What is the difference between 6061 T6 aluminum and 6061 T651?

Answer 1

Al 6061 T6 possesses the similar chemical & mechanical properties as Al 6061 T651.

T651 designates that the extrusion has to undergo stretching or permanent set to relieve the extrusion from residual stresses for a uniform machinability. Apart from that, there is no change in the chemical or mechanical properties. Al 6061 T6 is same as Al 6061 T651 & usage of T6 instead of T651 will not affect any functional aspect of the component.