The Role of Ultrasonically Induced Acoustic Streaming in Developing Fine Equiaxed Grains During the Solidification of an
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THEW S. DARGUSCH, and DAVID H. STJOHN are with the Centre for Advanced Materials Processing and Manufacturing, The University of Queensland, St Lucia, Queensland, 4072, Australia and also with the Defence Materials Technology Centre (DMTC), The University of Queensland, St Lucia, QLD 4072, Australia. Contact email: [email protected] QIANG WANG is with the Centre for Advanced Materials Processing and Manufacturing, The University of Queensland and also with the School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an, 710055, Shaanxi, China. NAGASIVAMUNI BALASUBRAMANI is with the Centre for Advanced Materials Processing and Manufacturing, The University of Queensland. MA QIAN is with the Centre of Additive Manufacturing, School of Engineering, RMIT University, Melbourne, VIC 3000, Australia. DMITRY G. ESKIN is with the Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University London, Uxbridge UB8 3PH, UK and also with the Tomsk State University, Tomsk, 634050 Russia. Manuscript submitted May 31, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
INTRODUCTION
APPLICATION of high-intensity ultrasound to the processing of metallic melts has attracted research interest for many years as summarized in monographs.[1,2] Over the last two decades, there has been a revival of interest in studying the fundamentals and to develop technologically viable methods of implementing ultrasonic melt processing in industrial casting processes. To date, ultrasound has been well demonstrated at the laboratory scale to refine a broad range of metals and alloys including Mg alloys,[3,4] Al alloys,[1,3,5–7] steel,[2,8,9] Zn,[10] and a TiAl alloy.[11] The influence of ultrasonication on the refinement of microstructure is based on the physical phenomena caused by high-intensity ultrasound propagation in the melt, in particular acoustic cavitation and acoustic streaming.[1–3] Although it has been recognized that acoustic streaming plays an important role in many ultrasound-assisted industrial processes including degassing, melt cleaning, homogenization, filtration, and waste treatment,[12,13] it has not received as much attention as cavitation. Previous studies have dealt with
the immediate effect of the collapse of the cavities or bubbles but overlooked the effect of acoustic streaming on solidification and grain formation. The lack of focus on acoustic streaming may explain why the exact mechanisms of ultrasonic treatment (UST) refinement are still being debated.[7] It has been reported that acoustic streaming generates convection with typical velocities in the range 0.2 to 0.8 m s 1 with rapid attenuation as the distance from the ultrasonic source increases.[14–19] Acoustic streaming assists in equilibration of the temperature field in the liquid phase and interacts with the solidification front when the freezing range is wide.[15] Recent research highlighted the effect of acoustic streaming in generating convection patterns that facilitate the formation of refined equiaxed zones.[20] It also s
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