| The importance of processing metals in their semisolid state is increasing quite rapidly due to the distinct advantages the process offers over similar methods of near-net-shape making. However, despite the attractive features of the process, its implementation to industrial applications is hampered by technical problems primarily due to the complex rheology of the material.; In this work we have developed three mathematical models addressing the transient rheology of this complicated material. The models have been implemented into computational codes to predict the material behavior in problems such as the flow in a straight channel and in a sudden square expansion.; As a first attempt in the modeling, and consistent with the experimental evidence, the behavior of a SSM is represented by a Herschel-Bulkley fluid. In this investigation, by using analytic solutions for antiplane shear flow in a corner between two rigid walls, we discuss the ability of regularized Herschel-Bulkley models in determining the topography of the yield surfaces. Results are shown for different flow parameters and compared to the exact solutions.; The time evolution of the three-dimensional yielded/unyielded surfaces has been investigated in a sudden-square expansion with a downstream-to-upstream expansion ratio of 2:1. The results show that during the evolution of the flow, two core regions and dead zones at the corners are formed. The extent of the core regions decreases with the pressure gradient and the Reynolds number and increases with the power-law index.; We propose a constitutive relation based on the same Herschel-Bulkley fluid model mentioned above, now however the rheological parameters are functions of the solid volume fraction and a structural parameter that evolves with the processing history. The framework of the proposed theory is validated using experimental data. The model is implemented into a 3-d computer code to predict the flow field, yield surfaces, and distribution of parameters in a channel flow. The results indicate that the break-down of links is rather small in the corners and in the core region of the channel.; The models described above are quite effective in representing the bulk behavior of the flow of semisolid metals. However, more detailed information about the microstructure of the material is obtained by taking into account the two-phase nature of the slurry. In this investigation, we used a two-phase model to predict the isothermal flow behavior of semisolid metals in a sudden 3-D square expansion. The model considers that the liquid-phase behaves as a Newtonian fluid while the solid-phase behaves as a Herschel-Bulkley fluid. The model is implemented numerically using a classical Galerkin-Finite element formulation with a segregated solution procedure. The results show that segregation of phases exists resulting in particle crowding in the corners of the channel. (Abstract shortened by UMI.)... |
