###### Abstract  
This study examines the use of ultrafine (\(\sim\)128 nm) microsilica (composed of a mixture of amorphous and microcrystalline silicon dioxide phases) particles, an industrial waste product, as a reinforcing material to create aluminum matrix composites (AMCs) via ultrasonic-assisted stir casting followed by T6 heat treatment. This study aimed to improve the mechanical properties of pure aluminum, which has insufficient strength for most engineering applications. The main objective of this study is to develop environmentally and economically efficient AMCs with improved properties, namely, the balance between strength and ductility, for further application in caliber rolling processes. Attention is also paid to minor reinforcements using a low concentration of microsilica (\(\sim\)0.36%wt), which minimizes the problems with the wettability of the reinforcing material particles. The composites reinforced with ultrafine microsilica exhibited enhanced mechanical performance, including a 59.7% increase in Vickers microhardness and a significant improvement in tensile strength, reaching 73 MPa. Additionally, T6 heat treatment synergistically improved ductility to 60.3% elongation while maintaining high strength, achieving a balanced performance suitable for forming processes. The study results confirm that using microsilica as a reinforcing material is an effective way to improve the performance of aluminum alloys, while minimizing costs and solving environmental problems. aluminum matrix composites (AMCs); pure aluminum; microsilica; T6 heat treatment; mechanical properties; ultrasonic-assisted stir casting 2020 IEEE/RSJ International Conference on Intelligent Systems (IEEE), Vol., pp. 2020-2020, 202  
techniques in solid, semi-solid, or liquid co-states [4]. Among these techniques, the liquid-phase approach, known as stir casting, is notable for its simplicity and cost-effectiveness [5]. The method is continuously being improved [6], and current developments in stir casting aim to enhance the quality of AMCs by integrating additional technologies, such as vacuum assisting [7], die casting [8], and ultrasonic exposure. The ultrasonic-assisted stir casting method demonstrates benefits, including improved dispersion of reinforcing particles, reduced porosity, increased structural uniformity, and improved mechanical properties of composites [9; 10; 11]. Various classes of traditional reinforcing materials, including oxides [12], nitrides [13], carbides [14], and borides [15], have been used to produce AMCs via stir casting. These materials have attracted considerable attention because of their exceptional hardness, high wear resistance, and temperature stability, which are superior to those of most other materials. However, despite their outstanding properties, traditional reinforcing materials are limited by their high cost and difficult access in some regions. These factors have stimulated the search for and development of alternative reinforcement components that provide similar performance characteristics at a lower cost and better accessibility. Recently, there has been increasing interest in agricultural [16] and industrial waste [17] as reinforcing materials for AMCs. Such wastes represent an economically attractive and environmentally sustainable solution, reducing costs and waste accumulation and promoting the development of recycling technologies. Among the agricultural wastes used in AMCs, the most frequently used are ash residues from the processing of cereal crops such as rice husk ash [18] and coconut shell ash [19]. Industrial wastes, including fly ash [20], red mud [21], spent catalysts [22], and magnesite [23], are widely used as reinforcing materials. These materials exhibit promising physical and mechanical properties that can significantly improve the performance of the matrix alloys. Industrial wastes represent more promising reinforcing materials for AMCs than agricultural wastes because of their unique physicochemical properties and high efficiency in improving the matrix performance.