Final answer:
Electron Beam Welding (EBW) uses a focused beam of electrons as a heat source for fusion welding, where electrons emitted from a tungsten element in an electron gun are accelerated to create a high-energy beam.
Step-by-step explanation:
The welding process that uses a focused beam of electrons as a heat source for fusion welding is Electron Beam Welding (EBW). In this process, thermal electrons are emitted from a heated element, generally made of tungsten, in an electron gun. These electrons are then accelerated through a potential difference, creating a highly collimated beam of electrons with kinetic energy that can be adjusted according to the selected value of the potential difference. The intense heat generated by the electron beam is sufficient to melt the materials being joined, allowing them to fuse upon cooling.