Aluminum AlZnMgCu: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

AlZnMgCu alloys belong to the 7xxx series of aluminum alloys, where zinc is the principal alloying element and magnesium and copper are significant secondary elements. These alloys are heat-treatable and derive their strength primarily from precipitation hardening through solution treating, quenching, and artificial aging sequences. Typical high-strength members of this family, such as AA 7075, offer some of the highest strength-to-weight ratios available among wrought aluminum alloys, while trading off absolute corrosion resistance and ease of welding compared with lower-strength families. They are widely used in aerospace, defense, high-performance sporting goods, and certain automotive structural applications where high static or fatigue strength is a critical design driver.

Major alloying elements in Al–Zn–Mg–Cu systems are zinc (promotes age-hardening precipitates), magnesium (forms strengthening precipitates with zinc), and copper (increases strength but can reduce corrosion resistance and increase SCC susceptibility). Minor additions such as chromium and zirconium are used for grain refinement and recrystallization control to retain strength in thermomechanically processed product forms. These alloys are chosen over 6xxx or 5xxx series when peak strength and fracture toughness per unit mass are prioritized, and over stainless steels when weight reduction with comparable stiffness and fatigue performance is desired. Selection is frequently governed by a trade-off between mechanical performance (strength, stiffness, fatigue) and the need for corrosion mitigation measures such as coatings, cladding, or overaging.

Manufacturing considerations strongly influence the choice of a specific AlZnMgCu grade and temper. Heat treatment capability, availability in product forms (plate, sheet, extruded profiles), and the ability to perform post-weld or post-forming treatments determine whether a given component can realize the alloy’s full potential. The combination of high strength, reasonable machinability, and adaptability to standard aluminum joining and finishing processes makes AlZnMgCu alloys a practical choice for engineered structures where mass efficiency is critical.

Designers must also account for environmental and lifecycle constraints when specifying AlZnMgCu. Corrosion protection strategies, susceptibility to stress corrosion cracking (SCC) under tensile stresses and specific tempers, and the sensitivity of properties to thickness and thermal history are all factors that influence material selection, fabrication route, and in-service maintenance plans. The net result is a high-performing alloy family that is indispensable where a premium weight-sensitive design objective exists and where adequate mitigation strategies for corrosion and weldability are implemented.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed condition; maximum ductility and formability
T4	Low–Medium	Medium	Good	Good	Solution treated and naturally aged; intermediate condition
T6	High	Low–Medium	Fair	Poor–Fair	Solution treated and artificially aged for peak strength
T73 / T76	Medium–High	Medium	Improved	Better than T6	Overaged tempers for improved SCC resistance and toughness
T651	High	Low–Medium	Fair	Poor–Fair	T6 with stress-relief by stretching or compressive treatment
H112 / H116	Variable	Variable	Variable	Variable	Commercially controlled tempers for partial control of properties
H14	Medium	Low	Fair	Poor–Fair	Strain-hardened and partially annealed; used for extrusions and sheets

Temper state has a dominant effect on mechanical performance, corrosion resistance, and forming behavior of AlZnMgCu alloys. Peak-aging tempers such as T6 maximize tensile and yield strengths but significantly reduce ductility and make forming and welding more challenging without post-process restoration.

Overaging to T73/T76 reduces the driving force for stress corrosion cracking and improves resistance to exfoliation and intergranular attack at the expense of a measurable drop in yield and ultimate tensile strengths. Selection of temper is therefore a balance between required peak strength and environmental durability.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.40	Impurity; promotes fluidity in casting and may form intermetallics that reduce toughness
Fe	≤ 0.50	Impurity; forms brittle intermetallics that can reduce ductility and corrosion resistance
Mn	≤ 0.30	Minor grain structure modifier; limited in 7xxx to avoid detrimental intermetallics
Mg	1.5 – 3.0	Strength contributor; forms MgZn2 precipitates with Zn during aging
Cu	0.5 – 2.5	Increases strength and toughness, but raises susceptibility to corrosion and SCC
Zn	3.5 – 8.0	Primary strengthening element; higher Zn increases peak strength via precipitation
Cr	0.04 – 0.35	Controls recrystallization and grain structure, improves toughness and resistance to grain growth
Ti	≤ 0.20	Grain refiner during solidification and thermomechanical processing
Others	Balance Al + trace	Trace additions and residuals (e.g., Zr) used for grain control and texture modification

The alloy chemistry of Al–Zn–Mg–Cu is optimized to promote the formation of fine GP zones and MgZn2 (η′/η) precipitates during aging, which are the primary hardening features. Copper modifies the precipitation sequence and contributes to increased peak strength and toughness, but it also changes electrochemical behavior and increases the potential for localized corrosion and SCC unless mitigated by temper selection or cladding.

