Texture features and strengthening mechanisms in welding nugget zone of SSFSWed thick-plate Al–Li alloy joint

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In order to investigate the microstructure evolution, texture features, and strengthening mechanisms at different positions in the welding nugget zone (WNZ) of thick-plate joints in depth, stationary shoulder friction stir welding (SSFSW) tests were conducted on a 12-mm thick 2195-T8 Al–Li alloy with rotating speeds of 300–500 rpm. At 300 rpm, the results demonstrate that WNZ is mostly made up of equiaxed grains orientated in <111> and <101>//TD, with a homogeneous grain size distribution along the plate thickness. In contrast, at 500 rpm, the crystal orientations convert to <001> and <101>//TD, and the grain size at the bottom of WNZ reaches only 39.3% of that at the top. At 300 rpm, WNZ is predominantly composed of shear textures and remains a large number of needle-like precipitates T1 and θ′, both of which contribute to the improvement in mechanical properties. The weld zone exhibits higher microhardness and tensile strength along the plate thickness at a lower rotating speed, retaining a high level within 8 mm from the weld surface but gradually decreasing from 8 to 12 mm as the rotating speed increases. In addition, the strengthening mechanism of the whole WNZ is dominated by precipitation strengthening at 300 rpm, which is replaced by dislocation strengthening at 400 rpm. As the rotating speed increases to 500 rpm, grain refinement strengthening takes over as the dominant strengthening mechanism in the WNZ.