MIG welding is basically a 3d printing pen if you use it right. It feeds filler material from a spool through the welding tip.

Like Mjolinor said, it's surprising how long it has taken. There are heat and contaminate (air, the weld spot is usually surrounded by a bubble of inert gas, ie Argon for Tig and Mig) issues that would need to be addressed, but I can't see why It can't be done. One of my final 'projects' while learning to Tig weld was to butt weld two razor blades, sharp ends, together without using filler. I then was able to T weld the same. If I, a hobbyist welder, can obtain that level of accuracy by hand, I can only imagine what skilled welders can put together in something like this.

Of course, getting the object to come of the print bed may be an issue. I doubt hair spray or a glue stick is the answer.....