Final answer:
Friction Stir Welding is a solid-state joining technology that joins metals through the heat generated by the friction between a rotating tool and the workpieces, leading to a joint formed by the mechanical intermixing of the softened materials.
Step-by-step explanation:
The correct answer to the student's question about the relatively new welding method in which heating is caused by the friction of a rapidly rotating tool with a protruding tube is C. Friction Stir Welding. This is a solid-state joining process that uses a non-consumable tool to join two facing workpieces without melting the workpiece material. Heat is generated by friction between the rotating tool and the workpiece material, which leads to a softened region near the FSW tool. While moving the tool along the joint line, it mechanically intermixes the two pieces of metal at the place of the joint, then the softened metal can be joined by pressure, applied by the tool, typically a cylindrical shoulder and a threaded pin. It was primarily used on aluminum, and it is gaining popularity due to its environmental and health advantages over traditional welding techniques.