With the help of an electromagnetic stirring device and alloy melt quenching technology, the microstructure of semi-solid AZ91D magnesium alloy slurry stirred by a rotationally electromagnetic field was studied and the experimental results are shown as the following. The primary α-Mg grains are refined obviously when the slurry is stirred by a rotational electromagnetic field during continuously cooling and they are eventually changed to fine rosette grains or spherical grains. If the above semi-solid slurry is further stirred isothermally for some time, much more spherical primary α-Mg grains can be obtained. If the melt is first cooled down to a given semi-solid temperature and then starts being stirred by the rotational electromagnetic field, the primary α-Mg dendrites will be large, and a longer time will be taken and a larger stirring power will be needed for the secondary arms of the dendrites to be remelted on the roots to prepare an ideal semi-solid slurry. Theoretical analysis indicates that the strong flow motion leads to a more even temperature field and a solute field and stronger man-made temperature fluctuation in the AZ91D magnesium alloy melt so that the spherical primary α-Mg grains are increased in the slurry. Moreover, all the measures promoting the temperature fluctuation will be favorable to the formation of spherical primary α-Mg grains and all the factors increasing the arm’s root remelting difficulty will be favorable to the formation of rosette-type primary α-Mg grains.
Suitable microstructures required for semisolid casting were formed by using a vertical pipe.Different lengths of vertical pipe,slug dimensions and pouring time were used to investigate their influence on the microstructure of A356 alloy.The results indicate that at the same length of the vertical pipe,the morphology of the primary α(Al)gradually deteriorates by the enlargement in the slug size,but the deteriorating speed slows down with increasing pipe length.They also reveal that the increase in the pipe length improves the microstructure,whereas no further improvement appears when the pipe length reaches a certain value.The optimum length of the pipe obtained in the present work is 430 mm.The microstructure of larger slug poured at higher pouring temperature gets worse and it can be improved by moderately elongating pouring time.The relative mechanisms were also discussed.
