In friction stir welding (FSW) of two similar or dissimilar metals, the work materials are positioned in such a way against a rotating-traveling stirring tool that allow FSW operations to be performed. The operation is begun by pushing the bottom of rotating tool pin against the initial position of weldment to be produced along desired welding line. The push by the bottom of rotating tool pin on the surface of work materials to be welded is made in order to generate local heating by mechanical friction. This book examines the weldment formation in friction stir welding.