High strength aluminum alloys tend to be brittle and that would cause chipping at the leading edges of the auger (not a good thing). Also the lighter weight would actually be detrimental to the desired effect of a heavier mass of rotating metal having more energy to blow the snow (think of a flywheel). I think steel is probably the best material to use because of it's strength, durability. and cost.
"Man's mind stretched to a new idea, never goes back to its original dimension." -Oliver Wendell Holmes
