The Role of Ultrasonically Induced Acoustic Streaming in Developing Fine Equiaxed Grains During the Solidification of an Al-2 Pct Cu Alloy

Recent research and a simulation of heat transfer and solidification during acoustically generated convection showed that the location of the coolest liquid, and thus the place where the first grains are expected to form, is under the sonotrode. Further, the generated vigorous convection produces a very flat temperature gradient in the bulk of the melt facilitating the formation of a refined equiaxed structure throughout the casting. This study validates these findings through a series of experiments on an Al-2 wt pct Cu alloy, which evaluate grain formation under the sonotrode over time and relate this to the formation of the macrostructure of a cast ingot. Analysis of the results confirms the predictions of the simulation and shows that, for the conditions applied, most grains nucleated in the cavitation zone are swept into the melt by acoustically generated convection and, over a period of 70 seconds, the number of grains increase and they grow with spherical and globular morphology gradually filling the casting with refined equiaxed grains. It was found that the macrostructure of each casting is made up of three microstructural zones. A fine grained equiaxed zone forms from the bottom of the casting due to settling of grains during and after termination of ultrasonic treatment (UST), which increases in size with the increasing duration of UST. Above this zone, a coarse-grained structure is formed due to depletion of UST-generated grains on termination of UST. At the top of the casting, a zone of columnar grains growing from the top surface of the melt is formed. The latter two zones decrease in size with the increasing UST duration until 80 seconds, when the macrostructure consists entirely of the equiaxed zone.