective surface treatments mitigate SCC risk, but designers often avoid using peak T6 in highly aggressive saltwater exposure unless sacrificial protection or cathodic systems are present.

Galvanic interactions with dissimilar metals are more aggressive for 771 compared with less active aluminum alloys because of its higher open-circuit potential; isolation from stainless steel or copper and careful joint design are necessary. Compared with 6xxx-series alloys (e.g., 6061), 771 offers higher strength but typically worse baseline corrosion resistance and a greater need for protective measures in exposed service.

Fabrication Properties

Weldability

Welding of 771 is challenging in peak tempers because the weld and surrounding heat-affected zone (HAZ) typically experience dissolution of strengthening precipitates and may not regain strength without post-weld heat treatment. Fusion welding methods (TIG/MIG) are possible but require specialized filler alloys and often result in HAZ softening and reduced fatigue life; filler choices aim to balance strength and ductility and commonly involve Al-Mg or Al-Mg-Si based alloys to reduce hot cracking tendency. Resistance to hot cracking is a critical design constraint, and pre- and post-weld treatments plus controlled thermal cycles are often used to minimize residual stresses and strength loss.

Machinability

771 exhibits generally good to very good machinability among high-strength aluminum alloys, often comparable to 7075; it machines cleanly with appropriate tooling and coolant strategies. Carbide tooling is preferred at moderate-to-high cutting speeds with positive rake angles to produce short, controllable chips; feeds should be optimized to avoid chatter and to retain surface finish and fatigue-sensitive surface integrity. Surface finishes and residual compressive stresses introduced during machining strongly affect fatigue performance and should be controlled through process parameters and finishing passes.

Formability

Forming is best performed in low-strength tempers (O or T4) where ductility is highest; severe cold forming in T6 condition is not recommended due to limited elongation and increased risk of cracking. Typical minimum bend radii in T6 are larger than for 5xxx-series alloys, and designers should plan for springback and possible partial annealing operations. For complex shapes, warm-forming or solution treatment followed by controlled quench and forming in a near-T4 state provides a route to produce near-net shapes prior to final aging.

Heat Treatment Behavior

Solution treatment of 771 is performed at temperatures typically in the 470–485 °C range, held long enough to dissolve soluble phases and homogenize the microstructure. Rapid quenching from solution temperature to room temperature or to a cold bath is necessary to retain a supersaturated solid solution; quench rate sensitivity increases with section thickness, and inadequate quenching reduces attainable peak strength.

Artificial aging for T6 is commonly executed at temperatures between 120–160 °C for several hours to produce a fine distribution of η′ precipitates, resulting in peak hardness and yield. Overaging treatments (T73 or T7x) employ higher aging temperatures or longer times to coarsen precipitates and improve resistance to stress-corrosion cracking and dimensional stability at the cost of some tensile strength.

In service or fabrication where heat treatment is not an option, work hardening provides limited strength increases for non-heat-treatable alloys; because 771 is heat-treatable, cold work is generally used for minor shape changes rather than hardening. Full anneal (O) is accomplished by heating above solution temperature followed by controlled cooling to restore ductility and remove residual stresses.

High-Temperature Performance

Elevated temperature exposure leads to decreased strength as precipitates coarsen and dissolve; 771 shows significant loss of yield and tensile strength above ~120–150 °C. For continuous service, maximum recommended temperatures are often limited to ~100 °C to preserve mechanical properties and prevent accelerated overaging.

Oxidation is minimal compared with reactive metals, but surface films and coatings may degrade at elevated temperatures; protective measures and material selection must account for thermal cycling which can change residual stresses and HAZ behavior after welding. Creep resistance is modest; for components subjected to sustained loads at elevated temperature, alternative alloys or design allowances are advisable.

Applications

Industry	Example Component	Why 771 Is Used
Automotive	Lightweight suspension arms, structural reinforcements	High strength-to-weight ratio reduces unsprung mass and improves performance
Marine	High-strength fittings and racing hull components	When coated, offers high strength with acceptable weight for performance craft
Aerospace	Fittings, landing gear components, forgings	High tensile and fatigue strength for primary/secondary structural parts
Electronics	Heat spreaders and stiffeners	Good thermal conductivity combined with structural rigidity
Sporting Goods	High-performance bicycle frames, racquets	Combines stiffness, low mass, and fatigue endurance for competitive equipment

In summary, 771 is selected where high static and fatigue strength per unit mass are decisive and where corrosion and fabrication challenges can be managed by protective measures, special processing, or appropriate temper selection. Its application space sits where weight savings translate directly into performance or efficiency advantages.

Selection Insights

For engineers selecting materials, 771 is a design choice that prioritizes strength-to-weight and fatigue performance at the expense of intrinsic corrosion resistance and ease of joining. Use 771 when structural weight savings and high static strength are primary constraints and when manufacturing can provide controlled heat treatment and surface protection.

Compared with commercially pure aluminum (1100), 771 trades much higher strength for lower electrical conductivity and reduced formability. Compared with work-hardened alloys such as 3003 or 5052, 771 gives a significant increase in yield and fatigue strength but requires more careful corrosion protection and has reduced ductility. Compared with common heat-treatable alloys like 6061 or 6063, 771 delivers higher peak strength for structural applications; choose 771 when the extra strength is required and when the design can accommodate stricter welding and corrosion-control protocols.

Use a conservative approach for welded joints, select overaged tempers for corrosive environments, and validate performance with fatigue and SCC testing for critical components; this balances the material’s strong mechanical attributes with its fabrication and environmental sensitivities.

Closing Summary

Alloy 771 remains relevant where exceptional strength-to-weight and fatigue performance are central to design objectives, provided its limitations in weldability and corrosion resistance are addressed through temper selection, protective systems, and controlled fabrication processes. When properly specified and processed, 771 enables high-performance lightweight structures across aerospace, automotive, marine, and specialty sporting applications.