did not fully change into rosette-like primary α-Al still existed, as shown in Fig. 3b. In the edge area of the ingot, the microstructure consisted of dendritic- like crystals which did not fully change into rosette- like primary α-Al, as shown in Fig. 3c. As seen in Fig.4a, the A356 semi-solid alloy
prepared by the compound process had a diameter of 68.8µm in the central area, 77.6µm in the transition and 84.7µm in the edge area. The A356 semi-solid alloy prepared only by LSPSEMS (as shown in Fig. 3) had diameters of 85.6µm in the central area and 112.6µm in the transition area, where there were more rosette-like primary α-Al. The primary α-Al at the edge area consisted of fine and small dendritic grains which could not satisfy the requirement of rheocasting. The shape factor of the semi-solid A356 alloy prepared by the compound process was 0.83 at the center, 0.77 at the transition and 0.59 at the edge area. The shape factor in semi-solid A356 alloy prepared by LSPSEMS was 0.78 at the center, 0.54 at the transition and 0.28 at the edge area (Fig. 4b). The shape factor in the different areas of semi-solid A356 alloy prepared by the different processes was consistent with the observation on the primary α-Al morphology at the different areas. The results from Fig. 4 indicate the compound process could effec- tively improve the size and primary α-Al morphol- ogy at different areas in a semi-solid A356 alloy. Previous research indicated semi-solid A356
slurry with fine particle-like or globular-like primary α-Al could be prepared by LSPSEMS, but pouring
temperature has an important effect on the grain morphology and size of the primary α-Al. The mor- phology of the primary α-Al presented rosette-like grains that were course with high pouring tempera- tures; the morphology of the primary α-Al presented globular-like or particle-like microstructure and the
α相的形貌则由颗粒状或球状向着蔷薇状转变，间 或还伴有少量未完全转变成蔷薇状初生相的树枝状 晶，且尺寸较粗大，见图3b；铸锭的边缘部位，则 是由未完全转变成蔷薇状初生相的树枝状晶组成， 见图3c所示。
由图4a可见，复合工艺制备的半固态A356合 金铸锭心部的初生相颗粒平均尺寸为68.8μm，过 渡区的平均尺寸为77.6μm，而边缘的平均尺寸为 84.7μm，可以说整个铸锭的截面上初生相晶粒较 细小，分布较均匀。而A356合金经低过热度浇注 和弱电磁搅拌后(见图3)，其心部的初生相颗粒的平 均尺寸为85.6μm，而过渡区由较多的蔷薇状初生 相所组成，其等积圆的平均尺寸达到了112.6μm， 但边缘的初生相主要由细小的树枝晶构成，这种初 生相形貌是不能满足流变成形的需要，故未测算此 区域初生相的平均尺寸。这说明复合工艺对于细化 初生相的晶粒尺寸效果显著，尤其是对于铸锭边缘 的尺寸影响较大。复合工艺制备的A356半固态铸 锭心部、过渡区和边缘部位的初生相晶粒的形状因 子分别是0.83，0.77和0.59；而低过热度浇注和弱 电磁搅拌制备的半固态A356铸锭心部、过渡区和 边缘部位的初生相晶粒的形状因子分别是为0.78， 0.54和0.28，见图4b所示。不同工艺制备的半固态 A356铸锭各部位初生相形貌的形状因子计算结果 与该部位观察到的初生相形貌是一致的。图4的结 果表明复合工艺可以有效地改善半固态A356铸锭 不同部位的初生相的尺寸大小和组织形貌。 以往的研究表明，利用低过热度浇注和弱电磁搅
Fig. 4. Shown is a comparison of average equal-area-circle grain
diameter D and shape factor F of primary α-Al at different positions (A-center, B-transition, C-edge).
图4两种工艺条件下制备的半固态A356铸锭不同部位初生相等 积圆直径和形状因子的比较
May 2013
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