Peening or Peining I believe, often done with the ball end of a ball pein / ball peen hammer. Not quite sure of why the different spellings, perhaps US/UK.
I once saw a photo of a Napier aero engine (Air and Space Smithsonian I think) and the close-up showed every bolt beautifully hammered into an almost perfect dome over the end of the nut. From my experience it is great for keeping a nut on, but but not so fun if/when you need to get the nut off in a hurry