A GENERALIZED VISCOELASTIC MAXWELL MODEL FOR SEMISOLID THIXOTROPIC ALLOYS.

More advanced investigation of any thixoforming process gives evidence that apart from the typical thixotropy also some viscoelastic features can be observed, particularly at high volumetric solid fraction. From the micrographic analysis one can conclude that the elastic properties have the origin in the solid globules, which are deformed in a flow process. In any sudden increase of deformation rate one can observe characteristic stress overshoot proving the elastic properties. The globules and their agglomerates after elastic deformation slide along each other (the drag flow) and on the occasion break the physical bonds within the agglomerates giving rise to the thixotropy phenomenon resulting in the apparent viscosity decrease. For a quantitative description of the above features a modification of the Maxwell model was derived exhibiting both the viscoelasticity and thixotropy. So-called computer aided rheology approach was used and the material rheological parameters were iteratively adjusted until the numerical simulation results matched the existing experimental results. In an experiment described thoroughly in the literature [1] chosen here for the verification of the numerical model, a typically banded starting microstructure of HP9/4/30 steel was used. Load versus displacement curves under an actual thixoforming process were simulated here successfully.