Aluminum 6151: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

6151 is a member of the 6xxx series aluminum alloys (Al-Mg-Si class) and is classified as a heat-treatable, precipitation-hardenable alloy. Its chemistry is dominated by magnesium and silicon which form Mg2Si precipitates during artificial aging to provide significant strengthening.

The alloy combines moderate to high strength with good corrosion resistance and reasonable formability when supplied in softer tempers. Typical industrial uses include architectural extrusions, automotive trim and structural components, marine fittings, and general engineering sections where a balance of strength, surface finish and anodizability is required.

6151 is selected where a combination of higher strength than pure aluminum or work-hardened alloys is required without the cost or joining constraints of higher-strength 7xxx series alloys. It is often chosen over lower-strength families for structural or load-bearing extrusions and where post-fabrication anodizing or painting is planned.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed condition; best for forming and machining.
H14	Medium	Moderate	Good	Good	Strain-hardened to a specified strength; limited hardening.
T4	Medium	Moderate	Good	Good	Solution heat-treated and naturally aged; good baseline for artificial aging.
T5	Medium-High	Moderate	Fair	Good	Cooled from shaping and artificially aged; commonly used for extrusions.
T6	High	Moderate-Low	Fair	Good	Solution heat-treated and artificially aged to peak strength; standard structural temper.
T651	High	Moderate-Low	Fair	Good	Solution heat-treated, stress-relieved by stretching, artificially aged; used for plate/extrusion with reduced residual stress.

Temper significantly alters 6151's balance between strength, ductility and formability because precipitate size and distribution govern mechanical response. Softer tempers (O, H1x) allow deep drawing and tight bending radii, while T5/T6 produce peak strength and reduce elongation, requiring careful design for forming and joining.

Chemical Composition

Element	% Range	Notes
Si	0.4 – 0.9	Silicon combines with Mg to form Mg2Si precipitates; controls strength and casting/flow characteristics.
Fe	≤ 0.50	Iron is an impurity that forms intermetallics and can reduce ductility and corrosion resistance if elevated.
Mn	≤ 0.15	Manganese refines grain structure and can slightly raise strength without much loss of ductility.
Mg	0.6 – 1.2	Magnesium is the primary strengthening element when combined with silicon; controls age-hardening response.
Cu	≤ 0.15 – 0.30	Copper may be present in small amounts to tweak strength and hardening kinetics; too much lowers corrosion resistance.
Zn	≤ 0.25	Zinc is typically low; higher Zn shifts properties toward 7xxx-like behavior and increases susceptibility to SCC.
Cr	≤ 0.25	Chromium helps control grain structure and limits recrystallization during thermomechanical processing.
Ti	≤ 0.15	Titanium is used in trace amounts as a grain refiner during casting or ingot processing.
Others	≤ 0.15 total	Small residuals (e.g., Sr, B) are controlled to maintain consistent mechanical and surface behavior.

The Mg–Si ratio and absolute contents control the precipitation sequence (GP zones → β″ → β′ → β) and therefore determine peak strength, aging kinetics and response to solution treatment. Trace elements and impurities influence grain size, recrystallization behavior and susceptibility to intergranular corrosion or embrittlement.

Mechanical Properties

6151 exhibits classic precipitation-strengthened tensile behavior where yield and ultimate tensile strengths increase markedly after aging to T5/T6 conditions. The annealed (O) condition delivers good elongation and energy absorption, but load-bearing applications typically specify T6 or T651 to obtain stable, higher yield values.

Yield and ultimate tensile values are thickness- and temper-dependent; thin extruded sections reach peak property levels more uniformly than thick plates due to more uniform solutionizing and quench rates. Hardness tracks tensile strength; T6 tempers commonly show significant rises in Brinell or Vickers hardness compared with O or H1x tempers.

Fatigue performance of 6151 is generally acceptable for structural applications and improves with surface finish and compressive residual stresses imparted by cold work or shot-peening. The presence of coarse intermetallic particles (iron-rich phases) and surface defects are common fatigue initiation sites, so control of casting and extrusion cleanliness is important.

Property	O/Annealed	Key Temper (e.g., T6)	Notes
Tensile Strength	~100 – 150 MPa	~260 – 320 MPa	Values vary with section thickness and aging practice; T6 gives peak strength for structural use.
Yield Strength	~40 – 90 MPa	~220 – 280 MPa	Yield rise on aging is substantial; design should use temper-specific measured properties.
Elongation	~15 – 25%	~8 – 15%	Ductility drops as strength increases; small sections typically show higher elongation.
Hardness (Brinell)	~30 – 60 HB	~90 – 130 HB	Hardness correlates with precipitate state; surface treatments and cold work affect readings.

