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Friction stir processing (FSP) was applied to process a 2 mm thick Al-Mg-Si (6061-T6) alloy plate. The influence of the FSP with the same speed ratio on microstructure evolution, microhardness distribution, tensile properties and corrosion performances of the stirred zone (SZ) in the 6061-T6 alloy was investigated. Results show that there are obvious differences in the microstructure, such as grain morphologies, mean grain size, grain boundary distributions and precipitate distributions of the SZ prepared using the FSP with different processing speeds, which in turn affects the mechanical properties and corrosion resistance. The mean grain size of the equiaxed recrystallized grains of the SZ sample prepared by high speed (8000 r/min, 800 mm/min) is 9.5 μm, which is significantly refined compared to 23.2 μm of the conventional speed (1000 r/min, 100 mm/min) and 13.6 μm of the as-received 6061-T6 alloy. In addition, the distribution characteristics of the precipitates are more similar to that of the as-received 6061-T6 alloy. As a result, in addition to the slight change in corrosion resistance, the SZ sample prepared using the high speed FSP exhibits excellent mechanical properties. The maximum tensile strength and elongation of the SZ sample are 281.5 MPa and 34.8%, which are 86.3% and 122.1% of that of the as-received 6061-T6 alloy, respectively.