Microstructure, texture and mechanical properties of friction stir welded butt joints of 2A97 Al[sbnd]Li alloy ultra-thin sheets

Abstract

1.4 mm 2A97 AlLi alloy ultra-thin sheets were welded by friction stir butt-welding at different rotational speeds. The influences of rotational speeds on microstructure, texture and mechanical properties of joints were investigated in detail. Microstructure observation shows that the grains in the nugget zone and thermo-mechanical affected zone of joints have a significant refinement compared with the base metal. In addition, the grain size in the joints increased with increasing the rotational speed. Electron backscatter diffraction analysis reveals that the intensity of the original rolling texture including Brass, Copper and S components of the base metal was weaken after welding, and there were some newly generated recrystallization textures in the nugget zone and the thermo-mechanical affected zone of joints, such as Cube and Cube ND components. Moreover, the intensity of recrystallization texture of joints increased while that of rolling texture decreased with increasing the rotational speed. With the change of rotational speed, the proportion of LAGBs and recrystallized grains changed due to the differences of heat input. Tensile test results show that the mechanical properties of joints increased firstly to a maximum and then decreased with increasing the rotational speed from 600 rpm to 1200 rpm. The ductility of joints is also affected by the microstructure, grain boundaries and textures. The joints with the small-sized grains, large proportion of LAGBs and deformation textures possess the sound elongation. All joints failed in the weld center with the toughness or toughness-brittleness fracture modes.