The effect of ultrasonic treatment (UT) on the synthesis of Al–MgAl2O4 master alloy has been studied by the reaction of SiO2 precursor with Al-2wt% Mg. The results show that SiO2 reaction is enhanced after UT and the corresponding mechanism is proposed. Grain refinement studies in pure Al showed that an increase in addition level of MgAl2O4 particles resulted in two to threefold reduction in grain size; whereas, UT intensified the grain size reduction by 11–12 fold. Significant grain refinement on UT is attributed to the cavitation enhanced wetting which activates MgAl2O4 particles as potent nucleating site for Al. © 2015, The Indian Institute of Metals - IIM.