A study on semisolid processing of A356 aluminum alloy through vacuum-assisted electromagnetic stirring

In this study, a vacuum-assisted electromagnetic stirring system was designed and manufactured in order to minimize the defects such as porosities in the semisolid processing A356 aluminum alloy by electromagnetic stirring (EMS). A sleeve cooling system was manufactured and two different cooling media (air and water) were tested for the stirred and unstirred slurries. For the stirred slurry, the obtained cooling rates were lower than the values for the unstirred slurry in both cooling media. Also the effects of the different cooling types and the thickness of the slurry cup on the microstructural evolution were thoroughly studied. The maximum cooling rate with the least evidence of the liquid segregation for the stirred slurry was achieved in water-cooling condition when the slurry cup of 2 mm thickness was used. At the pouring temperature of 620°, the optimum EMS stirring current and time were obtained 30 A and 60 s, respectively. The vacuum level with the least presence of the porosity was determined at 50 kPa, while further increase in the vacuum level (decrease in the applied pressure) resulted in increase in the presence of the porosity. The last trend could be attributed to the possible effect of the stirring during the EMS process on the entrainment of bifilms into the semisolid slurry, which is discussed in details. © Springer-Verlag London Limited 2011