<p>Liquid stir casting is a widely used process for fabricating metal matrix composites due to its cost-effectiveness, efficiency, and ability to produce complex shapes. However, magnesium alloy composites produced by conventional stir casting often exhibit porosity caused by air entrapment and oxidation, poor wettability, and uneven dispersion of agglomerated particles, especially when using nanoparticles. This study aims to address these challenges and enhance the mechanical, thermal, and wear properties of AZ91 magnesium alloy composites by employing ultrasonic-assisted stir casting with nano silicon carbide (30 nm SiC) reinforcement. The process involved the addition of 0.5 wt% magnesium fluoride (MgF₂), continuous argon gas purging, and controlled stirring speed combined with ultrasonic treatment. The effects of processing conditions and nano SiC concentration on the surface morphology, mechanical, thermal, and wear properties of the composites were systematically investigated. Transmission electron microscopy revealed a uniform distribution of nano SiC particles within the AZ91 alloy matrix, resulting in reduced porosity and enhanced composite performance. The optimal composition, incorporating 6 wt% nano SiC, exhibited significantly reduced porosity (0.39%), increased microhardness (88 HV), improved tensile strength (293 MPa), a reduced wear rate (1.2 × 10⁻⁴ mm³/Nm), and decreased thermal conductivity (42 W/m·K). These improvements make the optimized AZ91-SiC composite a promising candidate for automotive component applications.</p>