Synthesis, characterization, and ultrasonic-assisted multi-objective machining optimization of marine waste-reinforced sustainable AZ91D magnesium hybrid composites

Developing sustainable lightweight materials using marine waste-derived reinforcements offers a promising pathway to resource-efficient manufacturing. This study investigates fabricating AZ91D magnesium hybrid composites reinforced with fish bone ash and seashell ash, along with graphite, zinc, silicon carbide, and boron carbide, using a stir-casting approach. Incorporating bio-waste-derived reinforcements aims to enhance the functional performance of magnesium alloys while promoting waste valorization and reducing dependence on conventional reinforcement materials. The fabricated hybrid composites were systematically evaluated for microstructural characteristics, mechanical behaviour, corrosion response, and machining performance. The machining behaviour was further optimized using a hybrid decision-making framework integrating Response Surface Methodology (RSM), Spherical Fuzzy Analytic Hierarchy Process (SF-AHP), and Fuzzy Measurement Alternatives and Ranking according to Compromise Solution (Fuzzy MARCOS). The study analyzed the effects of key ultrasonic-assisted turning parameters, including cutting speed, feed rate, and ultrasonic power, on surface roughness, material removal rate, energy consumption, and tool wear. The results show that incorporating marine waste-derived reinforcements and ceramic phases improves the structural integrity, wear resistance, and corrosion performance of the AZ91D matrix. Furthermore, the hybrid optimization methodology effectively identifies suitable machining conditions by balancing multiple conflicting performance responses. The proposed sustainable composite development strategy provides an integrated approach for converting marine waste into value-added engineering materials while improving machining efficiency. This work advances environmentally responsible magnesium-based hybrid composites for potential lightweight structural and advanced manufacturing applications.