The kWeld is designed to deliver a welding pulse that is extremely short, measured in milliseconds. It is very helpful to have electrodes that have a very low electrical resistance.
If the cables were clamped to the outside of the electrode, and the tip did not need to be easily and cheaply replaceable, pure copper would be a good choice. Customers have preferred having a replaceable tip, and the best design so far uses pure copper rods as the replaceable tip, but doing so requires drilling a socket into the conductive holder.
If the socket was shallow, there would not be enough physical contact between the pure copper tip and the conductive electrode holder. Machining a deep hole in pure copper is difficult, due to how soft the metal is. Tellurium C14500 Is reasonably machinable, but still has a conductivity of IACS 93/100 of pure copper.
The brass electrode holders on the popular Sunkko spot-welder are very easy to machine (cut and drill), but the conductivity of brass is roughly 28/100 IACS. I will certainly look into the two types you listed to see if they might provide some benefit over C14500
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Thanks for posting those options. The RWMA-2 option has a slightly lower conductivity of 85/100, so there is no benefit to using them in this application, however...
The GLIDCOP material is actually a useful option for high-energy welds. If the pure copper tips get so hot and soft that they begin sticking to the material, the "go to" electrode is Tungsten. Although Tungsten has a very high melting temperature, they also have a low conductivity rating of IACS 32/100. This means tungsten electrodes will get VERY hot.
The GLIDCOP material has a conductivity of 92/100, which is very good. For our purposes (spot-welding nickel buses onto 18650 cells) the pure copper tips are working well, but...should you begin using the kWeld for high-energy welds that soften the tips so much that they begin sticking to the material, GLIDCOP is a MUCH better material than pure tungsten. (WCu/ Tungsten-Copper RWMA-11 was my previous better option for high energy).