Friction stir processing (FSP) is a solid state process in which a non consumable stirring
(rotating) tool is plunged into a work piece up to half thickness which causes intense plastic
deformation material mixing and thermal exposure resulting in refinement of micro structural
properties enhancement of mechanical properties and homogeneity of the processed (nugget)
zone. The FSP technique has been successfully used for producing fine-grained structure and
surface composite modifying the microstructure of materials synthesizing composites like
metal-metal composites. The use of FSP generates significant frictional heating and intense
plastic deformation thereby resulting in the occurrence of dynamic recrystallization in the
stirred zone (SZ). Although there is still a controversy about the grain-refinement mechanism
in the SZ it is generally believed that the grain refinement is due to dynamic
recrystallization. Therefore the factors influencing the nucleation and growth of the dynamic
recrystallization will determine the resultant grain microstructure in the SZ. It has been
demonstrated that the FSP parameters tool geometry material chemistry workpiece temperature
vertical pressure and active cooling exert a significant effect on the size of the
recrystallized grains in the SZ.
