Papers by Author: Andreas N. Alexandrou

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.