A one phase thermomechanical model for the numerical simulation of semi-solid material behavior. Application to thixoforming

This work deals with the development of an appropriate constitutive model for semi-solid thixoforming processes of metallic alloys. These processes rely on a very specific material behavior called thixotropy that can be displayed by some metallic alloys heated up to their semi-solid state. It is a particular evolutionary behavior which is characterized by a solid-like behavior at rest and a liquid flow during shearing, thus by a decrease of the viscosity and of the resistance to deformation while sheared. An original one-phase thermo-elasto-viscoplastic constitutive model has been developed. The basic idea is to extend the classical isotropic hardening and viscosity models beyond the solid state by considering two more non-dimensional internal scalar parameters. The semi-solid state is treated as a particular case, and one of the main features of the proposed constitutive model is that it remains valid over a wide range of temperatures, starting from room temperature to above the liquidus in a continuous manner, thus allowing a continuous transition between classical solid and fluid behavior. Another feature is that, after the forming step, it is possible to simulate the cooling down of the component back to room temperature using the elastic–viscoplastic model. So it is possible to estimate residual stresses, something that is definitely impossible while using a fluid-like model or a rigid viscoplastic approach. The presented model is illustrated and validated by means of representative numerical applications, as two different extrusion tests are carried out and the computed predictions are compared to experimental results.