Final answer:
The subcategories of solid-state welding include ultrasonic welding, friction welding, explosion welding, resistance welding, electron beam welding, laser welding, forge welding, pressure welding, and infrared welding. The heat sources for welding can include gas, electricity, and laser.
Step-by-step explanation:
The subcategories of solid state welding are A. Ultrasonic Welding, Friction Welding, Explosion Welding, C. Resistance Welding, Electron Beam Welding, Laser Welding, and D. Forge Welding, Pressure Welding, Infrared Welding.
The heat sources for welding can include gas, electricity, and laser. Forge welding, used by blacksmiths for thousands of years, involves using heat from a forge fueled by gas or fire to join iron and steel pieces together.