Gas-Enhanced Ultrahigh-Shear Mixing: An Application to Molten Aluminum Alloys
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, FRANK BENKEL, and GABRIEL BIRSAN
A new mixing technology that explores an integration of ultrahigh shearing with gas injection, directly into the mixer shear zone, has been applied to molten aluminum alloys. The refining and homogenizing effects were assessed through microscopic observations of solidified structures. For the set of process parameters applied, the ultrahigh shear alone caused structural refinement, which doubled the sole effect of gas flotation. Combining ultrahigh shear with gas injection magnified the structural refinement, which substantially exceeded the individual effects, caused by gas flotation and ultrahigh shearing. In addition to matrix grain-size reduction by almost two orders of magnitude, the complex intermetallic compounds, being inherently coarse in conventional castings, were also refined. The results confirmed our earlier observations made through transparent media that an interaction of large volume of fine gas bubbles with the liquid, superimposed on ultrahigh shear, leading to intensive cavitation, generated in the cylindrical rotor–stator apparatus, drastically enhanced the treatment outcome. https://doi.org/10.1007/s11663-020-01803-1 Ó Crown 2020
I.
INTRODUCTION
HIGH-SHEAR mixing through imparting vigorous radial and axial flow patterns in solid–liquid–gas media, causing homogenization, emulsification, dispersed phase size reduction, and de-agglomeration is used throughout many industries, including chemical, pharmaceutical, food, plastic, ceramic, or rubber.[1] An application of high-shear mixing in metallurgy to liquid or semisolid metals affects solidification and may be used to improve casting integrity, grain refinement, reducing size of coarse intermetallic compounds, generation thixotropic morphologies for semisolid processing, homogenizing immiscible metals, or creating metal matrix composites.[2,3] The technique was found effective in nano-processing for dispersion of nanomaterials into low-viscosity formulations and in molecular level mixing to generate the graphene-based composites.[4] An improvement of properties achieved through high-shear treatment fueled a search for novel mixing technologies with more powerful energy dissipation and higher impact on processed materials. An idea of exploring gas bubbles is used in a new mixing technology, called gas-enhanced ultrahigh-shear (GE-UHS) mixing, developed by the present authors. The distinct feature of this invention is an integration of gas bubbles and
ultrahigh shear, which is achieved through an injection of external gas directly into the shear zone of the cylindrical mixer. In our previous report,[5] the concept and prototype design along with initial test results were described. As revealed via experiments with transparent media, combining gas injection and shear within the rotor/stator couple, essentially changed the treatment outcome through sharp increase in mixing effects. The water–air test does not allow, however, for full assessment of the mixing power of the new concept, especially in metallurgical applications. Altho
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