Welding copper is a pain in the butt due to rapid heat transfer. Copper gets EXTREMELY hot. It takes a LOT of amps. Have at least a 250 amp TIG welding machine with water cooled torch. Do a few weld puddles at a time then allow for cooling or the copper will heat warp. Reflective, insulated gloves won’t protect you enough. It is HOT and radiates that high heat, keep your hands 6” away from the weld. Use long ceramic TIG torch cups.
Weld with De-Oxidized copper only. There are many cheap grades of copper not processed for welding that contain copper-oxides. This makes them weld poorly, bubble, pit, and become porous. Liquid tight requires new, traceable, oxygen free copper. De-Oxidized copper is expensive and the price swings wildly. Use De-Oxidized copper TIG filler rod.
In production shops copper is submerge it in cold water with only the weld zone exposed, but it heats the water up rapidly, McDonalds coffee hot.
It is extraordinarily efficient at turning electricity into heat.
Welding heats the whole piece rapidly. Stop frequently to deal with the heat. This is why quality cooking pots and pans have copper bottoms.
It is difficult to weld thin copper. Thin copper is frequently soldered, 800 degrees or less. Welding copper takes time, weld about an inch, then allow cooling. A TIG machine with Pulse Control set at one pulse per second and with Down Slope or Back Slope Control set at about 1/3 background amperage helps control the heat. Cold copper will be softer after welding.
Helium shielding gas provides much better results than Argon shielding gas reducing porosity, pits, and bubbles. It is a must for liquid tight welding. It’s three times the cost of Argon. Helium is lighter than atmosphere so it rises from the weld, more gas volume is required. Argon is heaver than atmosphere so it smothers the weld maintaining an inert welding zone. It is an art to keep the copper weld pool going because it freezes, the molten, liquid weld puddle becomes hard when it needs to be flowing. If you develop the skill set to weld copper practice to retain your skill.
Use stainless steel sheet or weld blankets to reflect or shield the heat.
Here is the heat transfer science graciously sent to me by a user of my website. Heat transfer per unit area in the direction of transfer is proportional to the temperature gradient. Thermal conductivity, k, (W/m-K) is a characteristic of the material. The temperature gradient is expressed as q=k(dT/L) and provides the heat flux (rate of heat transfer per unit area). Pure aluminum has a k=237. Pure copper has a k=401. Alloys reduce these values. Some alloys of aluminum exceed the conductivity of copper alloys.
Since 1964, I have yet to weld an aluminum alloy that radiated heat as intensely as de-oxidized copper.