Aluminum 7150: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

7150 is a 7xxx-series aluminum alloy belonging to the high-strength Al‑Zn‑Mg‑Cu family used extensively in aerospace-grade structural applications. Its chemistry centers on zinc as the principal alloying element with significant contributions from magnesium and copper, and small additions of zirconium for grain structure control and recrystallization resistance.

The alloy is heat-treatable and primarily strengthened by solution heat treatment followed by quench and artificial aging to produce a dense dispersion of metastable eta (η′) and related precipitates. This precipitation-hardening mechanism produces very high yield and tensile strength compared with 1xxx–6xxx series alloys while retaining reasonable toughness when processed for fracture resistance.

Key traits of 7150 include very high strength-to-weight ratio, good fatigue crack growth resistance when appropriately overaged or thermomechanically treated, and moderate corrosion resistance that can be improved by overaging and cladding. Weldability and formability are limited in peak-aged tempers, so design and processing choices often trade formability for strength and fracture performance.

Typical industries include aerospace primary and secondary structures, high-performance defense components, and select high-strength industrial applications where weight saving and damage tolerance are critical. Engineers choose 7150 where combinations of high static strength, fatigue performance, and acceptable toughness outweigh decreased weldability and higher material cost compared with more common alloys.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High (20–30%)	Excellent	Fair	Fully annealed for maximum ductility and formability; rarely used in structural forgings
T6	Very High	Low–Moderate (8–12%)	Limited	Poor	Peak-aged for maximum strength; common for structural parts where forming is done prior to aging
T651	Very High	Low–Moderate (8–12%)	Limited	Poor	T6 plus stress relief by stretching; used for precision components to reduce residual stresses
T73	High	Moderate (10–14%)	Limited	Poor–Fair	Overaged condition to improve resistance to stress-corrosion cracking (SCC) at expense of peak strength
T76 / T7451 / T7751	Moderate–High	Moderate (10–15%)	Limited	Poor–Fair	Designed to balance SCC resistance, fracture toughness, and residual stress control for critical airframe uses

Temper dramatically shifts 7150’s balance of strength, toughness, and corrosion resistance. Peak-aged T6/T651 yields the maximum static strengths but increases sensitivity to stress-corrosion cracking and reduces ductility, whereas overaged tempers like T73 trade some strength for markedly improved SCC resistance and often slightly higher ductility.

Manufacturing sequence and intended service dictate temper selection: form major shapes in O or cold-formed tempers and then solution treat and age where possible, or select overaged tempers for components exposed to corrosive environments or requiring higher fracture toughness.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.12	Controlled low silicon to reduce intermetallics and maintain fracture toughness
Fe	≤ 0.12	Impurity limit; elevated Fe can form brittle intermetallics and reduce toughness
Mn	≤ 0.05	Minimal; not a principal strengthening contributor in this alloy
Mg	2.3–2.9	Primary precipitate former with Zn to create η′ precipitates for high strength
Cu	2.3–3.1	Raises strength and hardness; improves fatigue but can increase SCC susceptibility
Zn	6.3–7.5	Principal alloying element driving peak strength through η/η′ precipitates
Cr	≤ 0.04	Trace control; sometimes present to modify grain boundary behavior
Ti	≤ 0.08	Deoxidizer and grain refiner in cast/ingot processing
Others (Zr, V, etc.)	Zr 0.08–0.20; remainder trace	Zr additions are deliberate to form dispersoids that control recrystallization and improve grain structure and toughness

Each element plays a precise role: Zn and Mg combine to form the η′ precipitates responsible for the alloy’s high strength; Cu modifies precipitate composition and kinetics improving strength and fatigue resistance but can increase SCC risk; Zr and trace elements control grain size and recrystallization during thermomechanical processing and solution/quench steps, improving damage tolerance and enabling thicker sections to retain desirable properties.

Mechanical Properties

7150 exhibits very high tensile and yield strengths in appropriately aged tempers, combined with good fracture toughness and fatigue-crack-growth resistance when processed to minimize coarse grain-boundary precipitates. Yield behavior is typically linear-elastic to yield with limited yield plateau; the alloy demonstrates reasonable strain hardening up to fracture but lower uniform elongation in peak-aged tempers.

