Characterisation studies of linear friction welded titanium joints

The microstructure and texture evolution during linear friction welding (LFW) of pure titanium joints were investigated with scanning electron microscope, transmission electron microscope and electron back-scattering diffraction. The unique combination of high temperatures and strain rates during LFW causes limited continuous dynamic recrystallization in the weld center zone (WCZ), leading to a mixed microstructure of refined grains with severe elongated grains. Under the combined effect of axial pressure and shear stress, the texture across the weld line changes significantly. In parent metal, the c-axis of grains is parallel to the RD-TD plane and has an angle of 45° to TD, then in the thermomechanically affected zone the c-axis turns parallel to the welding interface and along the TD, finally in WCZ the c-axis turns to the ND and the P₁ 101¯0112¯0 texture forms. Compared with the ideal hcp shear textures, the present results show that material flow during LFW of titanium indeed arises from the simple-shear deformation and is governed by prismatic slip rather than twinning. The strong texture in LFWed joints is the cause of the anisotropic mechanical properties.