Influence of Remelting Process on Particle Agglomeration, Gas Porosity, and Mechanical Properties of SiC-Reinforced Aluminium Matrix Composites

Abstract Recycling silicon carbide-reinforced aluminium metal matrix composites (Al-MMCs) through conventional remelting presents a significant challenge because the process leads to melt quality and promotes the formation of defects that are difficult to reverse. In this study, industrial AlSi9Mg–20%SiC (Duralcan™ F3S.20S) scrap was remelted six times to examine how repeated melting influences melt cleanliness, chemical composition, microstructural stability, and mechanical performance. Progressive degradation was observed with each cycle. Magnesium levels decreased steadily from 0.546 wt.% in the reference billet to 0.343 wt.% after the sixth remelt, while measured silicon content increased due to non-uniform clustering of SiC particles. Correspondingly, the Density Index increased from 3.09% to 13.93%, indicating a gradual increase in porosity linked to hydrogen absorption and entrapped oxide films. Microstructural analysis showed that consecutive remelting cycles progressively increased SiC agglomeration, reducing the uniformity of particle distribution. SEM–EDX revealed that many of these agglomerates were associated with MgO layers and oxide bi-films, which also acted as nucleation sites for pore formation, collectively reducing the material’s load-bearing capability. As a consequence, the ultimate tensile strength decreased from 310 MPa in the reference billet to below 150 MPa by the sixth remelt. Hardness declined from approximately 105 HBW towards matrix-level values after the fourth cycle, while elongation dropped from 0.9% to 0.3–0.4% across the same range. Overall, the results demonstrate that repeated remelting severely degrades melt quality and disrupts reinforcement effectiveness, confirming that conventional remelting is unsuitable for recycling SiC-reinforced Al-MMCs.