One of the essential technologies in manufacturing fields such as aircraft, automobile bodies, and bridges is joining. Welding is one of the joining methods, and there are various techniques such as arc welding and resistance welding.

Recently, a high-profile next-generation technology is Friction Stir Welding (hereafter referred to as FSW). FSW was developed in 1991 by TWI (The Welding Institute) in the United Kingdom. It works by softening the materials to be joined using frictional heat generated by a rotating cylindrical welding tool, and then stirring them together to form a solid-state joint. Since the materials are not melted during the process, FSW achieves high fatigue strength and minimal welding deformation.