Scroll shoulder tool is a relatively new friction stir welding (FSW) tool introduced by The Welding Institute of UK in 1999. It offers extra advantages of eliminating the tilted tool axis that is usually used in conventional tool FSW and performing non-linear thick section FSW with a relatively simple machine. However, the forming mechanism of its shoulder flow zone still remains unidentified. This mechanism is fundamental in designing a proper scroll shoulder tool to produce a defect-free weld. This monograph, firstly, systematically reviewed FSW tool design, weld zone material flow pattern...