Microstructural evolution in semisolid forging of A356 alloy

A semisolid forging study using A356 alloy was carried out to investigate the evolution of microstructures affected by the solid fraction, the forging pressure and the addition of Sr. The semisolid slurry for forging was made by an electromagnetic stirring method. Microstructures were evaluated at two typical positions of the semisolid forged specimens: one is the region, where direct forging pressure is applied, and the other is the region, where the slurry was squeezed and extruded indirectly. Microstructural characteristics, such as the morphology of the primary Al particles and the eutectic Si, were changed with the positions observed. As the solid fraction increased, the morphology of the primary Al particles was transformed from rosette-like to globular. As the forging pressure increased, the size of eutectic Si became fine. An increase of the forging pressure is speculated to improve the interfacial heat transfer coefficient between the semisolid slurry and the mould wall, leading to an increase of the cooling rate on the growth of eutectic Si. The addition of Sr is effective in modifying the morphology for eutectic Si, especially when the forging pressure was low.