Influence of ageing time on the microstructural and mechanical behaviour of Al-Si-Mg/coconut shell ash metal matrix composite

The metal matrix composite of Al-6.7Si-0.2Mg alloy reinforced with measured amount of coconut shell ash with dimensions 146 mm in length and 32 mm in diameter was fabricated using stir casting followed by heat treatment of the composite involving solutionizing, water quenching and aging. The metal matrix composites were aged at 4 h, 8 h and 12 h at a temperature of 180 °C. A comprehensive analysis of the microstructure and mechanical properties of the cast metal matrix composites (MMCs) was conducted using optical microscopy, scanning electron microscopy, X ray diffraction technique, micro hardness test and tensile test. In comparison to as cast, MMC and MMCs aged at 12 h, MMCs aged at 4 h and 8 h presented a finer microstructure showing progressive refinement of the eutectic silicon phase. The maximum hardness value of approximately 130 HV was also observed for MMC aged at 8 h. The ultimate tensile strength and yield strength was highest at approximately 165 MPa and 52 MPa respectively for the MMC aged at 8 h. Density measurements showed minimal porosity for the aged MMCs. The fractography results confirmed ductile fracture for the 4 h and 8 h aged MMCs whereas prolonged aging to 12 h can lead to mixed ductile brittle behaviour.