Aluminum 712: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

Alloy 712 is a high-strength, heat-treatable aluminum alloy that is best classed within the 7xxx series family where zinc is the principal strengthener. Its chemistry is dominated by Zn-Mg-Cu additions that drive age-hardening, with trace additions of Cr/Ti or Zr often used for grain structure control and improved toughness. The principal strengthening mechanism is precipitation hardening following solution treatment and artificial aging, although limited work hardening can be used in some tempers to tune properties. Typical traits include high static strength and good stiffness for weight-sensitive structures, moderate thermal and electrical conductivity, and a trade-off of reduced general corrosion resistance and weldability relative to 5xxx and 6xxx family alloys.

Industries that use Alloy 712 are primarily aerospace and high-performance transportation where strength-to-weight ratio and fracture performance are prioritized, as well as some high-strength marine and specialty automotive applications requiring superior structural performance. The alloy is chosen over lower-strength alloys when design envelopes require high yield and tensile strength without resorting to exotic materials or heavier gauges. Engineers select 712 when the design demands high specific strength and fatigue resistance while accepting the need for controlled fabrication processes and corrosion mitigation strategies. When compared to 6xxx series alloys, 712 provides higher peak strength at the expense of formability and weld-degraded properties, making it a specialist material rather than a general-purpose structural alloy.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed; maximum ductility for forming
H14	Medium	Medium-Low	Good	Fair	Work-hardened to intermediate strength without aging
T5	Medium-High	Medium	Fair	Fair	Cooled from an elevated temperature shaping process and artificially aged
T6	High	Low-Medium	Limited	Poor	Solution heat-treated and artificially aged to peak strength
T651	High	Low-Medium	Limited	Poor	Solution treated, stress-relieved by stretching, then artificially aged
T73	Medium-High	Medium	Good	Improved	Overaged condition with improved SCC resistance and toughness

Tempering strongly governs the balance between strength and ductility for 712; O and H tempers are used when significant forming is required while T tempers maximize strength through controlled precipitation. Overaged tempers such as T73 are deployed to improve fracture toughness and resistance to stress-corrosion cracking at the expense of some peak strength.

Chemical Composition

Element	% Range	Notes
Si	0.10–0.40	Trace impurity, controls casting characteristics
Fe	0.10–0.50	Impurity that can form intermetallics affecting toughness
Mn	0.05–0.30	Minor; can improve grain structure and strength slightly
Mg	1.3–2.5	Primary co-alloying element with Zn to form MgZn2 precipitates
Cu	0.8–2.0	Strength enhancer and controls age-hardening kinetics
Zn	4.5–6.5	Principal strengthener in 7xxx-class alloys
Cr	0.02–0.30	Microalloying for recrystallization control and toughness
Ti	0.01–0.10	Grain refiner in wrought products
Others	Balance / impurities (each <0.05–0.5)	Residual elements (Zr, V, etc.) for grain control and trace effects

The Zn–Mg–Cu system defines the age-hardening response: Zn and Mg combine to form fine MgZn2 precipitates that provide most of the strengthening after aging, while Cu shifts precipitation kinetics and increases peak strength. Microalloying additions such as Cr, Ti or Zr are used to limit grain growth during solution treatment and to improve fracture toughness and fatigue resistance by stabilizing a fine sub-grain structure. Residual elements and impurities influence grain boundary phase formation and therefore impact SCC susceptibility and toughness.

Mechanical Properties

Under tensile loading 712 exhibits classic heat-treatable aluminum behavior where strength and ductility are strongly temper-dependent; solution-treated and peak-aged tempers develop high tensile and yield strengths with moderate ductility. Yield strength in peak tempers is typically a large fraction of ultimate tensile strength, which benefits dimensional stability under service loads but reduces forming window and increases springback. Hardness correlates well with tensile properties: hardness rises substantially after aging as coherent and semi-coherent precipitates develop; this hardening also influences machining characteristics and fatigue crack initiation. Thickness and section size affect peak attainable strength due to quench sensitivity; thick sections can have lower strength and toughness because of slower cooling and larger interdendritic precipitates.

Property	O/Annealed	Key Temper (e.g., T6)	Notes
Tensile Strength	~220–260 MPa	~520–580 MPa	T6 peak-aged values similar to other high-Zn aluminums; depends on section thickness
Yield Strength	~60–120 MPa	~460–520 MPa	Significant increase on aging; yield ratio is high in T6 states
Elongation	~18–26%	~6–12%	Ductility reduced after aging; O condition preferred for severe forming
Hardness	~50–75 HB	~140–165 HB	Brinell hardness rises substantially with aging and precipitation

Fatigue performance in well-processed 712 can be excellent compared with lower-strength alloys, as long as surface quality, residual stress state, and corrosion are controlled. Peak fatigue strength is achieved in T651 or overaged conditions that balance strength and crack-growth resistance, while aggressive peak-age states maximize static strength but can be more crack-sensitive.

