Dynamic recrystallization-driven improvement of corrosion performance in friction stir welded AZ61 alloy

This study examines the influence of tool rotational speed on the microstructural evolution and corrosion behaviour of friction stir welding (FSW) AZ61 magnesium alloy. Four spindle speeds—800, 1000, 1200, and 1400 rpm – were employed to examine the correlation between thermal input, microstructural evolution, and electrochemical response. Optical microscopy, SEM/EDS, and XRD analyses revealed that rotational speed has a significant impact on grain refinement, precipitate size, and phase composition. At 1200 rpm, the optimal microstructure was achieved, with fine equiaxed grains (∼3.89 µm) and uniformly distributed β-Mg₁₇Al₁₂ precipitates. This ensured balanced dynamic recrystallization and controlled thermal exposure. Corrosion tests, including long-term immersion, open-circuit potential (OCP), and potentiodynamic polarisation (Tafel), revealed that the 1200 rpm welds had the lowest corrosion rate (0.0253 mpy) and the most stable passive film formation. Excessive heat input at 1400 rpm caused precipitate coarsening and a slight degradation in corrosion performance, whereas insufficient heat at 800 rpm produced heterogeneous microstructures and severe pitting. The results indicate that an optimised rotational speed of 1200 rpm yields the optimal combination of microstructural uniformity, mechanical integrity, and corrosion resistance.