Aluminum 6951: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

6951 is part of the 6xxx series of aluminum alloys, which are broadly classified as Al-Mg-Si alloys and are heat-treatable by precipitation hardening. It belongs to a subgroup of 6xxx alloys that include modest copper and controlled amounts of zinc and chromium to raise strength and modify aging kinetics while maintaining good corrosion performance.

Major alloying elements in 6951 include magnesium and silicon (for the Mg2Si strengthening phase), with copper as a purposeful strength and hardening accelerator. Trace additions of chromium, titanium and controlled iron and manganese are used to control grain structure, recrystallization and dispersoids that affect toughness, HAZ behavior and fatigue life.

The principal strengthening mechanism is precipitation hardening combined with limited work hardening; solution treatment followed by artificial aging produces fine Mg-Si (and Cu-modified) precipitates that lock dislocations. Key traits include elevated strength-to-weight ratio relative to standard 6xxx alloys, good general corrosion resistance, reasonable weldability with some HAZ softening risk, and fair formability in softer tempers.

Typical industries using 6951 are aerospace substructures and fittings, defense hardware, high‑performance automotive components and some marine structural parts where a balance of strength, corrosion resistance and manufacturability is required. Engineers choose 6951 when higher peak strength and a favorable fatigue-to-weight trade-off are needed without the cost, fabrication constraints or anodizing issues of the highest-strength 7xxx series alloys.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed for forming and drawing
H111 / H11	Low-Medium	Medium-High	Very Good	Good	Light strain-hardened, retains some formability
H14	Medium	Medium	Good	Good	Single-stage strain-hardened for increased yield
T4	Medium	Medium-High	Good	Good	Natural aged after solution; intermediate properties
T5	Medium-High	Medium	Fair	Good	Cooled from shaping and artificially aged
T6	High	Low-Medium	Limited	Good (with HAZ softening)	Solution treated and artificially aged to peak strength
T651	High	Low-Medium	Limited	Good (with HAZ softening)	T6 plus stress relieving by stretching; common for aerospace
H24/H34	Medium-High	Medium	Fair	Good	Combined strain and artificial aging for controlled properties

Temper has a first-order effect on strength, ductility and formability in 6951, with O and H-series tempers used for heavy forming and deep drawing. T6 and T651 provide the highest static strengths and best fatigue performance but reduce formability and increase sensitivity to heat input during welding.

Chemical Composition

Element	% Range	Notes
Si	0.4–1.0	Combines with Mg to form strengthening Mg2Si precipitates
Fe	0.1–0.5	Impurity that forms intermetallics; controlled to limit embrittlement
Mn	0.05–0.3	Grain structure modifier; small amounts improve toughness
Mg	0.6–1.3	Primary strengthening element via Mg2Si; controls age hardening
Cu	0.6–1.5	Raises strength, accelerates aging and affects corrosion and HAZ
Zn	0.05–0.30	Minor addition to tweak strength and aging response
Cr	0.05–0.35	Controls recrystallization, dispersoid formation and HAZ stability
Ti	0.02–0.15	Grain refiner used during casting/extrusion processing
Others	Bal. (including trace Zr, B)	Small additions/impurities for process control

The chemistry of 6951 is tuned to favor Mg-Si precipitation with Cu acting as a strength and aging modifier. Silicon and magnesium ratios control the volume fraction and stability of Mg2Si precipitates, while Cu modifies the sequence and thermal stability of precipitates yielding higher peak strengths. Chromium and trace elements form dispersoids that limit grain growth and improve HAZ and fatigue properties.

Mechanical Properties

Tensile behavior in 6951 is characteristic of heat-treatable Al-Mg-Si-Cu alloys: a large increase in strength occurs after solution treatment and artificial aging as fine precipitates form. Yield to tensile ratios are dependent on temper and processing, and peak-aged conditions can show relatively high proof strengths with moderate elongation. Elongation in annealed conditions is high for forming, but drops substantially in T6/T651 where the material becomes more susceptible to localized necking.

Hardness follows the same trend as tensile strength and is a useful shop-floor proxy for temper verification; hardness increases by ~2–3× from O to T6 in comparable gauge material. Fatigue performance benefits from fine, uniformly distributed precipitates and a controlled microstructure; surface finish, residual stresses and thickness all strongly influence the fatigue limit. Thickness effects are significant because larger cross-sections lead to coarser precipitate distributions and slower quench rates, which can reduce peak achievable strength and change HAZ softening behavior.

