Rheological behaviour of partially solidified A356 alloy: Experimental study and constitutive modelling

Rheological behaviour of the mushy zone is dependent on the microstructure and related to parameters such as temperature, shear rate and shear strain/time. However, constitutive modelling of the rheological behaviour is difficult due to the diversity of the parameters. This study focuses on the influence of the microstructure and the above parameters on the rheological behaviour of partially solidified A356 alloy and establishing a constitutive model accordingly. During solidification, the evolution of α-Al phase leads to the changes of the apparent viscosity of mushy zone. Steady-state viscosity and peak viscosity measured by rotational rheometry reveal the thixotropy of the mushy zone. The thixotropic strength is found to increase with increasing shear rate and decreasing temperature. The peak viscosity, as well as the steady-state viscosity, is found to obey a power law model and is modelled using the power law model with the coupling of temperature and shear rate. Furthermore, a constitutive model with temperature, shear rate, shear tress and shear strain/time as parameters which describes the rheological behaviour of the mushy zone of partially solidified A356 alloy accurately and completely is established. Comparing with previously reported experimental results, the FEM simulation employing the constitutive model works well in enabling visualization of the viscoelastic filling process of A356 alloy in mushy zone. The constitutive model provides a solution for characterizing the forming behaviour of other mushy zone/semi-solid aluminium alloys with corresponding rheological parameters.