In this work, we have introduced power ultrasound in the sump during hot-top DC casting of 152 mm Ø billets to influence the aluminium grain growth morphology . Grains in the central part of the billet are highly dendritic and elongated towards the sonotrode position in the sump. We suggest that ultrasound generates acoustic flow in the sump counteracting natural convection , thereby affecting the direction of grain growth; this is supported by numerical modelling . We found that macrosegregation can be decreased by tuning the position of the sonotrode in the sump and the ultrasonic power. Apart from the central part of the billet , the grain size was found to be significantly refined compared with the reference cast. This could be an effect of forced dendritic fragmentation by ultrasonic melt processing .