Property	O/Annealed	Key Temper (e.g., T6/T651)	Notes
Tensile Strength	110–180 MPa	320–420 MPa	Wide range dependent on gauge, aging and exact chemistry
Yield Strength	55–110 MPa	280–380 MPa	Proof stress increases markedly with artificial aging
Elongation	18–30%	8–15%	Ductility drops in peak-aged tempers; gauge affects numbers
Hardness	25–50 HB	90–140 HB	Hardness correlates with precipitate density and distribution

Physical Properties

Property	Value	Notes
Density	2.78 g/cm³	Typical for Al-Mg-Si family alloys
Melting Range	555–640 °C	Solidus to liquidus approximate interval
Thermal Conductivity	~140–160 W/m·K	Slightly reduced from pure Al due to alloying
Electrical Conductivity	~30–38 %IACS	Lower than pure aluminum; temper dependent
Specific Heat	~880–910 J/kg·K	Close to pure aluminum value
Thermal Expansion	~23.5–24.5 µm/m·K	Typical linear expansion coefficient for Al alloys

These physical properties make 6951 attractive where light weight and thermal management are required, although conductivity is traded off versus alloying for strength. Thermal expansion and conductivity should be considered in assemblies involving dissimilar materials, as mismatch can drive thermal fatigue and joint stresses.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.4–6.0 mm	Strength decreases slightly with thicker gauges	O, H14, T4, T6	Used for formed panels and skins
Plate	>6 mm up to 100 mm	Slower quench sensitivity; lower peak strength vs thin gauge	O, T6	Structural components and machine bases
Extrusion	Profiles up to several meters	Good strength in T6/T651 after aging	T5, T6, T651	Complex profiles for fittings and rails
Tube	Varied diameters, thicknesses	Welded or seamless; properties depend on processing	Hx, T5, T6	Fluid lines, structural tubing
Bar/Rod	Diameters up to 200 mm	Homogenized microstructure needed for uniform properties	O, T6	Fittings, machined components

Form factor and processing route control cooling rates and recrystallization, which influence final strength and toughness in 6951. Extrusions and thin sheet can be rapidly quenched and readily reach peak tempers, while thick plate requires careful process control and potentially thermomechanical treatments to approach similar properties.

Equivalent Grades

Standard	Grade	Region	Notes
AA	6951	USA	Designation used in manufacturer/product literature
EN AW	No direct equivalent	Europe	Closest common alternatives: EN AW-6061 / EN AW-6082 depending on use
JIS	No direct equivalent	Japan	No single standardized JIS grade maps directly to 6951
GB/T	No direct equivalent	China	Local alloys with similar chemistries may be specified; verify data

There is no always-exact cross-reference to a single international designation for many proprietary variants like 6951; users should verify chemistry and property data instead of relying solely on nominal equivalence. In practice, 6061 or 6082 are often used as functional analogues for design comparisons, but copper content and aging response in 6951 will produce different peak strengths and HAZ sensitivity.

Corrosion Resistance

In atmospheric environments 6951 offers solid-general corrosion resistance comparable to other 6xxx alloys, helped by the formation of a stable oxide layer and a subdued copper content compared with 2xxx or 7xxx series alloys. Localized corrosion resistance is good but dependent on heat treatment and surface finish; peak-aged tempers containing copper can have reduced pitting resistance compared with low-copper alloys.

Marine exposure requires care: 6951 performs acceptably in offshore and splash-zone conditions but may need coatings, anodizing or sacrificial protection for prolonged immersion in aggressive seawater. Stress corrosion cracking susceptibility is moderate and is influenced by temper and residual stresses; peak-aged conditions with tensile residual stress are more vulnerable than solution-treated or stress-relieved material.

Galvanic interactions follow standard aluminum behavior: when coupled to more noble metals (stainless steel, copper/brass) 6951 will preferentially corrode unless electrically insulated; anodizing improves surface resistance and reduces galvanic coupling. Compared to 5xxx magnesium-strengthened alloys, 6951 trades some innate marine resistance for higher strength and better age-hardenability.

Fabrication Properties

Weldability

Welding 6951 by common fusion processes (TIG, MIG) is feasible, but the alloy shows some HAZ softening due to dissolution and coarsening of strengthening precipitates. Selection of filler alloys with compatible strength (e.g., 4043, 5356 depending on joint requirements) and pre/post-weld heat treatments can mitigate some strength loss. Hot-cracking risk is moderate and is influenced by joint design, travel speed and cleanliness; proper joint fit-up and control of weld pool composition are important.

