the different between 6082 t6 aluminum sheet and 6082 t651

Aluminium alloy 6082 is a medium strength alloy with excellent corrosion resistance. It has the highest strength of the 6000 series alloys. Alloy 6082 is known as a structural alloy. In plate form, 6082 aluminum sheet is the alloy most commonly used for machining. As a relatively new alloy, the higher strength of 6082 has seen it replace 6061 in many applications.

Aluminium alloy EN AW 6082, BS L115 also corresponds to the following standard designations and specifications but may not be a direct equivalent:
Alloy 6082, UNS A96082, ASTM B209
DIN AlSi1MgMn, 3.2315

BS L115, 6082 is supplied in a wide range of tempers:

O – Soft
T6 – Solution heat treated and artificially aged
T4 – Solution heat treated and naturaly aged to a substantially stable condition
T651 – Solution heat treated, stress relieved by stretching then artificially aged
T6511 – Solution heat-treated, stress-relieved by controlled stretching and minor straightening is allowed after stretching to comply with standard tolerances then artificially aged.