Role of Acoustic Streaming in Formation of Unsteady Flow in Billet Sump during Ultrasonic DC Casting of Aluminum Alloys

Abstract

The present work investigated melt flow pattern and temperature distribution in the sump of aluminum billets produced in a hot-top equipped direct chilling (DC) caster conventionally and with ultrasonic irradiation. The main emphasis was placed on clarifying the effects of acoustic streaming and hot-top unit type. Acoustic streaming characteristics were investigated first by using the earlier developed numerical model and water model experiments. Then, the acoustic streaming model was applied to develop a numerical code capable of simulating unsteady flow phenomena in the sump during the DC casting process. The results revealed that the introduction of ultrasonic vibrations into the melt in the hot-top unit had little or no effect on the temperature distribution and sump profile, but had a considerable effect on the melt flow pattern in the sump. Our results showed that ultrasound irradiation makes the flow velocity faster and produces a lot of relatively small eddies in the sump bulk and near the mushy zone. The latter causes frequently repeated thinning of the mushy zone layer. The numerical predictions were verified against measurements performed on a pilot DC caster producing 203 mm billets of Al-17%Si alloy. The verification revealed approximately the same sump depth and shape as those in the numerical simulations, and confirms the frequent and large fluctuations of the melt temperature during ultrasound irradiation. However, the measured temperature distribution in the sump significantly differed from that predicted numerically. This suggests that the present mathematical model should be further improved, particularly in terms of more accurate descriptions of boundary conditions and mushy zone characteristics.

Characterization of acoustic streaming in water and aluminum melt during ultrasonic irradiation

“…On the contrary to the cavitation bubble and ultrasound pressure distribution, characteristics of acoustic streaming in aluminum melt have been investigated in much lesser extent. A proper understanding of acoustic streaming features is important for casting technology including Direct Chill (DC) casting process [12] , [13] , [14] . Because of the above mentioned experimental difficulties in the case of molten metals, numerical simulation with combination with water model experiments remains the main investigation approach in this area.…”

Free surface entrainment of oxide particles and their role in ultrasonic treatment performance of aluminum alloys

“…This effect is commonly called modification. Besides, when ultrasonic vibrations are introduced into the sump of a DC caster, the acoustic streaming and turbulent oscillations alter characteristics of the mushy zone [19] , [20] , [21] that can be effective in preventing hot-tearing in high-speed casting of aluminum alloys [22] and in improving the quality of billet surface [18] .…”