Interesting side note on the "early/mainstream" days of American aluminum frames: Gary Klein developed his large diameter aluminum frames and received a patent on the design. Some other companies (notably Schwinn) actually paid Mr. Klein license fees to build large diameter aluminum frames. Other companies, notably Cannondale, did not buy a Klein license.
Mr. Klein took Cannondale to court for patent infringement. The cycling community was interested in whether the dimensions of frame tubing was something that could be patented, but in the end the question was decided on entirely different grounds. Bringing in large diameter aluminum frames built by
Bill Shook and
Harriet Fell (wife of the late Sheldon Brown) which predated Mr. Klein's, Cannondale successfully argued that the Klein patent was null and void due to "prior art" - you can't patent something that's already in existence and known publicly.
tcs
IIRC, there were also welded oversize aluminum bikes built by Roger Durham (of Bullseye hubs) built in the early '70's. I think these were also brought up as "prior art".
Googling around, I found this 1890s LuMiNum with cast aluminum frame (Bicycle Quarterly Vol. 8, No. 1):
The Monarch Silver King bikes of the '30's and '40's were also cast aluminum. There's a couple on EBay right now:
