The rotor of the induction motor has a core which is made of electrical steel.
The bars which constitute the squirrel cage are typically made of aluminium or copper. The bars are placed in slots on the rotor core. There is no need for insulation between the bars and the core as the voltage developed in the squirrel cage is very low.
The rotor bars are skewed in order to prevent magnetic locking. Magnetic locking is also known as cogging.
Magnetic Locking can also be prevented by ensuring that the number of rotor slots is not equal to the number of stator slots.