Friction Stir Welding (FSW) is a solid-state welding process that uses a non-consumable tool to join two pieces of metal together by applying pressure and friction, creating a weld joint. The tool rotates and moves in a linear motion along the joint line, causing the metal to soften and flow, forming a strong bond between them. This process is used in many industries, such as aerospace and automotive, where it is ideal for joining aluminum, copper and other non-ferrous metals.