Achieving synergistic strength-ductility improvement in as-cast Ca-modified Mg–Mg2Si composites through multi-pass FSP and Cu addition

Multi-pass friction stir processing (MPFSP) overcomes intrinsic brittleness in Mg–3Si-0.5Ca-xCu (x = 0, 1, and 2 wt %) composites via synergistic Cu alloying and severe plastic deformation (SPD). MPFSP fragmented coarse Mg2Si dendrites (∼72.16 ± 38.08 μm in as-cast condition) into well-refined particles, achieving average size of less than 9.21 ± 0.46 μm after 3 FSP passes in the Cu-free composite (AC). Notably, the ACU1 composite (1 wt % Cu) exhibited exceptional refinement of Mg2Si particles to less than about 4.53 ± 0.23 μm (80.66 % reduction compared to AC sample) and near-zero porosity (0.21 %). This promoted dynamic recrystallization (DRX) and α-Mg grain refinement, particle-stimulated nucleation (PSN), and Zener pinning, enabling record properties: 158.70 MPa yield strength (YS), 274.02 MPa ultimate tensile strength (UTS), and 11.57 % fracture strain (FS). Conversely, ACU2 sample (2 wt % Cu) showed mechanical properties saturation following the second FSP pass (∼184 MPa UTS and ∼6.7 % FS) probably due to 12.64 % lower thermal conductivity (compared to ACU1 composite) causing localized overheating, triggering Ostwald ripening (Mg2Si coarsened to 7.63 ± 0.38 μm), particle coalescence, grain coarsening, and higher porosity (0.86 %). Fractography confirmed a transition from brittle/cleavage fracture mode to ductile fracture mode in ACU1 composite. This work establishes composition-controlled thermomechanical processing to break strength-ductility trade-offs in Mg matrix lightweight composites.