EFFECT OF STIRRING PARAMETERS ON THE THIXOTROPIC BEHAVIOR OF SEMI-SOLID AZ91D MAGNESIUM ALLOY

摘要

The thixotropic behavior of semi-solid AZ91D slurry solidified and stirred isothermally was studied through a Couette type viscometer and the thixotropic behavior of semi-solid AZ91D slurry reheated and stirred isothermally was also studied through the same apparatus. After stirred, continuously cooled and further held isothermally, the semi-solid AZ91D magnesium alloy slurry reaches the steady state. Then the stirring is stopped, the slurry is rested isothermally for a given time, and finally the slurry is stirred again at a given shearing rate. The result shows that the apparent viscosity of the semi-solid AZ91D magnesium alloy slurry increases after isothermally held, but it quickly fall down to a steady value after stirred again and takes on a sharp shear-thinning behavior; with the same shearing rate and the rest time increasing, the steady apparent viscosity increases because of the agglomeration of the primary grains, and the time required for the slurry to reach the steady state also becomes longer; if the solid fraction increases, it takes a longer time for the slurry to reach the steady apparent viscosity with the same shearing rate and the same rest time; if the solid fraction and the rest time are the same, but the shearing rate rises, it takes a shorter time for the slurry to reach the steady apparent viscosity and the final steady apparent viscosity also decreases. When the semi-solid AZ91D magnesium alloy slurry reaches the steady state, the slurry is cooled down again and solidified thoroughly. Then the frozen slurry was reheated to the former temperature during its isothermal stirring, and soaked for some time and finally stirred at a given shearing rate. The reheating and stirring experimental result shows that if the soaking time is shorter or longer, the steady apparent viscosity of the reheated semi-solid AZ91D magnesium alloy slurry is higher. However, If the soaking time is proper, the steady apparent viscosity will be lower. Moreover, the steady apparent viscosity of the reheated semi-solid AZ91D magnesium alloy slurry is generally lower than that of the semi-solid AZ91D magnesium alloy slurry directly prepared by earlier cooling and stirring.