Rotor current paths and lamination in an induction motor
Steel/Iron/Ferrous material are used for the rotor laminations to facilitate created a magnetic circuit, a key requirement of an electrical machine. 
The magnetic circuit is not meant to carry any electrical current. This is why they are laminated to reduce the loop size for the induced eddy currents. The thinner the laminations the smaller the eddy currents, great the efficiency 

The slots are filled with electrically conductive material to facilitate creating an electromagnet. Aluminium can be used (for weight) but usually copper is used (conductivity). Sometimes silver (when weight and conductivity are paramount). 

These windings are electrically insulated from the ferrous stator.