Trace elements such as chromium and zirconium are deliberate microalloying additions used to pin grain boundaries and control recrystallization during hot working and thermal cycles. Control of impurities such as iron and silicon is essential because their intermetallic phases are sites for crack initiation and localized corrosion in high-strength tempers.

Mechanical Properties

AlZnMgCu alloys show a wide envelope of mechanical behavior depending on temper, product form, and thickness. In peak-aged T6 temper, these alloys exhibit high ultimate tensile strengths and correspondingly high yield strengths, with ductility typically in the single-digit to low double-digit percentage range. In annealed or solution-treated tempers the same alloy will present much higher elongation and lower yield, enabling forming operations that would be impossible in the T6 condition.

Fatigue behavior is generally excellent for AlZnMgCu when manufactured with controlled grain structure and minimal surface defects, making it favorable for cyclic loading applications. However, fatigue and fracture toughness are sensitive to residual tensile stresses and microstructural heterogeneities; overaging (T73/T76) can improve fatigue crack growth resistance at the expense of reduced static strength. Thickness effects are pronounced: thicker sections often show reduced strength due to slower quench rates and coarser precipitate distributions, necessitating processing controls such as quench-inhibiting measures or tailored aging cycles.

Hardness follows tensile behavior, with peak-aged tempers exhibiting substantially higher hardness values than annealed or naturally aged states. Heat input from welding or localized high-temperature operations will produce soft zones (HAZ) through dissolution or coarsening of strengthening precipitates, which reduces local yield and fatigue strength and often necessitates post-weld heat treatment or design accommodations.

Property	O/Annealed	Key Temper (e.g., T6/T651)	Notes
Tensile Strength	250 – 350 MPa	480 – 620 MPa	Wide range depends on alloy variant and thickness; T6 near peak values
Yield Strength	120 – 300 MPa	410 – 540 MPa	Yield increases markedly with aging; yield/UTS ratio varies with temper
Elongation	12 – 20%	5 – 15%	Ductility reduced in peak-aging; forming is easier in O/T4 states
Hardness	60 – 95 HB	135 – 165 HB	Hardness corresponds to precipitate density and temper; measured values depend on scale

Physical Properties

Property	Value	Notes
Density	2.78 – 2.82 g/cm³	Slightly lower than steel; excellent mass efficiency for structural parts
Melting Range	~480 – 635 °C	Solidus−liquidus interval depends on zinc and copper content; avoid service near eutectic melting
Thermal Conductivity	120 – 160 W/m·K	Lower than pure Al but still high compared with steels; thermal path design benefits
Electrical Conductivity	20 – 35 % IACS	Reduced relative to pure Al due to alloying; thickness and temper have small effect
Specific Heat	~870 – 910 J/kg·K	Approximate value near room temperature for design thermal mass
Thermal Expansion	23–24 µm/m·K (20–100 °C)	Typical aluminum expansion; important for multi-material joint design

AlZnMgCu alloys retain many of the favorable physical characteristics of aluminum, notably low density and relatively high thermal conductivity compared with ferrous materials. These properties make them attractive in applications requiring thermal management and lightweight structural members, but designers must account for lower electrical conductivity relative to pure aluminum when specifying for electrical applications.

Thermal stability and melting characteristics set practical limits for thermal exposure during processing and service. The precipitation-strengthened microstructure is sensitive to temperature: prolonged exposure above approximately one-third to one-half of the melting temperature (by absolute scale) will produce softening and loss of mechanical integrity, which is particularly relevant for welding, brazing, and high-temperature service.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.4 – 6.0 mm	Good through-thickness control needed for thick sheets	T6, T651, T73	Common for structural skins and panels; quench sensitivity affects thick sections
Plate	6 – 200 mm	Strength can drop with thickness due to slower cooling	T6, T651, T73	Heavy plates require controlled quench and tempering to retain properties
Extrusion	Complex profiles, various wall thicknesses	Extruded microstructure benefits from post-extrusion aging	T6, T73, H112	Used for high-strength structural profiles and fittings
Tube	thin to thick-walled	Welding and forming affect local properties; pressure capability high in T6	T6, T73	Heat exchangers and structural tubing where strength-to-weight is critical
Bar/Rod	diameters up to several hundred mm	Machinability good; large sections require thermal processing	T6, O, T73	Used for forgings, machined components, and aerospace fittings

Processing routes differ across product forms: sheets and plates are usually solution-treated on a production scale and then quenched and aged, while extrusions require careful control of quench rates and sometimes direct aging to achieve desired combinations of properties. Plate thickness and quenchability are critical design inputs; when maximum temper uniformity is required, thinner sections or post-process homogenization may be specified.

