Effect of oscillation frequency on microstructure of A356 slurry prepared by semi-solid metal forming with flow

A novel semi-solid processing and forming integrated process, semi-solid metal forming with flow process (SSMF), was proposed, and the effect of oscillation frequency (0-50 times/min) on the microstructure of an A356 semi-solid slurry prepared using this process was studied. The microstructure and properties of the semi-solid A356 alloy samples were investigated using metallography, scanning electron microscopy, and energydispersive spectroscopy. The melt-flow velocity field and temperature field applied during the SSMF slurry preparation were investigated using numerical simulations. The results showed that the oscillation frequency had a significant effect on the primary alpha-Al microstructure. With the increase in oscillation frequency, the primary alpha-Al grains of the A356 alloy gradually transformed into a non-dendritic and near-spherical microstructure. The average grain size decreased nonlinearly from 98.1 mu m to 47.6 mu m, the shape factor increased from 0.19 to 0.88 (by 78.4%), and the uniformity deviation was reduced to 10.38 mu m. A semi-solid slurry with good performance can be prepared by controlling the oscillation frequency during the SSMF process, and the resulting grain morphology of the semi-solid microstructure was fine and uniform. The mechanism of non-dendritic microstructure formation was elucidated: firstly, the melt flow caused by oscillation changes the distribution of the solute; secondly, the melt flow promotes the fracture of necking dendrites. Furthermore, the collision nucleation mode of the SSMF was proposed, and the relationship between the oscillation frequency and nucleation rate was obtained.