The application of heat and deformation during welding significantly affects the development and spatial distribution of mechanical properties and microstructures in welded joints. This study evaluates the differences between shouldered and shoulderless tools in the micro-stir friction welding of 0.8 mm thin plates. Employing a suite of advanced characterization methods, including white light interferometry, electron backscatter diffraction, and scanning electron microscopy, this research weld surface formation, joint microstructure distribution, and fracture characteristics. The enhancement mechanism of mechanical properties is explained through the Hall-Petch relationship and Taylor"s hardening law. The findings indicate that the main reason for the increased yield strength observed in shoulderless tool joints is the combined mechanism of dislocation and fine-grain strengthening. Specifically, the utilization of shouldered tools resulted in a smooth weld surface, with an average grain size of 11.24 μm and a high-angle grain boundary content of 16.80% in the nugget zone. The primary texture components observed were the {011} <100> Goss and {112} <111> copper textures, yielding a maximum texture strength of 3.70. Simultaneously, the fracture dimples exhibited a reduction in size and increased depth; whereas, welds produced with shoulderless tools displayed slight burrs on the surface. The experimental results demonstrate that the average grain size in the nugget zone of these joints is significantly reduced to 0.59 μm, while the high-angle grain boundary content reaches 34.34%. This process is accompanied by the formation of {111} <110> shear and {001} <110> rotated cubic textures as the main components, resulting in a significant increase in maximum texture strength to 6.65.