Physical Properties

Property	Value	Notes
Density	2.78 g/cm³	Typical for Al–Zn–Mg–Cu alloys; favorable strength-to-weight ratio
Melting Range	~500–645 °C	Solidus–liquidus span depends on Zn/Cu levels and secondary phases
Thermal Conductivity	120–150 W/m·K	Lower than pure Al; reduced by alloying and precipitates
Electrical Conductivity	28–38 % IACS	Reduced compared with pure aluminium due to solute and precipitates
Specific Heat	~0.90 J/g·K	Near the specific heat of most wrought aluminum alloys
Thermal Expansion	23–24 µm/m·K	Coefficient of thermal expansion representative of Al alloys

The physical properties make 712 attractive for weight-sensitive structural parts that require thermal stability and adequate heat dissipation, although electrical and thermal conductivities are lower than those of purer aluminum grades. The melting and solidification ranges influence casting and welding behavior; solidification intervals promote formation of intermetallic phases that must be controlled through alloy tuning and process control.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.3–6.0 mm	Strength consistent in thin gauges; good for formed panels	O, H14, T5, T6	Used where high specific strength and light gauge forming are needed
Plate	6–150+ mm	Strength and toughness sensitive to thickness; quench sensitivity in thick sections	O, T6, T651, T73	Heavy sections require tight thermal control during quench and aging
Extrusion	Wall thickness 1–20 mm	Extruded profiles can achieve high strength but are limited by quench rate	T5, T6 (post-aging)	Complex sections may require direct-aging or paint-bake cycles
Tube	OD 10–300 mm	Mechanical properties depend on fabrication route and reduction	O, T6	Seamless or welded tubes require post-process heat treatment for peak properties
Bar/Rod	Diameter 3–150 mm	Bars respond well to solution/quench/age sequences; section size controls properties	O, T6	Used for fittings, machined parts, and highly stressed components

Different product forms require tailored processing to achieve target properties; thin sheet can be rapidly cooled and aged to peak conditions while thick plate demands specialized quenching or overaging strategies to reduce residual gradients. Extrusion and rolling history influence recrystallization behavior and the final anisotropy; consequently, designers must consider directional properties and the effect of cold work or stretch straightening on the delivered tempers.

Equivalent Grades

Standard	Grade	Region	Notes
AA	712	USA	Industry designation for the high-strength Zn–Mg–Cu wrought alloy family
EN AW	No direct equivalent	Europe	No single EN AW designation matches 712 exactly; closest analogs are AW-7075 and AW-7050
JIS	No direct equivalent	Japan	No exact JIS counterpart; similar performance overlaps with A7075-series alloys
GB/T	No direct equivalent	China	Chinese standards may provide similar high-strength Zn–Mg–Cu alloys but not a direct 712 equivalent

Subtle differences between 712 and nearby standardized grades arise from exact Zn/Mg/Cu ratios and microalloying additions that change precipitation sequences and quench sensitivity. Even small shifts in Cu or Zn levels can alter peak-aged strength, fracture toughness, and susceptibility to stress-corrosion cracking, so direct substitution should be validated with mechanical testing and corrosion assessment. Regional standards often provide close-performing alternatives, but buyers should verify temper designations, product form qualifications, and property certificates before specifying direct replacements.

Corrosion Resistance

Alloy 712 exhibits moderate atmospheric corrosion resistance in non-aggressive environments but is more susceptible to localized corrosion and pitting than 5xxx or annealed 6xxx alloys due to its higher Zn and Cu content. In marine or chloride-rich environments the alloy requires protective measures such as paint systems, anodizing, or cathodic protection; otherwise pitting and exfoliation can accelerate component degradation. Stress-corrosion cracking (SCC) is a well-recognized risk for high-strength Zn–Mg–Cu alloys and is sensitive to metallurgical condition, residual stress, and aging state; overaging (e.g., T73) or residual-stress relief via stretching reduces SCC susceptibility. Galvanic interactions with dissimilar metals are important: 712 is anodic to stainless steels and cathodic to magnesium, so isolation or compatible fasteners and coatings should be specified to avoid galvanic corrosion.

Compared with 5xxx family alloys (Mg-based), 712 trades corrosion robustness for higher strength; 5xxx series typically resists marine environments better without heavy protective systems. Versus 6xxx alloys, 712 typically offers higher static strength but poorer general corrosion and weld-zone performance, requiring additional surface protection in exposed applications.

Fabrication Properties

Weldability

Welding of Alloy 712 by conventional fusion methods (TIG/MIG) is challenging because the heat input modifies the precipitation state and produces HAZ softening, leading to significant loss of strength adjacent to the weld. Specialized filler alloys and process control reduce hot-cracking risk, but even with appropriate fillers the repaired or welded joint will usually not recover base-metal peak T6 strength; friction stir welding is often preferred to retain higher mechanical properties and to minimize porosity and cracking. Pre- and post-weld treatments, including controlled preheating, quench/aging or local mechanical stress-relief, are commonly required to manage distortion and to optimize joint performance.

