Evaluation of microstructure, mechanical, and tribological behavior of agro-waste hybrid reinforced high-performance Al-Si-Mg metal matrix composites

This study investigated the microstructure, mechanical, and tribological behavior of high-performance Al–9 wt%Si–1 wt%Mg metal matrix composites reinforced with hybrids of bush mango shell ash (BMSA) and palm oil bunch ash (POBA). The composites were fabricated via stir casting using single-ash reinforcements (2–10 wt%) and hybrid BMSA–POBA systems. Mechanical properties were evaluated while tribological behavior was assessed using pin-on-disc wear tests. Microstructural characterization was conducted using scanning electron microscopy (SEM). The unreinforced Al–Si–Mg alloy exhibited an impact strength of ∼30 ± 0.5 J, flexural strength of ∼65 ± 3 MPa, hardness of ∼170 ± 2.1 BHN, coefficient of friction of 0.17 ± 0.0033, and a wear rate of 4.03 ± 0.037 mm³ /N*m. With BMSA reinforcement, impact strength increased to a maximum of 54 ± 0.5 J at 6 wt% before decreasing to 45.5 ± 1.2 J at 10 wt% due to particle agglomeration, while POBA-reinforced composites reached peak impact strength of 49 ± 0.4 J at 8 wt%. Hybrid composites demonstrated superior impact performance, achieving maximum impact energy of 68 ± 0.8 J at moderate levels of reinforcement. Flexural strength improved significantly from 65 ± MPa to 178.75 ± 2.8 MPa and 146.25 ± MPa for 10 wt% BMSA and POBA composites, respectively, while hybrid composites attained a maximum flexural strength of 205.65 ± 4.2 MPa. Hardness increased steadily with reinforcement content, rising from 170 ± 2.1 BHN for the unreinforced alloy to 210 ± 2.6 BHN and 212 ± 1.4 BHN for 10 wt% BMSA and POBA composites, respectively. Hybrid composites showed the most pronounced hardening effect, reaching a maximum hardness of 344.5 ± 4.1 BHN. Tribological results showed a substantial reduction in friction and wear rate with the addition of reinforcement. The coefficient of friction decreased from 0.17 ± 0.0033 for the base alloy to 0.02 ± 0.0029 for 10 wt% single-ash composites and as low as 0.013 ± 0.0045 for hybrid systems. Similarly, the wear rate declined from 4.03 ± 0.037 mm³ /N*m for the unreinforced alloy to 1.08 ± 0.035 mm³ /N*m and 1.21 ± 0.04 mm³ /N*m for 10 wt% BMSA and POBA composites, respectively, with the hybrid composite achieving the lowest wear rate of 0.89 ± 0.026 mm³ /N*m. SEM analysis confirmed that optimal mechanical and tribological performance corresponded to uniform particle dispersion and strong matrix–particle interfacial bonding, whereas reduced performance at higher reinforcement contents was associated with particle clustering and micro-porosity. • Bush mango and palm bunch ash were used as low-cost, eco-friendly reinforcements in Al–9Si–1Mg alloy. • Hybrid BMSA–POBA composites outperformed single ash, giving best mechanical and wear performance at moderate loadings. • Impact and flexural strength improved with reinforcement due to better load transfer and crack deflection. • SEM showed uniform dispersion, improved properties, while agglomeration and porosity reduced performance at high loadings.