Gravity casting and semi-solid extrusion were employed to prepare Mg2Si-Al composites. X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), EDS analysis, Vickers hardness testing, and tensile testing were used to compare the microstructures and mechanical properties of the two composites, aiming to clarify the effects of the fabrication processes and aging times on their microstructures and mechanical performance. The findings show that semi-solid extrusion converts dendritic α-Al into spherical or ellipsoidal grains (60 ± 25 μm) and induces the spheroidization of Mg2Si reinforcing phases (29 ± 12 μm). Vickers hardness data show that both composites exhibit a rise-and-fall hardness trend with an increasing aging time, reaching a maximum at 8 h. At this aging stage, the semi-solid extruded composite has a Vickers hardness of 214 ± 32.71 HV, 22.99% higher than that of the gravity-cast composite under the same treatment. Tensile tests demonstrate that the semi-solid extruded composite attains its best tensile strength (254 MPa) and elongation (3.26%) at 8 h of aging. Compared with the semi-solid extruded composites aged for 4 h and 16 h, the 8 h-aged sample exhibits 32.30%/49.41% higher tensile strength and 52.34%/38.72% higher elongation, respectively. After 8 h of aging, the semi-solid extruded composite shows a 59.75% increase in tensile strength and an 88.44% increase in elongation compared with the gravity-cast composite.
Zhou et al. (Fri,) studied this question.