Effect of stirring speed on microstructure of A356 alloy cast through rheometal process

Synthesis of an A356 alloy slurry with 30 wt.% solid fraction was carried out by the rheometal process, previously known as a rapid slurry formation process. A detailed study of the A356 alloy at different slurry stirring speeds (400, 600, 800, 1000, and 1 200 rpm) was performed. Variation of the surface morphology of the enthalpy exchange material at different stirring times (0, 5, 10, 15, 20 and 25 s) was also investigated. The evolution of the cast microstructure was observed using optical and scanning electron microscopy. The microstructure exhibited a network of eutectic Si around the primary alpha-Al phase. The average grain size of the primary alpha-Al phase decreases with stirring speeds up to 1 000 rpm. For 1200 rpm, average grain size decreased, however, high turbulence led to entrapment of gas porosity. The experimental results revealed that as stirring times increased under isothermal conditions, the alpha-Al phase changed its morphology from dendritic to rosette to globular. The effect of flow pattern was investigated with the introduction of four baffles along the crucible wall. These baffles modified the fluid flow pattern such that the average grain size of alpha-Al phase was further refined.