Elongation to failure is markedly dependent on temper and product form; annealed or overaged tempers offer improved ductility, while peak-aged plates and forgings have lower elongation and can be susceptible to brittle fracture under high constraint. Hardness follows tensile trends and is commonly used as a shop control for temper verification; hardness distribution across thick sections can indicate quench efficacy.

Thickness and quench sensitivity strongly affect mechanical gradients: plates and thick extrusions are more likely to show reduced properties at mid-thickness due to slower quench rates, unless grain‑refining and Zr dispersoids are optimized. Fatigue performance benefits from fine, uniform precipitates and controlled residual stresses produced by T651/T7451-type tempers.

Property	O/Annealed	Key Temper (T6 / T651)	Notes
Tensile Strength	170–260 MPa	540–590 MPa	T6/T651 values are typical for well-processed wrought products; values decline with section thickness and overaging
Yield Strength	60–130 MPa	480–520 MPa	High yield strengths make 7150 suitable for highly stressed structural components
Elongation	20–30%	8–12%	Elongation reduces significantly in peak-aged tempers; overaging raises ductility modestly
Hardness (HB)	40–80 HB	150–175 HB	Hardness correlates with strength and is useful for incoming inspection and heat-treatment verification

Physical Properties

Property	Value	Notes
Density	2.81 g/cm³	Typical of high-strength Al‑Zn‑Mg‑Cu alloys; benefits weight-sensitive designs
Melting Range	Solidus ≈ 477 °C; Liquidus ≈ 635 °C	Alloying widens melting interval relative to pure aluminum
Thermal Conductivity	≈ 120–150 W/m·K	Reduced vs pure Al due to alloying; adequate for many structural applications but not optimal for high-performance heat sinking
Electrical Conductivity	≈ 30–40 % IACS	Alloying significantly lowers conductivity compared with pure Al
Specific Heat	≈ 0.88–0.92 J/g·K (880–920 J/kg·K)	Typical aluminium-range specific heat useful for thermal mass calculations
Thermal Expansion	≈ 23.0–24.0 ×10⁻⁶ /K	Similar to other wrought Al alloys; important for joint design with dissimilar materials

The physical properties reflect the alloy’s service envelope: relatively low density provides excellent specific strength, but alloying reduces thermal and electrical conductivities compared with pure aluminum and some 5xxx/6xxx series alloys. Thermal expansion must be considered in heterogeneous assemblies because differential expansion can drive fatigue and stress concentrations.

Thermal properties and melting range control heat-treatment cycles and dictate quench media and tooling temperatures; thermal conductivity also affects localized heating during machining and welding operations.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.5–6.0 mm	Susceptible to local softening if not aged properly	T6, T651, T73	Common for aerospace skins and stiffened panels; forming usually done prior to final age
Plate	>6 mm up to 150 mm	Quench sensitivity increases with thickness; mid-thickness soft zone possible	T6, T651, T73	Thick plates require controlled processing and Zr-containing alloys to retain properties
Extrusion	Cross-sections up to large profiles	Properties can vary with section thickness and quench path	T6, T651, T76	Extrusions benefit from rapid cooling and Zr dispersoids for property uniformity
Tube	Ø few mm to large diameters	Wall thickness controls quench and mechanical gradients	T6, T73	Used for aerospace tubing and structural frames with strict quality control
Bar/Rod	Diameter / cross-section dependent	Forging and rolling histories influence strength/toughness	T6, T651	Bars for highly stressed fittings and machined components; preheat and quench practices critical

Form affects not only available dimensions but also achievable properties due to quench kinetics and thermomechanical history. Sheet and thin extrusions more readily achieve target T6 strengths, whereas plates and thick forgings require tailored thermomechanical processing and dispersoid control (e.g., Zr) to prevent soft mid-sections and maintain fracture performance.

Designers must coordinate forming sequences, stress-relief, and final aging; forming should generally be performed before final solution heat treat and aging where feasible, and machining allowances should be set to permit local heating and surface condition control.