Applications influence chosen product form and temper; for example, aerospace structural skins use rolled sheet in T6/T651 with cladding or corrosion protection, whereas marine structural members often use overaged tempers and surface treatments. Machining allowances and distortion controls are also informed by product form and temper selection.

Equivalent Grades

Standard	Grade	Region	Notes
AA	7075 / AlZnMgCu	USA	7075 is the most common commercial representative of high-strength Al–Zn–Mg–Cu alloys
EN AW	7075	Europe	EN AW-7075 conforms to European alloy numbering; similar chemistries and tempers
JIS	A7075	Japan	Japanese designation for the 7075 family with region-specific processing tolerances
GB/T	7075	China	Chinese standard covers 7075 equivalents and heat-treatment specifications

Subtle differences between standards arise from tolerances on impurity elements, precise composition windows, and permitted mechanical property ranges in each temper. For critical aerospace or safety-of-life components, procurement specs will reference a particular standard and temper with specified testing and certification requirements to ensure interchangeability and reproducible performance.

Regional heat-treatment practices and allowed tempers (e.g., the designation of T651 vs T6511 vs T73) can impose differences in residual stress control and elongation targets even for alloys with nominally identical chemistries. Always cross-reference drawing callouts with supplier mill certifications and test reports.

Corrosion Resistance

AlZnMgCu alloys exhibit moderate general corrosion resistance in atmospheric environments, but they are more susceptible to localized corrosion (pitting and exfoliation) and stress corrosion cracking compared with 5xxx and some 6xxx series alloys. The presence of copper and a high Zn:Mg ratio tend to increase electrochemical heterogeneity and drive localized attack when exposed to chloride-bearing environments, making protective coatings, cladding (Alclad), or sacrificial anodic measures common in marine and coastal applications. Overaging to T73/T76 or cladding with high-purity aluminum layers substantially improves resistance to exfoliation and SCC, though this reduces achievable peak strength.

Marine exposure demands careful mitigation: in seawater and splash zones, unprotected high-strength AlZnMgCu alloys can lose performance rapidly due to pitting and SCC if tensile stresses are present. Design approaches include using sacrificial coatings, cathodic protection, selecting overaged tempers, and avoiding crevice-forming geometries. Fasteners and assemblies are typically isolated from dissimilar metals or use corrosion-resistant hardware to prevent galvanic acceleration of aluminum attack.

Stress corrosion cracking is a notable failure mode for high-strength tempers under sustained tensile stress in corrosive chloride environments. The susceptibility can be reduced by lowering yield strength (overaging), applying compressive surface stresses (peening), or altering alloy chemistry. Compared to 6xxx alloys (e.g., 6061), 7xxx alloys have higher strength but require more rigorous environmental control and design considerations to avoid SCC-related failures.

Fabrication Properties

Weldability

Welding of AlZnMgCu alloys is challenging in peak-aged tempers because the heat input dissolves or coarsens strengthening precipitates, producing a softened heat-affected zone with significantly reduced yield and fatigue strength. Fusion welding methods such as TIG and MIG are feasible for repair and fabrication, but the weld metal and HAZ will generally be considerably weaker than the parent T6 material unless post-weld solution treatment and aging are performed, which is often impractical for assembled structures. Filler alloys such as 5356 or 4043 are commonly used; however, 5356 (Al–Mg) offers better strength, and specially formulated fillers for 7xxx may be applied to minimize galvanic and strength mismatches. Hot-cracking is a risk when welding higher-zinc alloys, so pre-weld preparation, joint design, and controlled heat input are critical.

Machinability

Machinability of AlZnMgCu is generally good relative to steels, with predictable chip formation and low cutting temperatures, but the high strength and hardness of aged tempers increase tool wear compared with softer aluminum alloys. Carbide tooling with sharp geometry and positive rake angles is preferred to manage chip evacuation and reduce built-up edge; cutting speeds are higher than for steels but must be limited to avoid excessive surface temperatures that alter local temper. For tight-tolerance aerospace components, allowance for stress relief and distortion control during machining and subsequent finishing is essential to maintain dimensional stability and mechanical performance.

Formability

Cold forming is most effective in O, T4, or partially annealed tempers where ductility is sufficient for bending and deep drawing operations, while T6 and H14 tempers are less formable and more prone to cracking during severe bends. Minimum bend radii are governed by temper and thickness; a conservative rule for T6 sheet is to use a minimum inside bend radius of 1–2× thickness, while softer tempers may permit radii closer to or below 1× thickness depending on tooling and blank holding. When complex shapes are required, form in softer tempers followed by controlled heat treatment to restore strength, or design with hemming and incremental forming to avoid workpiece failures.

