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In this work we revisit the issue of obtaining true material constants for semisolid slurries. Therefore, we consider the circular Couette flow of Herschel-Bulkley fluids. We first show how true constants can be obtained using an iterative procedure from experimental data to theory and vice versa. The validity of the assumption that the rate-of-strain distributions across the gap share a common point is also investigated. It is demonstrated that this is true only for fully-yielded Bingham plastics. In other cases, e.g., for partially-yielded Bingham plastics or fully-yielded Herschel-Bulkley materials, the common point for the fully-yielded Bingham case provides a good approximation for determining the material constants only if the gap is sufficiently small. It can thus be used to simplify the iterative procedure in determining the material constants.

The objective of this presentation is to show and explain why semisolid slurries can fill thin sections at seemingly unlimited flow velocity; the suitability of SSM parts with very thin sections is a characteristic of the process that is often overlooked by the industry. This fact provides a unique opportunity for new advanced applications of the process, not possible by any of the existing metal-mold casting methods.

An essential element for the integration of a semisolid process in the production of complex commercial components is the availability of accurate mathematical and computational tools that could describe both the rheological behavior and the material characteristics of the suspension, which are strongly affected from its internal structure and its evolution during deformation. In this study we considered the squeeze flow experiment, which is a standard method used to determine material properties of semisolid slurries, where a fixed amount of material is compressed from its topside either under constant load or constant velocity, while the bottom side remains fixed. Through high fidelity computational modeling we simulated the classical compression experiment by including the effects of thixotropy in order to demonstrate its role in determining material constants. More specifically a structural viscoplastic model based on the Bingham plastic constitutive equation is proposed. The yield stress is assumed to vary linearly with the structural parameter which follows a first-order rate equation accounting for the material structure break-down and build-up. The development of the yielded/unyielded regions in relation to material structural changes is analyzed. Furthermore, we performed also simulations, where the compression is interrupted for a short time, in order to study the material internal structure after a short relaxation time.

A standard method used to determine material properties of semi-solid slurries is the squeeze flow experiment in which a fixed amount of material is squeezed under constant force or velocity. The relation between the force and the displacement provides information about the rheology of the slurry. The thixotropy and the time response of the sample however is rarely, if ever, taken into consideration. In this work we study how thixotropy affects the flow characteristics and consequently the predicted material properties. We show that depending on the method of compression and the thixotropic constants the flow can be significantly different. Therefore the predicted material constants can vary and hence cannot be unique.

When SSM material is subjected to a sudden transient the rate of buildup (aging) is negligible compared to the rate of breakdown (shear rejuvenation). While this is generally true, due to prolonged processing or the geometry the local shear rate in some regions may become equal or lower than the critical value, where aging becomes as important as shear rejuvenation. In this work we simulate in detail shear rejuvenation and aging in semisolid slurries. Using a standard thixotropic model used widely for modeling SSM suspensions but a novel computational method we reveal and confirm numerically for the first time shear banding. The phenomenon is found to be time dependent where the material first yields fully and then, after a certain time the yielded front retreats to form two distinct bands -one yielded and one unyielded. This phenomenon must be accounted for in the evaluation of the material constants since the time scale of the process is similar to the time scale of the phenomenon.

Due to the distinct rheology of semisolid slurries the process has well established advantages over competing near-net-shape manufacturing technologies. Despite the obvious advantages of the process its adoption by the casting industry, however, has been slow. This is primarily due to lack of confidence of how these slurries flow in die cavities. The added cost associated with the specially prepared slurry has also affected the process commercial success. Nevertheless despite these problems attention to the semi-solid metal process has indeed increased over the years. The main focus of this review is the modeling of semisolid slurries. The objective here is to present an overview of relevant aspects of modeling by focusing on the physics of the slurry and by stressing consistent mathematical and analysis methods to determine the material constants.

A rotating vane-cup rheometer is used to determine the rheological properties of semi-solid slurries, and a procedure is established for characterizing the rheology with emphasis given to the proper and self-consistent evaluation of the material constants.

The problem of steady-state propagation of a finger or a bubble of inviscid fluid through a Hele–Shaw cell filled by a viscous non-Newtonian, including visco-plastic (Bingham) fluid is addressed. Only flows symmetric relative to the cell axis are considered. It is shown that, using a hodograph transform, this non-linear free boundary problem can be reduced to the solution of an elliptic system of linear partial differential equations in a fixed domain with part of the boundary being curvilinear. The resulting boundary-value problem is solved numerically using the Finite Element Method. Finger shapes are calculated, and the approach is verified for one-parameter family of solutions which correspond to the well-known Saffman–Taylor solutions for the case of a Hele–Shaw cell filled by a Newtonian fluid. Results are also shown for fingers with non-Newtonian fluids. In the case of a cell filled by visco-plastic (Bingham) fluid, it is shown that stagnant zones propagate with the finger, and that the rear part of the finger has constant width. The same approach is applied to finding a two-parametric family of solutions for steady propagating bubbles. Results are shown for bubbles in Hele–Shaw cell filled by power-law and Bingham fluids.