It was shown on laboratory and industrial scale that ultrasonic melt treatment (UST) significantly refines structure of aluminium alloys and improves the quality of castings. However, despite considerable efforts which have been made over decades in the field of ultrasonic processing of aluminium melts, quite a few problems remain unclear. One of them is addressed in this project. The aim of the project is to understand which mechanism is responsible for cavitation-aided grain refinement. It is expected that the knowledge gained as a result of this work can be used in directchill, shape and die casting. The paper describes an experimental setup and first results on the correlation between parameters of UST, solidification conditions and degree of structure refinement. In separate experiments, a model Al-Cu alloy with different amount of solidification sites is solidified with and without UST. The final microstructure is analyzed.