Aluminum 7051: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

7051 is a member of the 7xxx series of wrought aluminum alloys, placing it in the high-strength, zinc-bearing family of Al-Zn-Mg-Cu systems. The alloy is engineered for high specific strength and performance in applications where strength-to-weight ratio and fatigue resistance are primary drivers rather than maximal corrosion immunity or joining ease.

Major alloying elements in 7051 are zinc and magnesium with controlled additions of copper and trace levels of chromium and titanium for grain and recrystallization control. The alloy is heat-treatable: its peak properties are achieved through solution treatment, rapid quenching, and controlled artificial aging to precipitate fine metastable Mg-Zn phases that provide precipitation strengthening.

Key traits include very high tensile and yield strengths relative to common structural aluminum grades, moderate-to-poor general corrosion resistance compared to 5xxx/6xxx families unless overaged, and limited weldability and formability in peak tempers. Typical industries using 7051 are aerospace and high-performance structures in defense, motorsport, and select high-end transportation sectors that require optimized strength, stiffness and fatigue performance.

Engineers choose 7051 over other alloys when component-level mass reduction and sustained high-stress capability are required and when manufacturing routes can accommodate heat treatment and specialized corrosion mitigation. The alloy is preferred where higher-strength 7xxx variants (e.g., 7075) do not meet specific temper stability or where 7051’s particular mix of Zn/Mg/Cu and microstructural control yields better crack-arresting fatigue behavior.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed, maximum ductility
T6	High	Low–Moderate	Fair–Poor	Poor	Solution-treated and artificially aged for peak strength
T651	High	Low–Moderate	Fair–Poor	Poor	T6 + stress relieved by stretching to reduce residual stresses
T73 / T76 / T7x	Moderate	Moderate	Fair	Poor–Moderate	Overaged tempers for improved SCC and corrosion resistance
Hxx (e.g., H111, H116)	Variable	Variable	Variable	Moderate	Strain hardened or strain hardened + partial anneal for intermediate properties

Heat treatment and tempering dictate whether 7051 is tailored for maximum strength (T6/T651) or for higher resistance to stress-corrosion cracking and improved toughness (T7x/T73/T76). The O temper is used for forming operations and secondary processing before final heat treatment, while overaged tempers sacrifice some peak strength to obtain significantly improved SCC resistance and stability during thermal excursions.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.40	Impurity from casting/processing; kept low to avoid brittle phases
Fe	≤ 0.50	Impurity; high levels reduce toughness and fatigue life
Mn	≤ 0.10	Typically low in 7xxx series; limited impact on strength
Mg	2.0–3.0	Combined with Zn to form strengthening precipitates
Cu	1.2–2.0	Increases strength and affects corrosion susceptibility
Zn	6.0–8.0	Principal strengthening element in the 7xxx family
Cr	0.04–0.35	Controls grain structure and helps resist recrystallization
Ti	≤ 0.15	Grain refiner for castings and certain wrought products
Others (each)	≤ 0.05	Trace impurities; balance to Al

Zinc and magnesium are the dominant strengthening pair, forming metastable eta (MgZn2) phases on aging that pin dislocations and increase yield and tensile strength. Copper raises strength further but tends to reduce corrosion resistance and raise susceptibility to localized attack and stress corrosion cracking; low levels of chromium and titanium help control grain structure and improve toughness during thermomechanical processing.

Mechanical Properties

7051 exhibits a strong dependence of tensile behavior upon temper. In solution-treated and artificially aged tempers (T6/T651) the alloy reaches very high tensile and yield strengths with attendant reductions in elongation; overaged tempers trade peak strength for improved toughness and SCC resistance. In annealed condition the alloy is ductile and readily formed but will not approach the high-strength levels required for many aerospace structural parts until thermally processed.

Yield strength, ultimate tensile strength, and elongation are thickness- and temper-dependent; thinner product forms generally attain higher strengths after aging due to faster quench rates and finer precipitate distributions. Hardness correlates with aging response: peak-aged tempers show elevated hardness values which fall under overaging or in the heat affected zone after welding. Fatigue performance is generally excellent for controlled microstructures but is sensitive to surface finish, residual stress, and local corrosion; proper treatment and design detail control are essential to maximize fatigue life.

