Processing Optimization of Sustainable AA6061–Fly Ash Composites by Compocasting

Abstract

Fly ash is an abundant industrial byproduct with potential as a particulate reinforcement in aluminum matrices, yet conventional stir casting often yields poor dispersion and weak interfaces. AA6061/fly ash composites containing 4–12 wt.% reinforcement were fabricated by compocasting (semisolid) and followed by material characterizations: XRD, SEM, fractography, hardness testing, and tensile testing. Reproducibility was assessed by analysis of variance (ANOVA), and performance was benchmarked against stir-cast counterparts. XRD detected no interfacial reaction products. SEM revealed a uniform dispersion of 20–50 μm particles, pore-free interfaces, and grain refinement, attributed to Zener pinning and heterogeneous nucleation. The microhardness doubled from 55 HV (unreinforced AA6061) to 110 HV (fly ash 12 wt.%), while the ultimate tensile strength increased from 140 to 249 MPa (+78%). Ductility decreased from 14% to 5%, consistent with the trade-offs associated with ceramic-particle toughening. Fractography revealed mixed-mode fracture surfaces with both intact and fractured particles, indicating robust interfacial bonding. ANOVA supported measurement reproducibility (p < 0.001). Relative to stir casting, compocasting yielded more uniform dispersion, lower porosity, and cleaner interfaces. Compocasting enables AA6061/fly ash composites with refined microstructures and substantially enhanced strength and hardness at the expense of reduced ductility. The process offers a practical route to valorize fly ash as reinforcement for weight-critical applications (automotive/aerospace) without deleterious interfacial reactions.