Aluminum 7175: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

7175 is a high-strength aluminum alloy belonging to the 7xxx series, which is characterized by zinc as the principal alloying element. It is designed primarily for aerospace and high-performance structural applications where strength-to-weight ratio and fatigue resistance are critical.

The alloy's major alloying additions include zinc, magnesium, and copper, with trace additions of chromium, titanium or zirconium used to control grain structure and recrystallization. These elements produce a precipitation-hardenable microstructure; 7175 is a heat-treatable alloy that attains its strength via solution heat treatment, quenching and artificial aging to form fine η (MgZn2)-type precipitates.

Key traits of 7175 include very high static strength and good fatigue performance for its class, moderate-to-poor intrinsic corrosion resistance compared with 5xxx/6xxx alloys, limited weldability, and reduced formability in peak tempers. Typical industries include aerospace primary and secondary structures, defense hardware, and highly loaded fittings where maximum structural efficiency is the priority. Engineers choose 7175 over other alloys when peak tensile and yield strengths and fatigue crack growth resistance outweigh demands for ease of joining, forming, or lowest material cost.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High (20–30%)	Excellent	Excellent	Fully annealed for forming and stress relief
H112	Moderate	Moderate (10–15%)	Good	Fair	Strain-hardened, used for limited forming and stabilization
T6	Very High	Moderate (6–12%)	Limited	Poor	Solution heat-treated and artificially aged to peak strength
T651	Very High	Moderate (6–12%)	Limited	Poor	T6 with stress-relief by stretching; common for aerospace plate
T73	High (but lower than T6)	Improved (8–14%)	Moderate	Poor	Overaged condition to improve corrosion & SCC resistance
T7651	High	Moderate	Limited	Poor	Controlled pre-age and stabilization for better toughness

Temper has a first-order effect on 7175 performance: peak-aged tempers (T6/T651) maximize tensile and yield strength but reduce ductility and formability and increase susceptibility to stress corrosion cracking. Overaging treatments such as T73 intentionally reduce peak strength to significantly improve resistance to SCC and improve fracture toughness, creating a trade-off between strength and environmental durability.

Selection of temper in production is driven by required final properties, subsequent forming or machining operations, and the intended service environment; for example, plate that must retain high static strength with minimal residual stress often uses T651, while components exposed to corrosive environments may be specified T73 or chemically treated finishes.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.10	Impurity; minimizes intermetallics and improves casting cleanliness
Fe	≤ 0.15	Impurity; excess reduces toughness and increases inclusions
Mn	≤ 0.10	Low; not a primary strengthener in 7xxx alloys
Mg	2.0–2.9	Combines with Zn to form strengthening MgZn2 precipitates
Cu	1.2–2.0	Increases strength and hardenability, can reduce corrosion resistance
Zn	6.0–7.5	Principal strength-contributing element through Mg–Zn precipitates
Cr	0.10–0.30	Controls grain structure and recrystallization, improves toughness
Ti	≤ 0.05	Grain refiner, used in ingot metallurgy and billet processing
Others (Zr, V, balance Al)	Trace / Balance	Zr or other microalloying may be present to stabilize microstructure

The combined Zn–Mg–Cu system is responsible for the precipitation hardening response in 7175; zinc and magnesium form η-phase precipitates that provide primary strengthening, while copper increases strength and affects precipitate morphology and electrochemical behavior. Chromium and trace zirconium or titanium additions inhibit grain growth during processing and control recrystallization, which is essential for maintaining toughness and fatigue resistance in thick sections.

Mechanical Properties

7175 exhibits very high tensile and yield strengths in peak-aged tempers, with tensile curves showing a pronounced yield point followed by limited uniform elongation. The alloy offers excellent fatigue strength and favorable crack-initiation resistance when produced with controlled microstructure and low inclusion content, making it well suited to cyclic-load structural components.

Yield strength and elongation are strongly temper-dependent; T6/T651 achieves the highest yield values but reduced ductility and toughness, while T73 reduces yield somewhat but enhances fracture toughness and resistance to stress-corrosion cracking. Hardness follows the same trend as strength, with peak-aged conditions showing substantial increases in Brinell or Rockwell values relative to annealed conditions.

Thickness and product form influence properties due to differences in cooling rates, residual stress, and grain structure; thick plate and forgings may require specialized thermomechanical processing and aging cycles to obtain uniform properties through section thickness and to limit quench-induced distortion or residual stresses.

