Raghu Ram N, Sivaji Babu K, B Bala Krishna
Abstract The present study investigates the effect of tool rotational speed on the microstructural evolution, phase transformation, mechanical response, surface residual stresses, and damping behavior of friction stir processed (FSPed) AZ61 magnesium alloy. To optimize the thermal-mechanical balance, experiments were conducted at four different rotational speeds (900, 1000, 1100, and 1200 rpm) under constant traverse conditions. Optical microscopy, SEM-EDS, and XRD analysis reveal that increasing the tool speed leads to dynamic recrystallization and redistribution of the β -phase, resulting in grain refinement and improved homogeneity up to 1100 rpm. The specimen processed at 1100 rpm had the finer grains (4.5 ± 2.3 μm), dense fine precipitates, and an ideal α -Mg + β -Mg 17 Al 12 phase balance. This condition resulted in the highest microhardness (≈108 VHN), maximum compressive residual stress (−450 MPa), and excellent damping capacity (tan δ ≈ 0.11 at 30 Hz). However, at 1200 rpm, excessive heat caused partial dissolution of the precipitate and grain coarsening, reducing both hardness and damping efficiency. The findings reveal a strong correlation among grain refinement, precipitate morphology, and internal stress-relaxation mechanisms, with a processing speed of 1100 rpm optimal for enhancing the mechanical and functional properties of FSPed AZ61 alloys.