Squirrel cage rotors of aluminum end rings solid state welded to aluminum conductor bars for use in electric motors and methods of making them are described. In one embodiment, the method includes: providing a laminated steel stack having a plurality of longitudinal slots; placing a plurality of aluminum conductor bars in the longitudinal slots, the conductor bars having first and second ends extending out of the longitudinal slots; contacting the first and second ends of the conductor bars with a pair of aluminum end rings under pressure; and oscillating the first and second ends of the conductor bars, the end rings, or both to form an oscillation friction weld.