Property	O/Annealed	Key Temper (T6 / T651)	Notes
Tensile Strength	~240–320 MPa	~540–590 MPa	Peak-aged tensile values typical for aerospace-grade 7xxx alloys
Yield Strength	~120–220 MPa	~470–520 MPa	Yield increases markedly with aging; reported ranges depend on product form
Elongation	20–30%	6–12%	Ductility is reduced in high-strength tempers
Hardness (HB)	~60–80 HB	~150–165 HB	Hardness correlates with precipitation strengthening level

Physical Properties

Property	Value	Notes
Density	2.80–2.82 g/cm³	Typical for high-strength aluminum alloys
Melting Range	~477–635 °C	Solidus ~477–490 °C, liquidus approaching pure Al ~635 °C
Thermal Conductivity	~120–140 W/(m·K)	Lower than pure Al due to alloying; still high relative to steels
Electrical Conductivity	~30–40 % IACS	Reduced conductivity relative to low-alloy Al due to solute atoms
Specific Heat	~0.90 kJ/(kg·K)	~900 J/(kg·K) at room temperature
Thermal Expansion	~23–24 µm/(m·K)	Similar coefficient to other Al alloys; consider for bolted joints

Despite heavy alloying, 7175 retains good thermal conductivity compared with steels, which enables moderate heat-sinking capabilities in some applications, though thermal performance is inferior to pure aluminum. Electrical conductivity is reduced by solute scattering; 7175 is not chosen where electrical conduction is a primary requirement. The combination of relatively low density and high strength produces an excellent strength-to-weight ratio, which is the main driver for its selection in high-performance structural roles.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.5–6 mm	Good in thin gauges; may be limited by formability	O, H112, T6	Used for secondary structures; not ideal for severe stamping in T6
Plate	6–150+ mm	Strength varies with thickness due to quench sensitivity	T651, T73	Aerospace structural plate produced with controlled quench and aging
Extrusion	Limited	Limited commercial availability; strength depends on section	T6, T73 (rare)	7xxx extrusions are less common; require careful homogenization
Tube	Varied	High strength possible in drawn/processed tubes	T6, T651 (fabricated)	Used for high-load tubular fittings; welding and joining are constrained
Bar/Rod	Dia. up to 200 mm	Good axial properties if properly heat-treated	T6, T73	Used for forgings, machined components, and high-load pins or fittings

Processing differences change the achievable microstructure: plate and sheet are typically produced by rolling and require precise quench control to avoid overaging or soft zones, while forgings and bars may use different homogenization and solution profiles to control grain size. Extrusions and complex profiles are less common for 7175 due to hot cracking and recrystallization tendencies, making 6xxx series alloys preferable when complex extruded shapes are required.

Equivalent Grades

Standard	Grade	Region	Notes
AA	7175	USA	Aluminum Association designation for the alloy chemistry
EN AW	7175	Europe	EN AW-7175 used in European specifications; processing origins influence property tables
JIS	A7175	Japan	Similar composition; JIS often includes additional production controls
GB/T	2A7175 / 7175	China	Chinese standards reference compositional and mechanical ranges comparable to AA 7175

Subtle differences between regional standards typically relate to allowable composition tolerances, specified tempers, and qualification testing procedures rather than gross chemistry. Manufacturing routes (ingot metallurgy vs. continuously cast and homogenized billets) and national heat treatment practices can impart small differences in impurity levels, grain structure control and typical mechanical properties; engineers should specify required mechanical and environmental performance rather than rely solely on nominal grade names.

Corrosion Resistance

In atmospheric exposure, 7175 exhibits moderate corrosion resistance but performs worse than 5xxx and many 6xxx alloys due to its higher copper content, which increases electrochemical activity. In clean, painted, or otherwise protected environments performance is acceptable for many structural uses, but the alloy benefits from cladding (alclad) or robust conversion coatings when long-term atmospheric exposure is expected.

Marine or salt-spray environments accelerate pitting and intergranular corrosion in high-strength 7xxx alloys; 7175 is susceptible to localized attack unless overaged (T73) or treated with protective coatings and sealants. Fastener and joint design must minimize crevices and incorporate sacrificial protection or insulating materials when dissimilar metals are present.

Stress corrosion cracking (SCC) is a known risk for high-strength tempers of 7xxx alloys, particularly in the presence of tensile residual stresses and corrosive media. Overaging, post-weld heat treatment and strict control of surface finishes and chemistries are common mitigation strategies. Galvanic interactions with steels or copper-bearing alloys are adverse; designers should avoid direct contact with more noble materials or provide insulation.

Fabrication Properties

Weldability

Welding 7175 is challenging; fusion welding (TIG/MIG) typically results in severe loss of strength in the heat-affected zone and a high risk of hot cracking. When welding is unavoidable, specialized filler alloys and strict pre/post-weld thermal control may be used, but riveted, bolted or adhesive-bonded joints are preferred for maintaining structural integrity. Heat-affected-zone softening often requires local mechanical reinforcement or post-weld treatments that are difficult to apply without degrading other properties.

