size of grain was fine at the low pouring tempera- ture. In the contrast experiment, due to the higher pouring temperature of 650C, even though the power of stirring reached 354W, the microstructure morphology of globular-like or particle-like could be obtained in the central area of the ingot, where there were three obvious structural areas. From another point of view, during preparation of the semi-solid A356 slurry by the compound process, such as using a pure copper rod to produce the local chilling effect, even if the pouring temperature as high as 650C,
the amount of globular-like or particle-like primary α-Al with finer microstructure in the semi-solid slurry prepared greatly increase. The transition area in the ingot also disappeared, and the uniformity of microstructure morphology in the ingot was obvi- ously improved. A pouring temperature as high as 650C in the compound process, on the condition of some chilling measurements taken, could be suitably raised to a temperature needed for rheocasting.
Conclusion It was feasible to prepare semi-solid slurry of
A356 alloy by the compound process. The pouring temperature could be suitably raised in this tech- nique to be convenient for real operation. The primary α-AL could be effectively fined when semi-solid slurry of A356 alloy is prepared by the compound process. The semi-solid primary phase consisted particle-like or globular-like α-AL in the ingot, and the microstructure uniformity was greatly improved .The size of primary α-AL pre- pared by the compound process was even finer than that prepared by LSPSEMS. During the preparation of semi-solid A356
alloy slurry by the compound process, the solidified layer contained globular-like finer primary grains was formed on the surface of the pure copper rod through the local chilling produced by the rod. The grains were easy to drift away from the surface of
the rod into melt to become the nucleation sites for α-AL under the forced convection. During the compound process, the mechanism of
grain refine was concerned with that the rod quicken up temperature reduction in the center of melt, per- haps the temperature field in which the temperature is gradually decreased from the wall of mold to the center of melt. There was higher surviving rate of nuclei in this temperature field, like as increasing the number of nuclei. Moreover, the rod could rapidly dissipate heat from the melt, or the increase of the local cooling rate in the melt. The crystals as nuclei could not grow up for short time so that the even finer grains were formed in the melt.
拌技术可以制备出初生α-Al的形状呈球状和颗粒 状、尺寸较细小的A356合金半固态浆料；但是， 浇注温度(过热度)对A356合金的初生α-Al的晶粒形 貌和尺寸有重要影响；浇注温度高(过热度大)，初 生α-Al的形状呈蔷薇状，尺寸较粗大；而浇注温度 低(过热度小)，初生α-Al的形状呈球状和颗粒状， 尺寸细小。在对比实验中，由于浇注温度偏高，达 650 ℃，即使弱电磁搅拌的功率达到354 W，只 能保证铸锭心部的组织形貌为颗粒状或球状，且铸 锭中三个区域明显。如果在低过热度浇注和弱电磁 搅拌制备A356合金半固态浆料的过程中，利用紫 铜棒加强液态合金的局部激冷，即使浇注温度高达 650 ℃，半固态浆料中的颗粒状或球状初生α相的 数量也大大增加，尺寸更加细小。铸锭中的过渡区 域也基本消失，组织形貌的均匀性显著增加，可保 证流变成形对半固态浆料形貌的要求。另外，在复 合工艺中，浇注温度达650 ℃，这意味着适当地提 高浇注温度，亦可保证流变成形所需半固态浆料对 初生相形貌的要求。这在实际生产中，对于方便操 作具有非常现实的意义。
结论
应用复合工艺制备半固态A356合金浆料是可行 的，浇注温度可适当提高，便于实际操作。 复合工艺可有效地细化初生α-Al相的晶粒；半固 态铸锭组织主要由颗粒或球状初生α-Al相组成，且 组织的均匀性大大提高；复合工艺可比常规低过热 度浇注和弱电磁搅拌工艺制备出形貌更圆整、尺寸 更细小的初生α-Al相。
在复合工艺中，紫铜棒可在液态合金中产生强烈 的激冷效应，在其表面形成由细小的团球状颗粒构 成的凝固层；在熔体强制对流的作用下，这些晶粒 易从棒体表面游离，进入熔体中成为合金凝固时非 自发生核的晶核来源。
复合工艺中，晶粒细化的机理可能与紫铜棒在液 态合金中加速了熔体中心温度下降，或许在熔体中 形成自搅拌筒壁向液心逐步降低的温度场有关。在 该温度场中，因搅拌和局部激冷所产生的晶核存活 率更高，如同增加了晶核数量；另外，紫铜棒可使 熔体中的热量迅速散去，相当于增加了熔体的局部 冷却速度，使得作为结晶核心的晶体在较短时间内 不能长大，结果形成了更为细小的晶粒。
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FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION May 2013
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