Rheology and microstructure of semi-solid aluminum alloys compressed in the drop-forge viscometer
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THE fluid-flow behavior of semi-solid metal alloys possessing non-dendritic microstructures has been studied using a variety of characterization techniques. Semi-solid alloys were originally produced by mechanically stirring molten metal as it cooled below the liquidus temperature; therefore, a natural device for examining the fluid-flow behavior was the Couette rheometer, or concentric-cylinder rheometer. The Couette rheometer creates globular semi-solid metal insitu during cooling, and numerous studies have been conducted which characterize the viscosity as a function of shear rate, cooling rate, particle size and morphology, and fraction solid.[1,2] Typical industrial semi-solid forming processes occur at transient, high shear rates and use alloys that were prepared by means other than mechanical stirring. Most commercial processes are conducted at shear rates of 102 to 104 s⫺1 in less than 0.1 seconds. However, rotational viscometers use in-situ prepared semi-solid material and are of limited use J.A. YURKO, formerly Postdoctoral Associate, Department of Materials Science and Engineering, Massachusetts Institute of Technology, is Staff Metallurgist, Idra Prince, Inc., Holland, MI 49424. Contact e-mail: [email protected] M.C. FLEMINGS, Toyota Professor Emeritus of Materials Processing, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted December 5, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
in transient, high-shear rate experiments. Other experimental approaches have also been utilized, including the back-extrusion method;[3,4,5] capillary viscometer,[6,7] indentation,[8,9] and vane techniques;[10] and parallel-plate rotation[11] and compression viscometers.[3,12–15] Although these techniques have more complicated fluid-flow geometries than a concentric-cylinder device, rheological data can be extracted that shed light on the behavior of semi-solid alloys at transient, high shear rates. The parallel-plate compression viscometer was originally used by Laxmanan and Flemings to study Sn-Pb semi-solid alloys in the fraction-solid range of 0.15 to 0.6 and for shear rates less than 1 s⫺1.[12] Other researchers used this device for studying semi-solid aluminum alloys and composites for similar fraction solid and low shear rates.[3,13] Kapranos et al. adapted the parallel-plate viscometer by increasing the compression speed of the plates to achieve shear rates as high as 200 s⫺1.[14] Semi-solid aluminum alloys were studied under a wide range of conditions, and the flow was characterized as a function of processing route, fraction solid, and compression velocity. The instrument has the unique ability to measure force during the initial moments of rapid compression. Recently, a novel compression-type viscometer which can achieve very high shear rates and strain was built at MIT.[16] The drop-forge viscometer (DFV) has a geometry similar to the parallel-plate viscometer, except the upper plate is suspended and then permitte
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