Establishment And Application Of Rheological Model For Semi-solid Light Alloy

On the basis of Chen and Fan's rheological model, the modified microstructural model of semi-solid metal slurry has been obtained. Then, the model is applied to study the rheological behavior of AlSi4Mg2 alloy and AZ91 D alloy.Firstly, we have studied the agglomeration and the deagglomeration of particles in a simple flow field and deduced the evolution equation of structural parameter n,which is defined as the average particle number in an agglomerate at a particular time t. The equation includes the internal and external parameters(such as shear rate,shearing time etc) which affect the agglomeration degree of particles. It has been shown that the microstructure of semi-solid metal slurries determines the rheological behavior exclusively, whereas the external flow conditions affect the rheological properties by changing its microstructure. The effective solid volume fraction is the bridge between the microstructure and the rheological characteristics. Through the effective volume fraction, both apparent viscosity and shear stress can be expressed as a function of the structural parameter n. Accordingly, the constitutive equations are determined.Secondly, we have studied the rheological behavior of AlSi4Mg2 alloy semi-solid metal slurries by the new model. We have studied the factors which affect the steady state behavior. It has been shown that the steady state viscosity and the average agglomerate size increase with the solid volume fraction and decrease with the shear rate. The prediction of the model is in good agreement with the experimental results in the literatures qualitatively. The importance of the effective solid volume fraction is shown by explaining the strong coupling between the viscosity and the microstructure. The external flow conditions such as shear rate etc change the agglomeration degree, that is, the effective solid volume fraction, then affect the viscosity. For the transient state behavior of AlSi4Mg2 alloy, we have analyzed the various factors which affect its transient state behavior during isothermal shearing and isothermal resting. It has been shown that there is a strong coupling between the transient viscosity and the microstructure of AlSi4Mg2 alloy; the microstructural characteristics is determined by the build-up and the break-down. The deagglomeration time is on the order of a few seconds, while the agglomeration time is on the order of a few hundred of seconds, that is to say, the deagglomeration kinetics is about two orders of magnitude faster than the agglomeration kinetics.Finally,we have studied the rheological behavior of AZ91 D alloy semi-solid metal slurries by the new model. The results agree qualitatively with the ones of AlSi4Mg2 alloy.