Machinability

In peak tempers 7175 machines well relative to many steels owing to its low density and good chip-cleavage behavior, but tool wear is affected by high hardness and work-hardening. Carbide tooling, rigid fixturing and conservative feeds with positive rake geometries are recommended; coolant use is important for maintaining dimensional stability and reducing built-up edge. Machinability indexes are generally lower than 2xxx aluminum alloys but competitive with other 7xxx series materials.

Formability

Forming is most effective in annealed (O) or strain-hardened (H) tempers; deep drawing and complex stamping are not practical in T6/T651 without prior anneal or warm-forming techniques. Bend radii should be increased relative to softer alloys, and springback is more pronounced due to higher yield strengths. Cold work increases strength further and can be used to obtain targeted mechanical properties when combined with appropriate aging cycles.

Heat Treatment Behavior

Solution heat treatment for 7175 typically occurs in the 470–480 °C range to dissolve strengthening phases into solid solution; time at temperature and section thickness govern homogenization. Rapid quenching is required to retain solute in supersaturated solid solution; inadequate quench rates introduce coarse precipitates and reduce peak-aged strength and toughness.

Artificial aging for T6 is commonly performed at ~120–140 °C for durations tuned to the section size to produce a fine dispersion of η-phase precipitates and maximize strength. Overaging treatments (T73) use higher aging temperatures or extended times to coarsen precipitates, trading some strength for significantly improved stress-corrosion cracking resistance and toughness.

T temper transitions are sensitive to prior cold work, quench rate and impurity levels; controlled stretching (to produce T651) reduces residual stresses and improves dimensional stability but requires tightly controlled parameters to maintain desired mechanical properties. Post-heat-treatment stabilization and solution/aging schedules are often specified for critical aerospace applications.

High-Temperature Performance

7175 experiences significant strength loss with increasing temperature; service temperatures above ~120 °C degrade precipitation strengthening and reduce yield and tensile strength substantially. Creep resistance at elevated temperature is limited compared to heat-resistant alloys; long-term loading at temperatures over 100–125 °C must be validated for dimensional stability and life.

Oxidation is not a major failure mode at typical service temperatures, but thermal exposure accelerates overaging and changes precipitate distributions, which can reduce fatigue life and SCC resistance. In welded structures, HAZ regions are particularly vulnerable to strength loss and should be avoided for high-temperature load-bearing service.

Applications

Industry	Example Component	Why 7175 Is Used
Aerospace	Fuselage fittings, wing carry-through fittings	Exceptional specific strength and fatigue resistance for primary structure
Defense	High-load structural forgings and weapon mounts	High static strength and toughness in demanding load cases
Automotive	Performance chassis components (limited)	Strength-to-weight for performance and racing applications
Marine	Structural brackets and fittings (protected)	Good strength-to-weight where corrosion protections are applied
Electronics	Structural frames	High modulus and stiffness for lightweight frames (thermal roles limited)

7175 is selected when structural efficiency, fatigue life and fracture toughness at high static stresses are the governing requirements; its deployment is concentrated in aerospace and defense where material cost is justified by performance. Protective finishes and conservative design against SCC are standard practice when the alloy is used outside controlled environments.

Selection Insights

Use 7175 when maximum achievable strength and fatigue resistance are the primary design drivers and when manufacturing and corrosion protection strategies are in place to manage its joining and environmental limitations. Specify T6/T651 for highest static strength and T73 or other overaged tempers when environmental durability and SCC resistance are critical.

Compared with commercially pure aluminum (e.g., 1100), 7175 trades much higher strength and fatigue resistance for reduced electrical conductivity and formability; choose 1100 when conductivity and deep-draw formability are essential and loads are low. Compared with work-hardened alloys such as 3003 or 5052, 7175 offers substantially higher strength but generally worse formability and potentially worse corrosion behavior in chloride environments; select 5052/3003 for forming-intensive or marine-exposed, non-structural parts. Compared with heat-treatable 6xxx alloys like 6061/6063, 7175 delivers higher peak strength and fatigue performance but at greater cost and with poorer weldability; prefer 6061 where welding, economy and moderate strength suffice.

Closing Summary

7175 remains a key material for high-performance structural applications where top-tier strength-to-weight and fatigue resistance are required and where manufacturing processes can accommodate its limited weldability and corrosion sensitivity. With appropriate temper selection, surface protection and design attention to joining and stress concentration, 7175 delivers a combination of mechanical performance that is difficult to match with lower-alloyed or non-heat-treatable aluminum grades.