Abstract
Computational fluid dynamics (CFD) can be used to predict die filling. However, input parameters must be obtained from rheological experiments (see Chap. 6. In some cases, the data that has been used is from steady state experiments, where the material has been maintained at a particular shear rate for some time. In cases where the process involves taking material from rest into the die in a fraction of a second, it is difficult to see how this can be appropriate. The material changes viscosity by several orders of magnitude in that fraction of a second [23, 32]. Observations of transient rheological behavior under rapid changes in shear rate are therefore more relevant for input data for modeling. Modeling of semisolid processing is generally continuum modeling, where the macroscopic behavior is predicted with the internal structure represented by a few internal variables. Continuum modeling for semisolid can be categorized into onephase or two-phase and as finite difference or finite element. In one-phase modeling, a single non-Newtonian equation of state for the viscosity takes into account the interrelationship between the liquid and the solid through the way in which they behave when they are in combination. In two-phase modeling, each individual phase (the liquid matrix or the solid particles) is characterized with separate equations. The two-phase approach is more complicated, requires significantly higher computational time and much higher effort on obtaining the experimental parameters for input. It is, however, more physically realistic and does allow for important (and undesirable) phenomena, such as liquid segregation to be modeled. Liquid segregation is where liquid gathers in one part of the component in preference to another during processing. Figure 7.1 shows a classic but extreme example. The material is being forced vertically upwards into a die and has to flow around a corner to reach the end of the die. The sharp corner (the dotted line in the inset shows the die shape) causes the liquid to separate from the solid and to gather in one place. In the micrograph, the quenched liquid is dark gray in comparison with the solid. The die has not filled properly. Liquid segregation is deleterious, leading, for example, to inhomogeneous mechanical properties after solidification.
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© 2009 Springer-Verlag Berlin Heidelberg
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Kirkwood, D.H., Suéry, M., Kapranos, P., Atkinson, H.V., Young, K.P. (2009). Modeling of Semisolid Processing. In: Semi-solid Processing of Alloys. Springer Series in Materials Science, vol 124. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-00706-4_7
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DOI: https://doi.org/10.1007/978-3-642-00706-4_7
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