Heat Treatment Behavior

AlZnMgCu alloys are classical heat-treatable alloys; primary thermal processing steps include solution treatment, quenching, and artificial aging. Solution treatment is typically performed near 470–480 °C to dissolve soluble Zn and Mg into a supersaturated solid solution, followed by rapid quenching (water or polymer quench) to retain that supersaturation. Artificial aging (T6) is performed at temperatures around 120–160 °C for established durations to precipitate fine η′ and GP-zone-type precipitates that provide peak hardness and strength.

Natural aging (T4) can provide intermediate strength and is the starting point for some fabrication sequences, while overaging treatments (T7/T73/T76) intentionally coarsen precipitates to reduce susceptibility to stress corrosion cracking and improve fracture toughness and dimensional stability. T651 designation indicates a T6 temper with controlled stress relief (stretching or compressive treatment) after quenching, which mitigates distortion for precision components. Process control of quench rate is critical; thick sections that quench slowly may not achieve the same supersaturation and therefore display lower achievable strength.

Non-heat-treatable behavior in H-tempers involves work hardening through strain; however, the high-strength AlZnMgCu family is usually engineered around heat-treatment rather than strain hardening for strength development. Annealing restores ductility through recrystallization and dissolution of strengthening phases, enabling forming operations prior to re-aging.

High-Temperature Performance

The elevated-temperature strength of AlZnMgCu alloys declines rapidly with increasing temperature; significant strength loss is observed at temperatures above ~100 °C, and utility for load-bearing applications above ~150 °C is generally limited. Precipitates responsible for strengthening coarsen or dissolve at elevated temperatures, leading to softening and reduced yield and fatigue resistance. Oxidation at moderate temperatures is minimal for aluminum relative to steels, but protective oxide films do not prevent microstructural coarsening.

For welded components, the HAZ behavior at elevated temperatures is a principal concern: local overaging or dissolution of strengthening precipitates causes persistent soft zones that can govern failure in high-temperature cycling or creep-prone applications. Design for elevated-temperature exposure requires selection of more thermally stable alloys or incorporation of thermal barriers and frequent inspection intervals.

Applications

Industry	Example Component	Why AlZnMgCu Is Used
Aerospace	Wing skins, fuselage frames, fittings	Exceptional strength-to-weight and fracture toughness in structural members
Marine	High-strength hull fittings, masts	High static strength with corrosion mitigation measures; weight savings critical
Automotive	High-performance chassis components, suspension parts	Weight reduction and stiffness where mass efficiency improves dynamics
Defense	Armor housings, missile bodies	High-strength, lightweight structural solutions for payload efficiency
Sports & Recreation	High-end bicycle frames, climbing equipment	High specific strength and fatigue resistance for performance gear

AlZnMgCu alloys remain the material of choice where the design objective emphasizes maximum structural efficiency per unit mass and where controlled fabrication and corrosion mitigation practices can be employed. The alloy family supports critical components across industries where both static and fatigue loading dominate design requirements.

Selection Insights

When choosing AlZnMgCu for a component, prioritize it when strength-to-weight and fatigue resistance are primary requirements and when fabrication can accommodate heat treatment and corrosion protection. If maximum ductility, conductivity, and simple welding are priorities, commercially pure aluminum (e.g., 1100) will outperform AlZnMgCu in those specific metrics at the expense of structural capacity.

Compared with work-hardened alloys like 3003 or 5052, AlZnMgCu offers much higher static and fatigue strength but typically requires more robust corrosion protection and has reduced formability in peak tempers. Compared with common heat-treatable alloys such as 6061 or 6063, AlZnMgCu usually provides higher peak strength and often better fatigue performance, but it can be more expensive, less weldable without post-weld treatment, and more susceptible to stress corrosion without overaging or protective measures.

Use AlZnMgCu when design life under cyclic loading, stiffness-per-mass, and minimized part weight outweigh increased finishing and corrosion-control costs. Select overaged or clad variants for aggressive environments, and reserve peak-aged tempers for components where in-service corrosive exposure is limited or well controlled.

Closing Summary

AlZnMgCu alloys combine some of the highest strength and favorable fatigue characteristics available among wrought aluminum alloys, making them indispensable for weight-critical, high-performance engineering applications. Responsible use requires attention to temper selection, corrosion mitigation, and fabrication controls to realize the alloy’s performance without compromising durability in service.