Microstructure and rheological behavior of semi-solid state AZ91D alloy in continuously cooling process

The microstructure and rheological behavior of the semi-solid state AZ91D slurry in continuously cooling and stirring process were studied by using a Couette type viscometer. The results show that the apparent viscosity of the semi-solid AZ91D magnesium alloy slowly increases originally and then sharply with increasing the solid fraction. It also has a sharp dependence on the cooling rate and the shearing rate operated on the slurry. Decreasing the cooling rate or increasing the shearing rate will lower the apparent viscosity. The analysis of the microstructure of the semi-solid state AZ91D alloy shows that this results from the solid phase shape becoming more globular due to stirring more sufficiently under lower cooling rate, or due to the larger stirring strength under higher shearing,rate.