6061 aluminium alloy (Unified Numbering System (UNS) designation A96061) is a precipitation-hardened aluminium alloy, containing magnesium and silicon as its major alloying elements. Originally called "Alloy 61S", it was developed in 1935. It has good mechanical properties, exhibits good weldability, and is very commonly extruded (second in popularity only to 6063). 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).

6061 Aluminium alloy composition by mass:

The mechanical properties of 6061 greatly depend on the temper, or heat treatment, of the material. Young's Modulus is 69 GPa (10,000 ksi) regardless of temper.

Annealed 6061 (6061-O temper) has maximum ultimate tensile strength no more than 150 MPa (22 ksi), and maximum yield strength no more than 83 MPa (12 ksi) or 110 MPa (16 ksi). The material has elongation (stretch before ultimate failure) of 10–18%. To obtain the annealed condition, the alloy is typically heat soaked at 415 °C for 2–3 hours.

T4 temper 6061 has an ultimate tensile strength of at least 180 MPa (26 ksi) or 210 MPa (30 ksi) and yield strength of at least 110 MPa (16 ksi). It has elongation of 10-16%.

T6 temper 6061 has been treated to provide the maximum precipitation hardening (and therefore maximum yield strength) for a 6061 aluminium alloy. It has an ultimate tensile strength of at least 290 MPa (42 ksi) and yield strength of at least 240 MPa (35 ksi). More typical values are 310 MPa (45 ksi) and 270 MPa (39 ksi), respectively. This can exceed the yield strength of certain types of stainless steel. In thicknesses of 6.35 mm (0.250 in) 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 25 °C (77 °F) is around 152 W/m K. The fatigue limit under cyclic load is 97 MPa (14 ksi) for 500,000,000 completely reversed cycles using a standard RR Moore test machine and specimen. Note that aluminium does not exhibit a well defined "knee" on its S-N curve, 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.

Different aluminium heat treatments control the size and dispersion of Mg
₂Si precipitates in the material. Grain boundary sizes also change, but do not have as important of an impact on strength as the precipitates. Grain sizes can change orders of magnitude based upon stress and can have grains as small as a few hundred nanometres, but are typically a few micrometres to hundreds of micrometres in diameter. Iron, manganese, and chromium secondary phases (Fe
₂Si
₂Al
₉, (Fe, Mn, Cr)
₃SiAl
₁₂) often form as inclusions in the material.