Piece of cake if you know how.
As said depends what it is, and shape as you can clamp it down to stop warp.
You apply heat gum, that stops most of the heat going elsewhere.
I have welded m/bike casings where chain has jumped etc and taken a chunk, not had a problem yet as tig is so controllable, bloody hot but very localised too.
I have even welded a few areas around where bearings went in these cases with no noticeable distortion.
I always heat the surrounding area on lighter alloys, depends what alloy you are talking about.
tabetha