Abstract： Semi-solid A12Y/AZ91 magnesium matrix composites slurry was prepared by ultrasonic vibration, effect of ultrasonic vi bration temperature and time on microstructure of semi-solid slurry was investigated. The results showed that with the ultrasonic vibration temperature decreasing the solid volume fraction of semi-solid slurry increased. The best ultrasonic vibration temperature was 600 ℃. With the increase of ultrasonic vibration time, the average grain diameter of primary a-Mg particles decreased firstly, then increased, the average shape factor increased gradually and decreased slightly after 90 s, and a few rosette dendrites were observed after 120 s. The best semi-solid slurry with average grain diameter of 75 μm and shape factor of 0.7 were gained after the melt was treated by ultrasonic vibration for about 60 s at near liquidus temperature （600 ℃）. At last, the microstructure evolution mechanism of semi-solid magnesium matrix composites slurry was analyzed by the theories of thermodynamics and kinetics.
The rheological behavior of semisolid Mg_(2)Si/AM60 composites prepared by ultrasonic vibration treatment was investigated.The effects of primaryα-Mg solid content,the ultrasonic power and the content of reinforcement phase Mg_(2)Si on the apparent viscosity of semisolid composites were discussed.The results show that the apparent viscosity of the semis olid composites increases with the increase in a-Mg solid content.Meanwhile,the apparent viscosity decreases with the increase in ultrasonic power,and the reduction amplitude is higher when the solid content is higher.In addition,the apparent viscosity increases with the increase in Mg_(2)Si content.Based on experimental data fitting,the rheological model of semisolid Mg_(2)Si/AM60 composites prepared by ultrasonic vibration was established.
The effects of Sm additions （0, 0.5 wt.%, 1.0 wt.%and 1.5 wt.%） on the eutectic Si and β-Al5FeSi phases of ADC12 as-cast alloys were studied by optical microscopy （OM）, scanning electron microscopy （SEM） and differential thermal analysis （DTA）. The experimental results showed that Sm was an effective modifying agent for the eutectic Si of ADC12 alloy, when 1.0 wt.%-1.5 wt.%Sm was added to the alloy, the coarse acicular eutectic Si was modified into fine particle or short rod structure. Moreover, the appropriate addition of Sm （about 1.0 wt.%） had marked effects on shortening the length of needle-likeβ-Al5FeSi phase. Whereas, Sm was less effective on modifying the needle-likeβ-iron to the Chinese script or sphericalα-iron phase. The modification mecha-nism was also discussed.
The influence of Sm （Samarium） content on microstructure and mechanical properties of recycled die-cast YLl12 aluminum alloys was investigated. The results show that many small Sm-rich particles form in the recycled die-cast YLl12 alloys with Sm addition. At the same time, the secondary dendrite arm spacing in the YLl12 alloys modified with Sm is smaller than that of the unmodified alloy. The eutectic Si of recycled die- cast YL112-xSm alloys transforms from coarse acicular morphology to fine fibres. Mechanical properties of the investigated recycled die-cast YLl12 aluminum alloys are enhanced with Sm addition, and a maximal ultimate tensile strength value （276 MPa） and elongation （3.76%） are achieved at a Sm content of 0.6wt.%. Due to the modification of eutectic Si by Sm, numerous tearing ridges and tiny dimples on the fractures of tensile samples are observed.
