Question

which non metal is used to weld metals?

Answer 1

Carbon rods are used in welding as electrodes, flux material and inert gas (Argon or Carbon dioxide to shield from oxygen) are also used in the process. Possibly some filler material is also used.

Some times electrodes are also used as filling materials. Carbon and steel alloy called carbon steel and cast iron - for welding these materials, carbon (non-metal) may be used.
Flux is a non metal used to protect the joint from direct exposure to atmosphere.