Caracterisation rheologique des alliages de magnesium a l'etat semi-solide

The rheological characterization of magnesium alloys has been successfully studied with both Couette and Squeezing Flow viscometers. With these two devices, the apparent viscosity was determined using different cooling rates and different shear rates. The microstructural evolution from dendritic to globular has a significant impact on the rheological behaviour of the slurry since the dendritic network is transformed into spherical solid particles with increasing shear rate, reduces the specific area.; Two magnesium alloys were tested: AZ91D which is the most common casting alloy and AJ62x due to its good creep resistance.; Several shear regimes were chosen using the Couette rheometer in order to identify the rheological behaviour of the magnesium alloy at semi-solid state. The continuous cooling tests describe the evolution of the apparent viscosity during the solidification. This regime was used to evaluate the structural changes during solidification as a function of shear rate and rate of temperature decrease. For isothermal tests at constant shear rates in the semi-solid state, it was shown that the apparent viscosity decreases with time, characteristic of a thixotropic behaviour which also depends on the intensity of shearing. The evolution of the microstructure during this regime was also observed, starting from the initial agglomerated structure to the regularly distributed smaller particles. The shear step change tests applied on the semisolid alloys describe the evolution with abrupt change in shear rate, simulating the condition when the slurry is injected in a complex mould. This study shows that the slurry exhibits quasi-newtonian behaviour at high shear rates. The last regime tested using the Couette rheometer consists of the continuous change of the shear rate for isothermal (constant solid fraction) conditions. Hysteresis loops of increasing and decreasing shear rates were generated. The results showed that the level of thixotropy depends on initial conditions such as the time to reach the maximum shearing and the time of rest before shearing.; During the squeezing flow tests in the semi-solid state used for higher solid fraction, a segregation of the liquid phase is observed for dendritic alloys. Samples with a rheocast structure have a better distribution of solid particles during squeezing flow. The segregation of liquid and solid phases was investigated by determining the local solid fraction in the sample, as a function of radial position.