Microstructural investigation of friction stir welded pure titanium

As a relatively high melting point material, Ti can be joined by the friction stir welding (FSW) using a sintered TiC welding tool and a water cooling system. The FSW zones of Ti were classified by the weld nugget (WN), the linear transition boundary (TB) and the heat affected zone (HAZ). The WN was characterized by the presence of considerable amount of twins and high density of dislocations. The TB was a linear region of rapid microstructural change from the WN to the HAZ and the HAZ was characterized by slight grain growth. The grain structures of the weld zone were closely related to the hexagonal close packed (HCP) crystal structure of Ti because the twin became dominant deformation mode. The hardness in the WN showed a scattered value, with higher hardness being observed in the region having denser twin structures.