•An analytical model is developed to predict the cavitation induced undercooling, grain nucleation and the solidified grain size.•Cavitation bubble implosion in metallic melts were imaged in-situ by ultrafast synchrotron X-ray imaging.•The model takes into account of ultrasound input intensity, cavitation bubble size and the effect of melt temperature.•The solidified grain size of different alloys were calculated using this model and compared with the experimental data. Microstructural refinement of metallic alloys via ultrasonic melt processing (USMP) is an environmentally friendly and promising method. However, so far there has been no report in open literature on how to predict the solidified microstructures and grain size based on the ultrasound processing parameters.In this paper, an analytical model is developed to calculate the cavitation enhanced undercooling and the USMP refined solidification microstructure and grain size for Al-Cu alloys. Ultrafast synchrotron X-ray imaging and tomography techniques were used to collect the real-time experimental data for validating the model and the calculated results. The comparison between modeling and experiments reveal that there exists an effective ultrasound input power intensity for maximizing the grain refinement effects for the Al-Cu alloys, which is in the range of 20-45 MW/m2. In addition, a monotonous increase in temperature during USMP has negative effect on producing new nuclei, deteriorating the benefit of microstructure refinement due to the application of ultrasound.