6061 T6 Forged Aluminum

T6 temper 6061 has been treated to provide the maximum precipitation hardening (and therefore maximum yield strength) for a 6061 aluminum 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.^[10] 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. A material data sheet ^[11] defines the fatigue limit under cyclic load as 97 MPa (14 ksi) 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.

Microstructure[edit]

Different aluminum 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 nanometers, but are typically a few micrometers to hundreds of micrometers in diameter. Iron, manganese, and chromium secondary phases (Fe
2Si
2Al
9, (Fe, Mn, Cr)
3SiAl
12) often form as inclusions in the material.^[12]

Grain boundaries in extruded plate 6061 aluminum alloy

Grain sizes in aluminum alloys are heavily dependent upon the processing techniques and heat treatment. Different cross-sections of material which has been stressed can cause order of magnitude differences in grain size.^[13] Some specially processed aluminum alloys have grain diameters which are hundreds of nanometers,^[14] but most range from a few micrometers to hundreds of micrometers.^[15]