Aluminum risers are cut out of extrusions,not forged.I would personally like to see a forged, aluminum riser.That would be the most stable and strongest riser one could get.
I know that Bowtech and , pretty sure Martin, claim their risers are machined from forged billets of aluminum.
From Bowtech
It begins with a single-piece riser crafted of forged aluminum and then machined into shape.
Martin advertises this on numerous magazine back covers.