Microstructure and mechanical properties modification of friction stir welded Ti60 titanium alloy via controlled heat dissipation and forced cooling

To obtain a uniform microstructure in the stir zone (SZ) along the thickness direction, back plate with low thermal conductivity and forced cooling on the weld top surface were conducted on the friction stir welded titanium alloy joints. The microstructures, tensile properties, and microhardness distributions across the joint cross-section were characterized in detail. Results indicated that defect-free welds were fabricated at the rotational speed of 150 r/min regardless of welding speeds. When water cooling was applied, fine acicular, refined lamella, and equiaxial α phase can be observed in the top, middle and bottom of the SZ, respectively. Moreover, a more uniform grain size distribution along the thickness direction of SZ was obtained in the water cooling joint. Thus, the UTS of the water cooling joint increased to 1176 MPa accompanied with an excellent ductility. The underlying strengthening and plasticizing mechanism was attributed to the formation of {101¯1} twins induced by water cooling in the SZ.