yup. fairly easy.
guessing you are welding thin pipe, so yes argon should be just fine, a 2% thoriated will be great, and some copper welding wire.
if you are welding thicker plate/pipe helium may work better, as well as preheating.
We do it now and again, we use a foot petal, not sure you could do it without. Jaya's right on 2% when we first started playing with it we used striped #8 electrical wire.
Gas: Argon
you want to run reverse polarity with a tungsten one size larger than you normally would, this will limit the maximum amount of heat you can put in the copper and helps out.
Helps to preheat with OA to about 300 deg or else you'll have issues with lack of fusion because copper is such a great heat sink.
Metarinka,
Reverse polarity (DCEP) is normally not used for welding of most materials due to the limited current capacity of the tungsten. You can approximate using 10% of DCEN current, e.g., for a 1/8" tungsten where a current of 150-400 amps can be used, for DCEP the maximum would be 15-40 amps. You can't weld very thick materials with this amperage level, especially copper.
This is true, however for machines that don't have thumb knobs or foot pedals (or where it would be inconvenient to use them) We used reverse polarity to limit the heat input into the copper. Because the melting point of copper (~1980 deg) is much lower than that of steels, even though copper is a very good thermal conductor we found that using straight polarity you risked concentrating far too much heat in the puddle while waiting for the base material to heat up causing loss of alloy issues and the like. I.E it's more a case of waiting longer than a case of trying to make the puddle hotter. I stand by what I said but I only ever brazed thin copper sheet that maxed at 1/8th so maybe I'm off for techniques on thicker material.
A preheat is generally specificied in for coppper becuase it has a high coefficient of thermal expansion if you tack or align joints at room temeperature they have a tendency to close up BAD at welding temperature. This also helps with establishing a puddle. and getting good fusion with the base material.
Finally I was always told that technically you can't "Weld" copper only braze it since the definition of brazing is joining over 800 deg F with non ferrous alloys which would be the case if you using some sort of copper filler. That's neither here nor there but I thought any joining process on copper and it's alloys (bronze, brasses) was considered brazing?
If the filler melts at a lower temperature than the copper base metal brazing would be the proper term. If the filler is actually copper, and has the same melting point as the base metal the process would be welding.
Another term frowned on by some, is braze welding. In this process a bead or fillet is deposited, The intent of this term is to difirentiate between the typical brazing process where filler is drawn into the joint by capilary action.
With regard to the ammount of heat input, the overall size of the workpiece becomes important due to the high themal conductivity, large parts absorb incredible ammounts of heat before You can get a molten puddle to form.
I have done this with up to 1/4" sheet stock. As far as fillers go, I don't have very much in the way of experience; we usually used some electrical wire or sheared a bunch of thin sheet into very narrow strips. I always ran a 2% thoriated, DCEN and higher current to get that heat in there. The problem with copper is that it conducts heat away from the weld very quickly, so hot and fast is the key. The cleaner, the better; make sure you clean your filler with some emery cloth and alcohol or acetone, too. And argon is fine. Good luck!
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