Transformer core material
There is no significant difference between the magnetic susceptibility of aluminium, and of free space. If that of free space is normalised to 1, transformer iron is in the 2000 to 5000 range, aluminium is 1.000016. So magnetically, the transformer might as well be air-cored.

Aluminium is a good conductor however, better than iron. Even transformer iron has to be formed into insulated laminations, wires or powder to reduce the losses caused by the effective shorted turns that loops of conductor within the core cross-section at right angles to the flux would cause. The same would need to be done with the aluminium core.