What is the difference between friction welding and friction stir welding?

Friction welding (or inertial friction welding) is a process where the heat required for welding is obtained by rubbing the parts to be joined together under axial pressure.
Friction stir welding (FSW) is a variant of friction welding. FSW is particularly suitable for joining parts that are not rotationally symmetrical. By means of a threaded pin, the material is heated and softened in order to ensure the joining of the materials and the quality of the weld.
Friction welding consists of only one phase: the friction of the parts to be welded against each other. In contrast, friction stir welding combines two phases: friction and stirring thanks to a rotating tool that penetrates the parts to be welded while advancing.