Machinability

Machinability of 6951 is typical of heat-treatable Al-Mg-Si alloys and is generally good in T6/T651 due to higher hardness giving stable chip formation. Carbide tooling with positive rake and adequate coolant yield the best cycle times; cutting speeds can be high compared with steels but must be selected to avoid built-up edge. Surface finish and dimensional control are excellent, but residual stresses from machining can affect fatigue-critical parts, so process planning and stress-relief operations may be required.

Formability

Forming is favored in softer tempers (O, H1x, T4) where drawability and bend radii are excellent; as the temper hardens (H2x, H14) formability reduces but springback control improves. In T6/T651 the alloy has limited cold forming capability and is prone to cracking on tight bends; solution treatment and re-aging or warm forming can be used to achieve complex geometries with minimal cracking. Recommended minimum bend radii and forming allowances should be established from trials for critical geometries.

Heat Treatment Behavior

As a heat-treatable alloy, 6951 responds to conventional solution heat treatment and artificial aging cycles. Solution treatment is performed at temperatures sufficient to dissolve Mg2Si and Cu-containing phases (typically in the 510–540 °C range for many 6xxx designs), followed by quenching to retain a supersaturated solid solution. Proper quench rates are critical for maximizing the volume of solute available for subsequent precipitation and peak strength.

Artificial aging (T5/T6) precipitates fine Mg-Si (and Cu-containing) phases; aging kinetics are faster and peak strengths higher with Cu modification, but thermal stability and overaging behavior must be controlled to avoid excessive property decline. T temper transitions are reversible within limits: parts may be solution-treated and artificially aged to T6, or partially cold-worked and artificially aged to H2x/H3x variants for tailored yield/toughness combinations. For non-heat-treatable processing paths, incremental work hardening and annealing are used to tailor properties where applicable.

High-Temperature Performance

6951 experiences a progressive loss of yield and ultimate strength above typical service temperatures; significant strength degradation is expected above ~150 °C as precipitates coarsen and dissolve. Oxidation in air is minor at moderate elevated temperatures due to a stable alumina layer, but long-term thermal exposure will modify precipitate structure and reduce fatigue and static strength.

Weld heat-affected zones are particularly susceptible to softening at elevated local temperatures due to precipitate dissolution and overaging; designing for lower thermal inputs, post-weld aging or localized heat treatments can restore a significant portion of original strength. For sustained elevated-temperature service, alternative high-temperature aluminum alloys or non-aluminum materials should be considered based on creep requirements.

Applications

Industry	Example Component	Why 6951 Is Used
Automotive	Structural reinforcements, chassis brackets	High specific strength, good fatigue behavior
Marine	Stiffeners and structural subframes	Balanced corrosion resistance and strength
Aerospace	Fittings, minor structural members	Favorable strength-to-weight and controlled HAZ with T651
Defense	Weapon mounts and hardened housings	High static strength with good machinability
Electronics	Enclosures and thermal spreaders	Good thermal conductivity and machinability

6951 is chosen in applications where a higher-strength 6xxx alloy is needed without moving to heavier or more corrosion-sensitive series. Its machinability and form-factor versatility allow designers to specify complex components that benefit from higher peak strength while maintaining acceptable corrosion performance.

Selection Insights

Use 6951 when you need higher peak strength than standard 6xxx alloys while retaining reasonable corrosion resistance and good machinability. It is a practical choice for medium- to high-strength components where fatigue life is a design driver and T6/T651 tempers are acceptable.

Compared with commercially pure aluminum (e.g., 1100), 6951 trades electrical and thermal conductivity and superior formability for a considerable increase in strength and fatigue resistance. Compared with work-hardened alloys such as 3003 or 5052, 6951 delivers much higher yield and tensile strengths but somewhat lower inherent marine corrosion resistance and less cold-formability. Compared with common heat-treatable alloys like 6061 or 6063, 6951 may be preferred when a slightly different balance of strength, aging response and fatigue performance is required, despite not always offering higher peak strengths than the highest-performing 7xxx or specialized 2xxx alloys.

Closing Summary

6951 remains relevant for modern engineering where an engineered balance of precipitation-hardened strength, fatigue performance and reasonable corrosion resistance is required. Its process flexibility across sheet, plate and extrusion forms combined with reliable machinability makes it a strong candidate for aerospace, defense and high-performance automotive applications where optimized strength-to-weight ratios are essential.