<p>The present study examines the effects of silicon carbide (SiC) nanoparticle reinforcement on the microstructure and mechanical characteristics of aluminium (Al) AA6061-T6 and magnesium (Mg) AZ91D joints produced by friction stir welding (FSW). The primary objective was to assess the influence of FSW process parameters and evaluate the role of SiC nanoparticles in improving weld strength and stability. SiC particles were introduced into pre-configured grooves, and experiments were carried out by varying volume fraction (Vf) (5%, 10%, 15%) of SiC, tool rotational speed (TRS) (600, 700, 800 rpm) and traverse speed (TS) (20, 30, 40 mm/min). Microstructural analysis revealed that the SiC nanoparticles significantly refined the grains in the stir zone (SZ). This refinement was primarily due to the pinning effect of nano-sized SiC particles, which restricted grain growth and facilitated dynamic recrystallization (DRX) during FSW, ultimately leading to a substantial reduction in grain size. Increasing SiC Vf from 5% to 15% enhanced friction stir-welded (FSWed) joints and exhibited improved mechanical characteristics. The optimal joint, within the investigated parameter range, was obtained at 700 rpm, 30 mm/min and 15% SiC, achieving an ultimate tensile strength (UTS) of 114.98 MPa, joint efficiency of 52.5%, and microhardness of 88.9 HV0.1 In contrast, the lowest UTS of 71.46 MPa, 32.63% joint efficiency and 67.87 HV0.1 microhardness was recorded at 600 rpm, 40 mm/min and 5% Vf SiC, mainly due to inadequate mixing and particle clustering, confirming the strengthening role of SiC. These findings illustrate the potential of SiC nanoparticle reinforcement in enhancing the mechanical properties of FSWed joints and demonstrate that the weld quality in AZ91D and AA6061-T6 joints strongly depends on parameter selection to ensure uniform particle distribution and defect-free welds.</p>