Evolution of grain structure, texture and mechanical properties of a Mg–Zn–Zr alloy in bobbin friction stir welding

The evolutions of grain structure, microtexture and mechanical properties during bobbin friction stir welding of Mg-5.89 wt%Zn-0.61 wt%Zr alloy were investigated. The electron backscattered diffraction analyses revealed that the grain structure evolution was a complicated process dictated by continuous dynamic recrystallization but also involved geometrized effects of strain and {10–12} twinning. Local texture varied significantly from the parent metal to the weld nugget. With increased access to the weld nugget, the initial <0001>||ND texture gradually reoriented towards <0001>||TD and finally transformed into a near-<0001>||WD texture. Arising of the near-<0001>||WD texture was associated with the extensive continuous dynamic recrystallization and the reorientations of the recrystallized grains under the effect of shearing deformation introduced by the rotating stir probe. Microhardness distribution exhibited weak grain size dependence. Transverse tensile elongation of the joint was lower than that of the parent metal owing to strain localization in thermo-mechanically affect zone (TMAZ)/weld nugget zone (WNZ) interface region. Decomposition of β₁' precipitates and texture evolutions in WNZ and TMAZ, as well as dislocations concentration and grain size mutation existed in the interface region corporately resulted in the inhomogeneous plastic deformation.