I suppose you would need to weld two hemispheres of plutonium together to make the hollow pit[1] for an implosion device. Or it could be safer to work on smaller pieces and then weld them together.
My guess is the same: that to add structural integrity during the first phase of implosion, so as to guide the implosion process better (rather than having a piece of the pit fly out, say), you'd want to weld the pieces with little plutonium columns between them. I doubt that's how modern bombs work.