Microstructural evolution and tribological and corrosion performance of friction stir welded AZ91 magnesium alloy at different rotational speeds

This study investigates the influence of tool rotational speed on the microstructural evolution, wear behavior, and corrosion performance of friction stir welded AZ91 magnesium alloy. The friction stir welding (FSW) process was carried out on a cylindrical threaded pin at rotation speeds of 700, 800, 900, and 1000 rpm, with the traveling speed held constant. The cylindrical threaded pin facilitated material flow, frictional heat generation, and dynamic recrystallization by increasing stirring. Optical microscopy results indicated grain refinement, with a decrease in grain size from 6.2 μm at 700 rpm to 4.1 μm at 900 rpm, corresponding to a reduction of about 33.9%. Further increasing the speed to 1000 rpm resulted in grain coarsening from 4.1 μm at 900 rpm to 5.9 μm, about 43.9% larger. The Scanning electron microscope (SEM) investigation showed fragmentation and rearrangement of β-Mg 17 Al 12 precipitates. The most uniform precipitates were distributed at 900 rpm, while there were much precipitation dissolution and precipitate-free regions at 1000 rpm. X-ray diffraction confirmed that the main phases are α-Mg, β-Mg 17 Al 12 , Al 8 Mn 5 , and MgZn 2 . The lowest wear depth and coefficient of friction (COF) values were achieved for the sample at 900 rpm, at about 1800 μm and 0.58–0.62, respectively. Electrochemical polarization testing revealed that the 900-rpm condition exhibited the most favorable corrosion response, with an Ecorr of 0.49 V and the lowest Icorr of 1.3 × 10 −3 A/cm 2 , indicating improved corrosion resistance owing to grain refinement, microstructure homogenization, and reduced microgalvanic coupling.