Friction stir welding tool insert

A friction stir welding (FSW) tool insert comprising PCBN, has a longitudinal axis of rotation 12, with coaxial shoulder 16 and stirring pin 14. The shoulder 16 diameter D, and pin 14 height H, satisfy a relative inequality, whereby D < 4.5H, i.e., the ratio between i) the largest linear dimension D (36, fig 1) of the shoulder 16 measured perpendicularly to the longitudinal axis of rotation 12; and ii), the pin height H (34, fig 1) measured parallel to the longitudinal axis of rotation 12 between a base of the stirring pin 14 and a maximum point of extension of the stirring pin 14; is less than 4.5. The integral polycrystalline cubic boron nitride tool may weld steel plates at least 12 mm thick, e.g. 20 mm. The shoulder 16 may have a spiral scroll 30. The pin height H may be 98% of the thickness of the plate.