Shear rate thickening flow behavior of semisolid slurries
- PDF / 942,765 Bytes
- 10 Pages / 597.28 x 785 pts Page_size
- 51 Downloads / 201 Views
I.
INTRODUCTION
INCREASING attention is being given to net shape processing o f components using technologies involving semisolid precursors. Semisolid processing involves permitting a melt to partially solidify before shape-making operations,tuzl The degree o f solidification may vary from 20 to over 90 pct solid, and the constitutive behavior o f the resulting slurry has a strong dependence on percentage solid. The semisolid is usually stirred while cooling into the two-phase condition, producing a slurry o f rounded solid particles that are smoother than normal dendrites. At solid fractions above 0.05 to 0.1, the slurry behaves as a non-Newtonian, history-dependent fluid. At higher solid concentrations (above 0.5 to 0.6), the slurry may act as a nonlinear viscoplastic solid w h i c h may be handled and processed via high-temperature forming methods used in traditional metal forming, such as forging, extrusion, injection, and rolling. Semisolids frequently exhibit what has been characterized as thixotropy, where the effective shear resistance o r apparent viscosity decreases as deformation occurs. This thixotropic behavior is generally assumed to result from the breaking up o f particle agglomerates due to continued shear deformations. [1,2.3] These agglomerates are characteristic o f stirred semisolids. An arrest o f flow or decrease in flow rate is accompanied by an increase in deformation resistance due to the reformation o f agglomerates. The material consequently exhibits a very strong coupling between flow behavior and the microstrnctural state o f the material. The experimental characterization o f semisolids has not normally corresponded to actual processing conditions, however. T o our knowledge, most o f the experimental characterization has involved "steady state" behavior, where the flow response is measured after tens o f minutes or hours o f constant shearing, tim The measured flow behavior from steady state experiments is shear rate thinning, pseudoplastic, where the apparent viscosity decreases with shear rate. Actual processing, such as die filling or forging, lasts seconds. Since the kinetics o f structure evolution tightly couple to flow behavior, there is no reason to expect that steady state shear rate
thinning behavior represents constitutive behavior during dynamic processing conditions. As we will demonstrate in Section IV, the short-term flow response is very different from the steady state response. The inherent constitutive behavior o f semisolid slurries depends on a n u m b e r o f macroscopic factors, including temperature, shear rate, and characteristics o f the slurry. These factors translate to a set o f state variables that govern the rheology o f the material. A subset o f these state variables is called internal variables o r structure variables and can include a measure o f the degree o f agglomeration a m o n g solid particles, fluid phase viscosity, particle size, particle morphology, and distribution o f particle sizes. Internal variables are distinct from external varia
Data Loading...
