Microstructural evolution and hardness response in the laser beam welded joints of pure titanium during recrystallization and grain growth

Microstructural evolution and hardness response in the laser beam welded joints of pure titanium during recrystallization and grain growth were investigated by means of electron backscatter diffraction and hardness measurement. The low-angle boundaries (LABs) and high local misorientations, in the as-welded heat affected zone (HAZ) and fusion zone (FZ), indicate the existence of plastic strains caused by the plastic strains due to the thermal stress and the non-equilibrium phase transformations. The recrystallization occurs in the HAZs during heat treatment, but the grain growth after recrystallization does not appear under the heat treatment conditions. Moreover as a result of the slightly high impurity contents in the as-welded FZ, no distinct recrystallization is found in the FZs in this work. During recrystallization in the HAZs, the coarse and unstrained grains with equiaxed shape and smooth boundary gradually consume the fine and strained ones with irregular shape and serrate boundary. Meanwhile, the LABs disappear and the low local misorientations replace the high ones in the modified grains. The recrystallization during heat treatment causes the decrease of plastic strains in the HAZs, which reflects on the descent of average hardness values based on the strain strengthening.