The bed is aluminum alloy. The frame is steel.
http://www.ford.com/trucks/f150/2017/
Scroll about half-way down the page.
Actually, the body and bed are aluminum alloy, not just the bed.
All the advertisements I heard from Ford stated that the frame was made from,”military grade aluminum”. You may know something that Ford’s Advertising Department or whatever the political correct term is doesn’t.
Bed and cab, basically the whole body, sounds like.