Summarize:

Depending on if you are using TIG (tungsten inert gas) or MIG (metal inert gas), and the thickness of your copper, you will need to adjust the ratio of your gas and temperature. As a general rule of thumb, copper under 2mm should have argon gas and up to 160A applied. If the copper is thicker, you will need to adjust the gas and amp by applying helium and upping the amp. Look online or welder's handbook to determine the best ratio for your copper. If the welding isn't going smoothly, try using 100% helium gas. It has a higher power compared to Argon, which allows for more heat to be applied to your copper. Since copper has a high thermal conductivity, high heat and preheating is necessary at thicknesses of over 0.01" (2 mm). Preheating is always measured in degrees and can be anywhere from 50 degrees Fahrenheit to 752 degrees Fahrenheit (10 degrees Celsius-400 Celsius ). To preheat, simply insert your copper into a furnace until the desired temperature increase has been found. Preheating allows the metal to cool slower after welding which works to produce a finer weld. This also reduces the risk of cracking. Once your safety gear is on and you're ready to go, initiate the arc. Hold the arc steady for 2-3 seconds until the puddle appears. When welding with copper, the arc should be at approximately 70 degrees to the copper. An arc is a shooting line of electricity created between the electrode and the material you're welding, copper. As soon as the puddle has appeared, gently apply a small amount of filler through the arc to weld the metals together. Apply the filler by gently probing it into the pudding as the arc welds the copper together. The filler will liquify and mix in with the two copper, helping the metals weld together. Your choice of filler rod will be dependent upon the type of welding you are doing, however, it should always be a copper or copper alloy rod. The longer your puddle is present, the more time the copper has to oxidize. This can prevent a clean weld, so move quickly to get the welding completed. Cooling too fast can mean your weld will break or crack. Slowed cooling is assisted by the preheating of copper. Allowing the copper to come to a heightened temperature before welding will keep the fresh weld warm. If welding indoors, it is acceptable to let the weld air-cool if it has been preheated. When welding in cold temperatures, you may to insulate your freshly welded copper to prevent it from cracking. The simplest way to slow cooling is covering the copper with a fibreglass blanket. Another method is stacking sandbags around the copper to help prevent heat loss. Never splash water or ice on a weld. If you don't have time to patiently wait for the heat to dissipate slowly, don't weld at all.
Use the proper gas ratio. Preheat the copper. Initiate an arc to get the puddle. Add filler into the puddle. Move fast. Allow the copper to cool.