Machinability

Machinability of 712 is generally good in the T6 condition relative to many high-strength aluminum alloys due to its reasonably uniform microstructure, but tool forces and chip control are higher than for softer alloys. Carbide tooling with positive rake geometry and high-speed steels with adequate coatings are recommended; cutting speeds and feed rates should be adjusted to balance tool life and finish, and coolant is advisable to control thermal input and prevent built-up edge. Surface finish and machining-induced residual stresses influence fatigue behavior, so final machining passes and stress-relief operations should be specified for critical aerospace components.

Formability

Forming is best performed in the O or soft H tempers where elongation and bendability are maximized; T6 and other peak-aged tempers have limited cold-forming capability and show greater springback and risk of cracking. Minimum bend radii depend on gauge and temper, but a conservative rule is to design bends with radii of 2–4× material thickness for T6 sections and 1–2× thickness for O-tempered material. Where complex shapes are required for high-strength parts, near-net-shape forming followed by heat treatment (ageing or solution/age sequences) is often the most practical manufacturing route.

Heat Treatment Behavior

As a heat-treatable alloy, 712 follows a standard solution treatment, quench, and artificial aging pathway to develop peak mechanical properties. Solution treatment temperatures are typically in the range of 470–490 °C to dissolve soluble phases, followed by rapid quenching to retain a supersaturated solid solution that can precipitate during aging. Artificial aging schedules vary depending on desired balance of strength and SCC resistance; a typical T6-type aging might use 120–130 °C for several hours to produce peak hardness, while overaging (T73) uses higher temperature or extended times to coarsen precipitates and improve fracture and corrosion resistance. T temper transitions can be used to tailor properties: reversion anneals and controlled natural aging steps affect subsequent artificial aging response and must be controlled to ensure reproducible property attainment.

Work-hardening plays a limited role compared with precipitation hardening for 712, but cold work can be used to augment strength in interim tempers (e.g., H1x series) if aging response is compatible. Full annealing returns the alloy to a readily formable O state and is used before severe forming operations.

High-Temperature Performance

Strength retention at elevated temperatures is limited for Alloy 712; significant softening occurs above ~120–150 °C because the engineered precipitate structure coarsens and loses coherency. For short-term exposures up to ~200 °C some residual strength may remain, but long-term service at elevated temperatures will reduce yield and accelerate creep and relaxation of residual stresses. Oxidation is minimal for aluminum alloys at moderate temperatures, but protective coatings can degrade and permit localized corrosion if thermal stability is insufficient. HAZ regions created by welding or other thermal cycles are particularly vulnerable to strength loss due to precipitate dissolution and re-precipitation, so thermal exposures during fabrication must be tightly controlled to maintain mechanical integrity.

Applications

Industry	Example Component	Why 712 Is Used
Aerospace	Fuselage fittings and wing carry-through structures	High specific strength and fracture toughness for primary structural parts
Marine	High-strength hull fittings and spars	High strength-to-weight and good fatigue resistance with proper corrosion protection
Automotive	High-performance chassis members and suspension components	Lightweighting where peak strength and stiffness reduce mass
Electronics	Structural frames and high-strength brackets	Strength and dimensional stability with moderate thermal conductivity
Defense	Projectile casings, structural brackets	High strength and good fatigue performance under cyclic loading

Alloy 712 is selected where a balance of high static strength, acceptable toughness, and a manageable fabrication pathway deliver clear performance advantages in safety- or weight-critical structures. Its use is most effective where additional corrosion protection and controlled fabrication processes can be budgeted.

Selection Insights

Alloy 712 is best selected when high static strength and stiffness are priority design drivers and the fabrication plan includes controlled heat treatment and corrosion protection. Compared with commercially pure aluminum (e.g., 1100), 712 trades off electrical and thermal conductivity and formability for much higher tensile and yield strength, making it unsuitable where maximum conductivity or deep drawing are required.

Compared with common work-hardened alloys such as 3003 or 5052, 712 offers markedly higher strength at the cost of reduced formability and greater sensitivity to marine corrosion; use 712 when the structural strength requirement outweighs ease of forming or inherent corrosion resistance. Compared with heat-treatable 6xxx alloys (e.g., 6061/6063), 712 provides higher peak strength but usually poorer weld-zone properties and corrosion resistance; choose 712 when maximum strength-to-weight is needed and when design allows for specialized joining or FSW and protective coatings.

Closing Summary

Alloy 712 remains relevant where designers demand a high-strength, heat-treatable aluminum with an excellent specific-strength envelope and good fatigue behavior, provided that fabrication controls and corrosion mitigation strategies are implemented. When used with appropriate tempers, joining methods, and protective measures, 712 delivers reliable high-performance solutions for aerospace, marine, and high-end transport applications.