By the time you get the aluminum thickness to equal the tensile strength of Chromoly or IT there is no real weight loss advantage and imo Aluminum is way to soft and brittle to be used in a application like that.
Every material has its place in certain application but aluminum does not belong in a steering stem application.
If ya dont believe me read the link i posted.
http://www.ihpva.org/projects/practicalinnovations/materials.html
My question is 3#s lighter worth your life?