Equivalent Grades

Standard	Grade	Region	Notes
AA	7150	USA	Designation per Aluminum Association for wrought alloy; widely used in aerospace specifications
EN AW	7xxx-series (no direct single-number)	Europe	No exact one-to-one EN equivalent; specify chemistry and temper per AMS/EN standards
JIS	A7xxx (approx.)	Japan	Japanese standards reference alloys in the 7000 family; equivalence requires chemistry and temper matching
GB/T	7A50 (approx.)	China	Chinese 7A5x family alloys are genomically similar; direct substitution requires verification by spec

There is no perfect cross-reference because regional standards package chemistry, residual limits, and permitted tempers differently. For critical aerospace components, engineers must match chemical ranges, heat‑treatment practice (including quench rates and stretch), and inspection criteria rather than rely solely on nominal grade names.

When sourcing internationally, require material certificates to include exact composition, tensile/yield values in the supplied temper, and details of heat-treatment and any mechanical stress-relief to ensure equivalence in performance and fracture behavior.

Corrosion Resistance

7150 exhibits moderate atmospheric corrosion resistance compared with more noble Al‑Mg alloys; in typical temper it can perform adequately with paint or conversion coatings. In marine or highly chloride-bearing environments it is more susceptible to pitting and intergranular attack than 5xxx or some 6xxx alloys unless overaged or clad.

Stress-corrosion cracking (SCC) is a primary concern for high-strength 7xxx alloys. Peak-aged T6/T651 tempers offer maximum strength but also maximum SCC sensitivity; overaging to T73 or selecting tempers engineered for SCC resistance (e.g., T76 family) is a common mitigation strategy for critical structures.

Galvanic interactions should be considered where 7150 contacts more noble cathodic materials (stainless steels, titanium): the aluminum will corrode preferentially unless electrically isolated or properly coated. Compared with 6xxx-series alloys (e.g., 6061), 7150 trades improved strength and fatigue performance for reduced inherent corrosion resistance and higher sensitivity to environmental cracking without protective measures.

Fabrication Properties

Weldability

Welding 7150 is challenging: fusion welding (TIG/MIG) can cause severe loss of strength in the heat-affected zone (HAZ) and is generally discouraged for primary structural members. When welding is necessary, filler alloys and post-weld solution/aging practices must be selected carefully; however, full restoration of parent mechanical properties by localized welding is generally not feasible.

Friction stir welding (FSW) and solid-state joining methods are preferred because they limit melting and can preserve more of the alloy’s temper properties, though HAZ softening still occurs. Fillers commonly used in aluminum joining (e.g., 4043, 5356) will not restore original base properties and can introduce galvanic considerations and differing electrochemical behavior.

Machinability

As a high-strength Al‑Zn‑Mg‑Cu alloy, 7150 has good machinability compared with steels but is more demanding than common 6xxx or 5xxx alloys due to higher strength and toughness. Tooling should use carbide inserts with positive rake and high feed to avoid rubbing; cutting speeds are typically 200–600 m/min depending on operation and coolant use.

Chip control can be good if proper tool geometry and coolant are used; however, work hardening is not a factor as with some stainless steels. Surface integrity and tool wear must be monitored because high hardness in peak-aged states can accelerate abrasive wear.

Formability

Forming is best performed in softer tempers or prior to final aging because T6/T651 conditions have limited ductility and springback. Minimum bend radii are larger in peak-aged conditions; typical bend radii for machined/formed structural elements should be conservatively specified (e.g., >2–3× thickness for tight bends in stronger tempers).

Cold forming followed by solution treatment and aging is a common route to achieve final geometry and mechanical properties; hot-forming processes and superplastic forming are rarely used with 7150 because of quench sensitivity and precipitate behavior that control final properties.

Heat Treatment Behavior

Solution treatment for 7150 is typically performed in the 470–500 °C range to dissolve alloying elements into a supersaturated solid solution while avoiding incipient melting of low-melting constituents. Rapid quenching to room temperature (or colder) is required to retain the supersaturated state; quench rate control is critical in thick sections to avoid mid-thickness softness.

Artificial aging follows quench. Typical T6 aging cycles use intermediate aging temperatures (e.g., 120