Ultrasonic melt treatment (UST) using a single sonotrode source in a launder is an efficient way to treat a large-volume melt. One key parameter is the melt processing temperature. Melt processing temperature affects the acoustic pressure generated by the sonotrode, which ultimately defines the cavitation development as well as the resulting acoustic streaming. Experimental results also show that processing temperature affects intermetallic number density and the final grain size. This work presents a numerical model covering acoustic cavitation, flow (including acoustic streaming), and heat transfer in direct-chill (DC) casting, to better understand this process. The UST effectiveness is quantified through the size of the high-pressure acoustic region and the melt residence time, a result reflected in experimental grain size data. The output of this work is useful for optimizing the selection of process parameters for UST DC casting.