Formation of liquid pools entrapped within solid grains of ZA27 alloys during partial remelting

The investigations were performed for the formation process of liquid pools entrapped within solid grains of three ZA27 alloys, produced respectively by either grain refinement, or traditional permanent casting or mechanical stirring during partial remelting at semisolid temperature of 460°C. The results show that the rapid coalescence of primary grains due to the merging of secondary arms during the initial stage of partial remelting is the main cause for the formation of the entrapped liquid pools. This coalescence resulted in the phenomenon that a high quality of eutectics, especially the η phase, was entrapped within the grains and then remelted to form liquid pool during subsequent heating. In addition, the growth of the η phase decomposed from the primary α' phase and β phase and the subsequent remelting are another cause for the refined and permanent mould casting alloys. Furthermore, the agglomeration of the solid grains also resulted in the entrapment of liquid in the interior of grains. Zn-26.21Al-1.95Cu-0.029Mg is used in the test.