6061 aluminium alloy

6061 is a precipitation-hardened aluminium alloy, containing magnesium and silicon as its major alloying elements. Originally called "Alloy 61S", it was developed in 1935.^[1] It has good mechanical properties and exhibits good weldability. It is one of the most common alloys of aluminium for general-purpose use.

It is commonly available in pre-tempered grades such as 6061-O (annealed), tempered grades such as 6061-T6 (solutionized and artificially aged) and 6061-T651 (solutionized, stress-relieved stretched and artificially aged).

Basic properties

6061 has a density of 2.70 g/cm³ (0.0975 lb/in³).

Chemical composition

The alloy composition of 6061 is:

• Silicon minimum 0.4%, maximum 0.8% by weight
• Iron no minimum, maximum 0.7%
• Copper minimum 0.15%, maximum 0.4%
• Manganese no minimum, maximum 0.15%
• Magnesium minimum 0.8%, maximum 1.2%
• Chromium minimum 0.04%, maximum 0.35%
• Zinc no minimum, maximum 0.25%
• Titanium no minimum, maximum 0.15%
• Other elements no more than 0.05% each, 0.15% total
• Remainder aluminium (95.85–98.56%)

Mechanical properties

The mechanical properties of 6061 depend greatly on the temper, or heat treatment, of the material.^[2] Young's Modulus is 10×10⁶ psi (69 GPa) regardless of temper.^[3]

6061

Annealed 6061 (6061-O temper) has maximum tensile strength no more than 18,000 psi (125 MPa), and maximum yield strength no more than 8,000 psi (55 MPa). The material has elongation (stretch before ultimate failure) of 25–30%.

6061-T4

T4 temper 6061 has an ultimate tensile strength of at least 30,000 psi (207 MPa) and yield strength of at least 16,000 psi (110 MPa). It has elongation of 16%.

6061-T6

T6 temper 6061 has an ultimate tensile strength of at least 42,000 psi (300 MPa) and yield strength of at least 35,000 psi (241 MPa). More typical values are 45,000 psi (310 MPa) and 40,000 psi (275 MPa), respectively.^[4] In thicknesses of 0.250 inch (6.35 mm) or less, it has elongation of 8% or more; in thicker sections, it has elongation of 10%. T651 temper has similar mechanical properties. The typical value for thermal conductivity for 6061-T6 at 77°F is around 152 W/m K. A material data sheet ^[5] defines the fatigue limit under cyclic load as 14,000 psi (100 MPa) for 500,000,000 completely reversed cycles using a standard RR Moore test machine and specimen. Note that aluminum does not exhibit a well defined "knee" on its S-n graph, so there is some debate as to how many cycles equates to "infinite life". Also note the actual value of fatigue limit for an application can be dramatically affected by the conventional de-rating factors of loading, gradient, and surface finish.

Uses

6061 is commonly used for the following:

• construction of aircraft structures, such as wings and fuselages, more commonly in homebuilt aircraft than commercial or military aircraft.^[6] 2024 alloy is somewhat stronger, but 6061 is more easily worked and remains resistant to corrosion even when the surface is abraded, which is not the case for 2024, which is usually used with a thin Alclad coating for corrosion resistance.^[7]

• yacht construction, including small utility boats.^[8]

• automotive parts, such as wheel spacers.

• some tactical flashlights

• aluminium cans for the packaging of food and beverages.

• SCUBA tanks (post 1995)

6061-T6 is used for:

• bicycle frames and components.

• many fly fishing reels.

• The famous Pioneer plaque was made of this particular alloy.

• the secondary chambers and baffle systems in firearm sound suppressors (primarily pistol suppressors for reduced weight and improved mechanical functionality), while the primary expansion chambers usually require 17-4PH or 303 stainless steel or titanium.^[9][10]

• The upper and lower receivers of many AR-15 rifle variants.

• Many aluminum docks and gangways are constructed with 6061-T6 extrusions, and welded into place.
• Material used in some ultra-high vacuum (UHV) chambers

• Many parts for remote controlled model aircraft, notably helicopter rotor components.

Welding

6061 is highly weldable, for example using tungsten inert gas welding (TIG) or metal inert gas welding (MIG). Typically, after welding, the properties near the weld are those of 6061-O, a loss of strength of around 80%. The material can be re-heat-treated to restore -T4 or -T6 temper for the whole piece. After welding, the material can naturally age and restore some of its strength as well. Nevertheless, the Alcoa Structural Handbook recommends the design strength of the material adjacent to the weld to be taken as 11,000 psi (75 MPa) without proper heat treatment after the weld. Typical filler material is 4043 or 5356.

Extrusions

6061 is an alloy used in the production of extrusions—long constant–cross-section structural shapes produced by pushing metal through a shaped die.

Forgings

6061 is an alloy that is suitable for hot forging. The billet is heated through an induction furnace and forged using a closed die process. This patricular alloy is suitable for open die forgings. Automotive parts, ATV parts, and industrial parts are just some of the uses as a forging. Aluminum 6061 can be forged into flat or round bars, rings, blocks, discs and blanks, hollows, and spindles. 6061 can be forged into special and custom shapes.^[11]

Standards

Different forms and tempers of 6061 aluminum alloy are discussed in the following standards:^[12]

• ASTM B 209: Standard Specification for Aluminum and Aluminum-Alloy Sheet and Plate
• ASTM B 210: Standard Specification for Aluminum and Aluminum-Alloy Drawn Seamless Tubes
• ASTM B 211: Standard Specification for Aluminum and Aluminum-Alloy Bar, Rod, and Wire
• ASTM B 221: Standard Specification for Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes
• ASTM B 308: Standard Specification for Aluminum-Alloy 6061-T6 Standard Structural Profiles
• ASTM B 483: Standard Specification for Aluminum and Aluminum-Alloy Drawn Tube and Pipe for General Purpose Applications
• ASTM B 547: Standard Specification for Aluminum and Aluminum-Alloy Formed and Arc-Welded Round Tube
• ISO 6361: Wrought Aluminium and Aluminium Alloy Sheets, Strips and Plates

References

Further reading

• "Properties of Wrought Aluminum and Aluminum Alloys: 6061 Alclad 6061", Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p. 102-103.