Microstructure and texture characterization in friction stir lap welded TIMETAL 21S

The evolution of microstructure and texture during friction stir lap welding (FSLW) of TIMETAL 21S (beta-type Ti-15Mo-3Nb-3Al-0.2Si, wt%) sheets were investigated through electron backscatter diffraction (EBSD). Excellent grain refinement is obtained through stir zone (SZ) thickness (1.2-1.8 mu m). The microstructure of the thermo-mechanically affected zone (TMAZ) is characterized by elongated deformed grains surrounded by small recrystallized grains indicating the occurrence of discontinuous dynamic recrystallization (DDRX). The micro-structure of heat affected zone (HAZ) is quite similar to the base metal (BM). The texture transformed from weak rolling-recrystallization texture in BM and HAZ to a typical shear texture with the domination of D1 or D2 components in the SZ and TMAZ area. A net shear texture gradient is formed across the SZ thickness which is connected with the heterogeneity of deformation. It is believed that the concomitant occurrence of grain size, dislocation and texture strengthening is responsible for the mechanical property distribution in different parts of FSLW joint.