AFAIK, Same-o same-o -- they have exactly the same mechanical properties
for machining and welding. the T651 is T6 material that's been
mechanically stretched or stamped to straighten it.

Richard Hanley wrote:

I have a part to make that calls for 6061 T6. I have some 6061 T651.
Can anyone tell me the difference between them?
Thank You Richard