electric motor casing

Typically iron castings are used unless there is a good reason for something else. Just look at the web site of a major manufacturer, for example,

"Automotive approved motors are available in both Standard High Efficiency, our Standard-E, and Premium Efficiency, our Super-E, designs. The Standard High Efficiency motors meet efficiency times power factor requirements of GM7EQ. The Premium Efficiency motors meet the efficiency times power factor of GM7EHQ. Available from stock in ratings from 1 HP to 100 HP. These cast iron motors meet automotive requirements for sound power levels."

"Washdown Duty Motors are available from stock in single and three phase AC and in DC designs. Horsepowers range from 1/2 to 20. Designs include motors having an electostatically applied epoxy finish, paint free with major components in 300 series stainless steel, and all stainless steel. Applications include food processing, packaging, pharmaceuticals, or applications where motors are regularly exposed to high pressure wash down." Pittsburgh Pete

Where weight and corrosion is important I have also run into a few aluminium motors.

While castings dominate high production volume motors, larger low volume motor casings are sometimes fabricated by welding.

The OP didn't say what size motors. Small DC motors I have seen are usually a deep-drawn steel cup, with a press-fit infection-molded cap.

To limit eddy currents in casing non-magnetic materials are used.