Figure 1. Tool pin geometry- (A) Four Flute (B) Triangular (C) Hexagonal

From

Corrosion Behaviour of Friction Stir Welded Aluminium Alloys AA6082-T6

H. S. Patil, S. N. Soman

American Journal of Materials Engineering and Technology. 2014, 2(3), 29-33 doi:10.12691/materials-2-3-1