Physical Properties

Property	Value	Notes
Density	2.70 g/cm³	Typical for Al–Mg–Si alloys; used for lightweight structural calculations.
Melting Range	~582 – 652 °C	Solidus/liquidus range depends on exact Si/Mg contents and impurities.
Thermal Conductivity	~160 – 180 W/m·K	Lower than pure Al due to alloying but still high compared with steels; good for heat-sinking.
Electrical Conductivity	~28 – 38 % IACS	Alloying reduces conductivity relative to pure Al; temper has modest influence.
Specific Heat	~0.90 J/g·K (900 J/kg·K)	Typical specific heat used for thermal modelling and transient heat calculations.
Thermal Expansion	~23 – 24 µm/m·K	Coefficient of thermal expansion similar to other Al alloys; important for bimetal joints.

These physical properties make 6151 attractive where a high strength-to-weight ratio and good thermal transport are needed together. Thermal and electrical conductivities are reduced from pure aluminum but remain favorable for heat transfer and lightweight conductor applications where mechanical demands exist.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.3 – 6 mm	Uniform in thin gauges; easily heat-treated post-fabrication	O, T4, T5, T6	Used for trim, panels, and brackets; excellent surface finish for anodizing.
Plate	6 – 50+ mm	Thick sections require longer solution and may show reduced peak properties	O, T6, T651	Thick plate may be limited by quench speeds; used for structural members.
Extrusion	Complex cross-sections, up to several meters	Very responsive to T5/T6 aging; thin walls reach properties quickly	T5, T6	Common for window frames, architectural profiles and structural extrusions.
Tube	Ø small to large, wall thickness varied	Produced by extrusion or drawing; properties similar to sheet/extrusion	O, T6	Used for structural tubing, pipelines for non-pressurized applications.
Bar/Rod	Ø few mm to 200+ mm	Homogeneous; solid bars allow consistent mechanical properties after heat treatment	O, T6	Used for machined fittings, fasteners, and extruded profiles later shaped.

Manufacturing route (extrusion vs plate rolling) strongly influences microstructure and anisotropy; extruded profiles typically have elongated grain structure and directional strength. Heat-treatment capability and achievable quench rates limit properties for thick sections, so design must consider section size, temper, and subsequent fabrication steps like stretching or machining.

Equivalent Grades

Standard	Grade	Region	Notes
AA	6151	USA/International	Recognized within the Aluminum Association system; specific spec details define limits.
EN AW	6151 (AlMgSi)	Europe	Often referenced as EN AW-6151 in European practice; chemical and mechanical limits follow EN standards.
JIS	A6151/A6061*	Japan	Japanese standards may reference nearest Al–Mg–Si grades; check specific JIS designation and temper.
GB/T	6151	China	Chinese GB/T designations commonly use the same numeric family, but tolerances may differ.

Exact equivalency between standards is non-trivial: chemical specification tolerances, required test data, and temper definitions can vary by standard body and product form. Engineers should cross-check certified mill test reports and mechanical property tables when substituting grades between regions.

Corrosion Resistance

6151 demonstrates good general atmospheric corrosion resistance comparable to other Al–Mg–Si alloys due to the protective Al2O3 passive film and relatively low copper and zinc contents. In mildly aggressive environments it performs well, and anodizing further improves appearance and surface protection.

In marine environments 6151 provides acceptable performance for above-water and splash-zone applications but requires design care; pitting and crevice corrosion can occur in stagnant saltwater, especially around fasteners and galvanic couples. Proper surface preparation, anodic or organic coatings, and selection of compatible fasteners are important for long-term durability.

Stress corrosion cracking (SCC) susceptibility is low to moderate for 6151 and is typically much less than for high-strength 7xxx series alloys. However, under tensile stresses and aggressive chloride environments some risk exists, particularly if localized overaging or microstructural inhomogeneity is present. Galvanic interactions favor using similar or more noble metals carefully; aluminum will corrode preferentially when mated with steels or copper unless isolated or sacrificial anodes are provided.

Fabrication Properties

Weldability

6151 is readily welded by conventional fusion processes such as MIG (GMAW) and TIG (GTAW) when design accounts for heat-affected zone (HAZ) softening. Filler alloys like ER4043 (AlSi) or ER5356 (AlMg5) are commonly used depending on the required strength and corrosion resistance; silicon-bearing fillers improve fluidity and reduce cracking. Post-weld heat treatment cannot fully restore T6 properties in the HAZ, and precautions against porosity and hot-cracking during joint preparation and welding parameters are necessary.

