Which welding process does not rely on an external power source?

Oxy-fuel welding is a process that relies on the combustion of fuel gases, typically acetylene and oxygen, to produce a flame that can melt metal. This method does not require an external electrical power source to generate heat, which distinguishes it from the other welding processes listed.

In contrast, MIG (Metal Inert Gas) welding, TIG (Tungsten Inert Gas) welding, and arc welding utilize electrical energy to create the necessary heat for melting and joining metals. These processes require a power source to generate an electric arc, which provides consistent and controlled heat for welding operations. In summary, the unique characteristic of oxy-fuel welding is its reliance on a chemical reaction for heat rather than an electrical power source, making it the correct answer.