I have been thinking abut direct-to metal 3d printing for a while and ended up asking about my favourite-so-far deposition method at robotics SE. http://robotics.stackexchange.com/questions/1944/arc-welder-for-3d-printing
Which is kinda odd thing to ask at robotics forum but hey, there is no 3d printing SE. I thought i might get more constructive comments from this community. So please share your thoughts 
I haven’t started prototyping it yet so if you know the proposed deposition method won’t work, please say so:)
Nothing like a great challenge.
Indeed - sounds like a great challenge starting an arc and maintaining it automatically, let alone putting down a relatively even layer of metal.
I was thinking about this as well, the concerns that I had/have are as follows.
1 When welding the part heats through, metal is much more thermally conductive then plastic, this could cause deformation and sag.
2 cost, this is not going to be cheap, welding wire and gas is more expensive the plastic.
3 attainable resolution, because the metal is molten, not semi solid it will spread more then the plastic does.
4 no spanning, because the arc requires a grounded conductor to work you cannot span gaps, probably not able to print any overhangs…
All of that being said, it would be possible to use the welder for additive manufacturing, but the part quality would most likely be worse then a part made via lost pla casting.
So, I’ve done some welding in my time and I would think a MIG head would be an appropriate place to start but then I might be missing something obvious…
Wouldn’t tig be a better bet .
How do you add the material with tig?
@Wayne_Friedt I was thinking the same question. Conceivably you could just use an extruder with hobbed bolt to feed a rod into teh arc but of course you’d then have to keep pausing the print to add new rods unless the thing had some sort of magazine and loading mechanism. MIG seems like a better option as the the MIG head isn’t really that different from a hot end - although you don’t want to let it touch the molten area obviously. And you can just use the standard spool of welding wire as the print material.There are also some pretty small MIG solutions out there - there are even some they use for jewelry I think…
@Camerin_hahn Thank you! I completely overlooked that molten steel is alot less viscous than heated plastic. No overhangs means it is back to the drawing board As to costs of plastic filaments, it is outrageous! here i can buy a kilo of high end copper covered 0.6mm welding wire for less than $16 while 1 kg of plastic filament is around $45 plus shipping.
You can get Tig rolls not cheap but there you go. Realistically its laser that are needed and adding less than we currently do. But heat is the major thing how do you dissipate that
Look up fronius cold metal transfer (cmt). Its a big method with an interesting wave form that cuts the arc as soon as its established. It deposits in droplets without major heat zone. Built for additive manufacturing like processes and open root processes
@andrew_butenko remember 1kg of steel is alot smaller volume then 1 kg of plastic