Machinability

Machinability of 6151 in softer tempers is good and approaches that of 6xxx family norms, allowing reasonable feed and speed ranges with modern carbide tooling. Chip formation is typically continuous to segmented depending on temper and section; use of positive rake tooling, coolant, and stable workholding improves surface finish. Higher-strength tempers (T6) impose increased tool wear; reduced depths of cut and higher rigidity are recommended.

Formability

Cold formability is excellent in O and H1x tempers, supporting deep drawing, tight bends and complex profiles. In T5/T6 tempers, formability is reduced and springback increases; these tempers are best formed prior to final aging or by using intermediate solution treatment. Recommended minimum bend radii depend on temper and thickness but commonly range from 1–3× thickness in annealed conditions and larger in T6.

Heat Treatment Behavior

As a heat-treatable 6xxx alloy, 6151 follows the typical solution-treatment, quench and precipitation-aging sequence. Solution treatment is performed at temperatures high enough to dissolve Mg2Si (typically in the range used for Al–Mg–Si alloys), followed by rapid quenching to retain a supersaturated solid solution; subsequent artificial aging at moderate temperatures precipitates strengthening phases.

Natural aging (T4) produces an initial gain in strength over time but not to the peak values of artificial aging. Artificial aging schedules (T5, T6) are selected to balance peak strength against toughness and to control distortion; overaging will coarsen precipitates and reduce peak strength while improving ductility and SCC resistance.

For designers, the T651 variant indicates solution heat treatment plus stretching to remove residual stresses before artificial aging, which is important for tight-tolerance extrusions and thick sections where distortion and residual stress can be problematic.

High-Temperature Performance

6151 loses a significant fraction of its room-temperature strength as service temperature rises above typical aging temperatures, with notable softening occurring above ~150–200 °C. Long-term exposure at elevated temperatures accelerates precipitate coarsening and reduces yield strength and fatigue resistance, restricting continuous service temperatures for structural purposes.

Oxidation in elevated-temperature air is minimal compared with ferrous alloys due to the stable alumina film, but aggressive environments and thermal cycling can promote scale spallation and localized corrosion. HAZ regions adjacent to welds are particularly vulnerable to strength loss and grain growth when exposed to heat, so thermal management and post-weld treatments are necessary for elevated-temperature applications.

Applications

Industry	Example Component	Why 6151 Is Used
Automotive	Trim, structural extrusions, lightweight brackets	Good strength-to-weight, surface finish and formability for stamped/extruded parts.
Marine	Deck fittings, rails, architectural elements	Balanced corrosion resistance and anodizability for visible hardware.
Aerospace	Secondary fittings, interior structures	Favorable strength-to-weight and good machinability for non-primary structures.
Electronics	Heat spreaders, chassis	High thermal conductivity combined with structural capability for housings.

6151 is commonly specified where designers need a mid-high strength aluminum that can be anodized or painted and that accepts conventional joining and machining. Its versatility across product forms and tempers makes it a go-to alloy for extruded architectural elements and moderate-load structural parts.

Selection Insights

For lightweight structural parts needing higher strength than commercially pure alloys like 1100, 6151 provides a clear improvement in yield and tensile strength while giving up some electrical conductivity and formability. Choose 6151 when mechanical load capacity and surface finish (anodizing) are priorities and when conductivity is not the primary requirement.

Compared to work-hardened alloys such as 3003 or 5052, 6151 offers higher achievable strength through aging while retaining comparable corrosion resistance in many environments. Opt for 6151 when peak strength and thermal aging capability are desirable; prefer 5052/3003 when formability and marine corrosion resistance under severe conditions are dominant factors.

Against close heat-treatable peers like 6061 or 6063, 6151 may be chosen for specific extrusion performance, surface finish or supply considerations despite often having similar or slightly different peak strengths. Engineers should evaluate temper-specific mechanical data, anodizing behavior and availability when choosing between 6151 and other Al–Mg–Si alloys.

Closing Summary

6151 remains a relevant and versatile Al–Mg–Si alloy for engineering applications that require a pragmatic mix of strength, corrosion resistance and surface quality. Its heat-treatable nature and broad availability in extruded and wrought forms make it a practical choice for architectural, automotive and marine components where balanced performance and finishability are required.