In present study, 6061 and A356 based nano-composites are fabricated by using the ultrasonic stirring technology (UST) in a coreless induction furnace. SiC nanoparticles are used as the reinforcement. Nanoparticles are added into the molten metal and then dispersed by ultrasonic cavitation and acoustic streaming assisted by electromagnetic stirring. The applied UST parameters in the current experiments are used to validate a recently developed magneto-hydro-dynamics (MHD) model, which is capable to model the cavitation and nanoparticle dispersion during UST processing. The MHD model accounts for turbulent fluid flow, heat transfer and solidification, and electromagnetic field, as well as the complex interaction between the nanoparticles and both the molten and solidified alloys by using ANSYS Maxwell and ANSYS Fluent. Molecular dynamics (MD) simulations are conducted to analyze the complex interactions between the nanoparticle and the liquid/solid interface. The current modeling results demonstrate that a strong flow can disperse the nanoparticles relatively well during molten metal and solidification processes. Molecular dynamics simulation results prove that ultrafine particles (<< 1 µm) will be engulfed by the solidification front instead of being pushed, which is beneficial for nano-dispersion. Experimental results confirm that the nanoparticles are dispersed reasonably well in the metal matrix, but some insignificant agglomeration still occurs. Besides, SEM/EDS results show that C element tends to gather around the grain boundary area where the Si eutectic phase is located.