Property	O/Annealed	Key Temper (T6 / T651)	Notes
Tensile Strength	200–300 MPa (typical)	480–600 MPa (typical range)	Strength varies with thickness, quench rate, and exact temper
Yield Strength	60–140 MPa (typical)	430–540 MPa (typical range)	Yield rises sharply with precipitation hardening
Elongation	15–30%	6–12%	Ductility reduced significantly in peak-aged tempers
Hardness	30–55 HRB	85–120 HRB (approx.)	Hardness tracks aging; overaging reduces hardness but improves SCC resistance

Physical Properties

Property	Value	Notes
Density	~2.80–2.82 g/cm³	Typical for high-strength Al-Zn-Mg-Cu alloys
Melting Range	Solidus ~480–500 °C, Liquidus ~635–650 °C	Alloy has a broad melting interval due to alloying elements
Thermal Conductivity	~120–150 W/m·K (at 20 °C, typical)	Lower than pure Al, influenced by alloying and temper
Electrical Conductivity	~30–40 % IACS (typical)	Reduced versus commercial-purity aluminum due to solute atoms
Specific Heat	~0.88–0.92 J/g·K	Similar to other wrought aluminum alloys
Thermal Expansion	~23–25 ×10^-6 /°C	Coefficient similar to other aluminum alloys, important for thermal design

7051’s physical properties mean it retains many of aluminum’s advantages—low density and good thermal conductivity—while trading some conductivity and thermal transport for increased mechanical performance. The alloy’s melting and solidification behavior requires care in processes involving elevated temperatures, including brazing and welding, to avoid unwanted microstructural changes.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.5–6 mm	High when peak aged; behavior depends on quench	O, T6, T651, T73	Common for structural skin and higher-strength panels
Plate	6–150+ mm	Strength can be reduced in thicker sections due to quench sensitivity	T651, T73, T76	Thick plates require tight quench control and often overaged tempers
Extrusion	Profiles vary	Strength influenced by section thickness and cooling rate	T6, T651	Complex cross-sections possible but quench control is critical
Tube	Pipes and aircraft tubing sizes	Similar trends with wall thickness; fatigue-sensitive	T6/T651	Used where high axial or hoop strength is needed
Bar/Rod	Diameters up to large forgings	Strength scalable with section size and temper	O, T6	Bars used for fittings, bolts, and forgings following solution/aging

Thin-gauge products achieve higher peak strengths due to faster effective quench rates while heavy sections are more subject to softening from slower cooling and coarser precipitate distributions. Processing routes differ: sheet and plate are commonly rolled and solution-treated in controlled furnaces, while extrusions require die design mindful of quench paths. Component selection should account for achievable temper state at intended section thickness.

Equivalent Grades

Standard	Grade	Region	Notes
AA	7051	USA	Designation in Aluminum Association; wrought alloy
EN AW	No direct one-to-one	Europe	No precise EN equivalent; 7075/7050 family members are closest
JIS	No direct equivalent	Japan	Common practice is to reference nearest 7xxx family grade
GB/T	No direct equivalent	China	Chinese standards contain high-strength Zn-Mg-Cu alloys but names differ

7051 does not always have a strict one-to-one designation in every national standard and may be represented by proprietary or closely related alloys in different regions. Engineers must confirm chemistry, temper definition, and mechanical property requirements when substituting from 7075, 7050, or other 7xxx family grades, since minor compositional differences and specification tolerances produce significant changes in SCC resistance and aging response.

Corrosion Resistance

In atmospheric environments 7051 shows moderate resistance that depends strongly on temper and post-treatment. Peak-strength tempers with higher copper content are more susceptible to localized corrosion and pitting than overaged tempers, while properly applied sealants and anodizing treatments can materially improve surface durability.

In marine and chloride-bearing environments 7051 requires protective measures because high-strength 7xxx alloys are prone to intergranular attack and stress corrosion cracking under tensile stress. Overaging to T7x tempers and the use of post-anodize sealing or coatings reduce susceptibility and are common mitigation strategies for marine applications.

Stress corrosion cracking is a principal design consideration: susceptibility correlates with tensile residual stress, local metallurgical condition, and exposure to aggressive environments. Galvanic interactions are significant—7051 is anodic to stainless steel but cathodic to common steels and copper alloys; design should avoid direct coupling or use insulating barriers and coatings. Compared with 5xxx and 6xxx families, 7051 delivers much higher strength at the expense of native corrosion resistance and SCC susceptibility unless specifically engineered to counteract those weaknesses.

Fabrication Properties

Weldability

Welding 7051 is challenging in peak-strength tempers because heat input dissolves strengthening precipitates and creates a softened heat-affected zone, significantly reducing local strength. Fusion welding (TIG/MIG) typically produces HAZ softening and high residual stresses; filler alloys engineered for 7xxx series are available but welds often require post-weld heat treatment or mechanical design compensation. Solid-state joining methods such as friction stir welding are frequently preferred because they limit peak temperatures, control microstructure, and provide superior joint properties relative to fusion welding for this alloy family.

