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• The phenomena occurring near or on the melt free surface during ultrasonic treatment for molten aluminum are examined. • Ultrasonic treatment can greatly promote the formation of alumina particles on the melt surface around sonotrode. • A part of formed alumina particles can be entrained from the free surface into the melt bulk. • The entrained alumina particles can serve as nucleation sites for primary Al3Zr compounds. Although the ultrasonic treatment of molten aluminum has been studied for long period, there is still much to be revealed for this process. Many studies have focused on the investigation of acoustic cavitation and streaming under the horn tip and their effects on the treatment efficiency. However, to the best of our knowledge, no attempt has been done to explain phenomena occurring near or on the melt free surface. Thus, the goal of this study is to investigate phenomena occurring at the free surface during ultrasound irradiation and clarify their possible influence on the ultrasound treatment performance. The results of high temperature and water model experiments reveal that ultrasound irradiation significantly promotes the formation of alumina particles on the melt free surface around sonotrode, and part of these particles can be entrained into aluminum melts. Furthermore, TEM observation results suggested that the entrained alumina inclusions can serve as nucleation sites for the primary Al 3 Zr compounds. Most importantly, the oxidation and entrainment of particles from free surface are likely to be controllable by the immersion depth of sonotrode into molten aluminum. [ABSTRACT FROM AUTHOR]