Microstructural characteristics of the residual liquid phase in semi-solid 2024 wrought aluminum alloy slurry in the rheo-diecasting process was investigated by optical microscope（OM）

scanning electron microscope（SEM） and X-ray diffraction（XRD）. The results indicate that the microstructure of rheo-diecasted 2024 wrought aluminum alloy is composed of spherical and near-spherical primary α-Al phase

and irregular eutectic θ（CuAl2） and S（CuAl2Mg） phase which is continuously distributed in grain boundary. The secondary solidification process consists of three stages: direct growth of secondary primary （α2） phase from the surface of the primary α-Al phase particles without re-nucleation

independent nucleation of α3 phase from residual liquid and followed by the equiaxed growth

and finally the eutectic reaction. Coupled growth is the main solidification process of eutectic action. The eutectic α-Al attaches to the eutectic θ（CuAl2） phase and S（CuAl2Mg） phase

and grows through the bridge-growth mechanism. The α-Al、θ（CuAl2） phase and S（CuAl2Mg） phase are precipitated alternately after each other

and α-Al+θ（CuAl2）+S（CuAl2Mg） phase will form a laminar eutectic structure.