Machinability

7051 is generally machinable with care; it machines similarly to other high-strength 7xxx alloys with moderate tool wear and tendency for built-up edge if feeds or speeds are incorrect. Carbide tooling with positive rake and high rigidity is recommended, and conservative cutting speeds relative to softer aluminum grades reduce thermal work-hardening and promote continuous chips. Surface finish and residual stress management are critical when parts are fatigue- or SCC-critical, so finishing passes and stress-relief operations are common.

Formability

In the annealed O condition 7051 is formable and can be deep-drawn or bent to moderate radii, but once heat-treated to high-strength tempers formability degrades and springback increases. Bending radii should be kept conservative in T6/T651 tempers and forming operations are normally done in O or partially annealed states prior to final solution treatment and aging. For complex shapes, warm forming or superplastic processes combined with appropriate pre- and post-heat treatments can be used, but these add process complexity and cost.

Heat Treatment Behavior

7051 is a heat-treatable alloy that follows the classical solution-treatment, quench, and aging path to achieve peak properties. Typical solution treatment temperatures are around 470–480 °C with rapid quenching to retain a supersaturated solid solution; quench sensitivity increases with section thickness, so heavy sections may require special quench media or interrupted quenching strategies.

Artificial aging for peak-strength T6-like conditions generally uses temperatures in the 120–180 °C range for several hours to precipitate fine Mg-Zn phases. Overaging (T7x/T73/T76 series) uses higher aging temperatures or longer times to coarsen precipitates and improve resistance to stress corrosion cracking and thermal stability, at the cost of some tensile strength. T651 designation indicates stress relief by stretching after quench; it is commonly specified for aerospace applications where dimensional stability and reduced residual stress are important.

High-Temperature Performance

7051 exhibits significant strength loss with elevated temperature; above roughy 120–150 °C the precipitation hardening microstructure begins to overage and mechanical properties drop. Continuous exposure to elevated temperatures accelerates coarsening of strengthening phases and reduces yield and fatigue strengths, so typical maximum service temperatures are kept below ~120 °C for long-term loading.

Oxidation is minimal compared with high-temperature alloys such as steels or titanium, but surface degradation and color changes occur at higher temperatures. The heat-affected zone from welding or high-temperature exposure will show weakened properties unless post-thermal processing or overaging steps are applied, so thermal exposure during fabrication must be controlled to preserve design-level performance.

Applications

Industry	Example Component	Why 7051 Is Used
Aerospace	Fuselage fittings, high-strength brackets	High strength-to-weight, good fatigue performance
Defense	Missile and ballistic structures	Reduced mass with maintained structural capacity
Motorsport / Automotive Performance	Chassis members, structural components	High specific strength for weight reduction in critical parts
Marine	High-performance structural elements	When combined with T7x tempers and coatings for SCC resistance
Electronics / Thermal Management	Structural frames where stiffness is critical	Good thermal conductivity relative to steels with lower density

7051 is most commonly employed where component mass must be minimized while meeting demanding static and fatigue loads, and where the production environment can deliver controlled heat treatment and corrosion protection. The alloy’s use is concentrated in sectors that accept higher material and processing costs in exchange for performance gains.

Selection Insights

For engineers evaluating 7051, prioritize it when high yield and tensile strength coupled with good fatigue resistance are decisive, and when processing routes (heat treatment, quench, coatings) are well controlled. Use overaged T7x tempers when stress-corrosion cracking or marine exposure is a concern and when some strength can be sacrificed for durability.

Compared with commercially pure aluminum (1100), 7051 sacrifices electrical and thermal conductivity and formability for a large increase in strength and stiffness. Compared with work-hardened alloys such as 3003 or 5052, 7051 provides far higher strength but typically lower corrosion resistance and far poorer cold-forming capability in peak tempers. Compared with common heat-treatable structural alloys like 6061, 7051 will typically deliver higher ultimate and yield strength for structural applications, but at the cost of greater SCC susceptibility and more stringent heat-treatment and joining constraints; choose 7051 when maximum strength and fatigue performance outweigh those trade-offs.

Closing Summary

7051 remains relevant for modern engineering where exceptional strength-to-weight, fatigue capability, and tailored temper stability are required and where fabrication processes can accommodate precise thermal schedules and corrosion mitigation. Its role in aerospace and high-performance structural applications underscores the continued value of carefully engineered 7xxx-series chemistries for demanding structural designs.