Dude, first off - you don't "weld" copper. Coppper is soldered or braised. Second - that coopper pipe has pressurized refrigerant in it! You want to blow up your face? You would need to drain the sytem with a special freon vacuum line (it's illegal to vent refrigerant into the air), and you may possibly be able to mend the pipe with a collar new seam, but this whole job is for someone who knows what the hell their doing. You try to weld in your attic and you will burn your house down!