reviving this old thread to share some info I just stumbled upon. no, this doesn't affec tultrasonic welds, but most of the time these power bricks aren't welded shut, but held together with a solvent.
if oyu don't care about the smell, wick some gasoline into the seam, wait 30 seconds, then wick some more into the seam and wait another minute or two, and it will cleanly pull apart. there are sprays you can buy that do this as well with less smell but they aren't as strong so your mileage may vary.