Maybe I am confused. The web page says this..
" These are highly machined 6061 aluminum wheels that are
electroplated with nickel alloy."

Under CBN Wheel 8-inch, 80 grit, 5/8" arbor on the web page it says As
long as the wheels are properly used as intended this should not cause a
problem as they are machined from one solid piece of steel. which is
what I have, but at least some of the others do say they are 6061
aluminum.