As the other poster said copper has a very high thermal conductivity. You would use straight polarity DC tig and high amps for 1/4" at least 250 or higher. You will toast your self when doing it as the heat coming back off the copper that thick is unbearable. You had better have a good water cooled tig torch and some thick gloves also. You would want to weld it in a well ventilated area (you don't want to breathe the copper fumes). AC can do it but it is difficult to get enough heat into the 1/4 copper on AC You would have to be up around 300 - 400 amps and would have to have a huge tungsten. I have welded .120" (~1/8") copper sheet at 200 amps dc straight polarity (3/32" tungsten) with no problems. You could preheat the copper with a torch (~ 500F) and possibly weld it at lower amps. Be careful and anyone in the welding area should have proper protection - eye, skin, hands, and so on.