Fig. 2. Shown is microstructure in the edge area at the top (a) middle (b) and bottom (c).
图2复合工艺制备 的A356半固态浆料 铸锭不同部位的边 缘组织
a
b
c
in different microstructure morphology. At present, a good solution to achieving uniform structure morphol- ogy in semi-solid alloys has not yet been found. To obtain the semi-solid slurry needed for rheo-
casting, some measurements were taken, such as rigorously controlling pouring temperature and the alloy’s cooling rate, or holding the liquid alloy in the liquid-solid range, to ensure morphology of the primary phase. The measurements taken can ensure the primary phase particles with the proper mor- phology will be obtained, but the operation becomes difficult and costly. On the basis of low superheat pouring and slightly electromagnetic stirring technol- ogy (LSPSEMS), authors applied local chilling to the semi-solid slurry of aluminum alloy prepared by LSPEMS, in which the compound process is formed, to realize the control of microstructure morphology and reduction of cost. Fig.1 shows the microstructure in the central area at the different positions of an A356 alloy ingot pre- pared by the compound process. It was seen in Fig. 1 that the microstructure in the central area, whether the position was from top or bottom, consists of particle- like or globular-like primary α-Al, and the grain size of the primary α-Al was finer. The morphology of the primary α-Al was not obviously changed along the direction of height in the ingot. It showed that the microstructure of the central area consisted of particle-like or globular-like primary α-Al, and the grain was fine when A356 alloy was prepared by the compound process. Fig. 2 shows the microstructure in the edge area at
the different position of an A356 alloy ingot prepared by the compound process. It was seen in Fig. 2 that little change occurred in the morphology of the micro- structure in this area compared with that in the central and the edge area. Some changes in microstructure in the edge area along the height of the ingot from top to bottom occurred. The microstructure in the edge
area of the top consisted of particular-like primary α-Al, rosette- like primary α-Al, and smaller amounts of rosette like primary α-Al (Fig.2a). Moreover, the amount of rosette-like primary α-Al slightly increased
差，以至于同一铸型中的液态合金的冷却速度不一 样，导致最终获得的组织形貌不一样。已有研究表 明，在低温浇注(如620和630℃)获得的铸锭中，由 若干个具有不同组织形貌的区域组成：铸件周边区 域为枝晶层, 由平均枝晶臂间距(DAS)为30 μm等轴 枝晶组成；第二个区域是一个逐渐过渡的区域，存 在α铝枝晶向着蔷薇状形貌转变，由蔷薇状和球状 晶体的混合物组成；在接近铸锭中心位置时蔷薇状 晶体的数量减少，显微组织完全由球状颗粒组成。 对于这类组织形貌不均匀的问题，目前还没有很好 的解决办法。
因此，如果利用单纯的低温浇注技术所获得的半 固态浆料若直接用于流变成形，尚不能满足流变成 形的要求，还须严格控制初生相的形貌。为了获得 符合流变成形要求的半固态浆料，人们采取了严格 控制液态合金的浇注温度和冷却速度或者使液态合 金在固液相区停留保温[5]等措施来保证初生相形 貌。这样虽然可保证获得形貌良好的初生相颗粒， 但操作变得困难，成本亦会提高。作者在已开发出 低过热度浇注和弱电磁搅拌工艺的基础上，将研究 把局部激冷技术应用到低过热度浇注和弱电磁搅拌 工艺中，形成新的工艺制备半固态A356合金浆料, 实现控制组织形貌、降低生产成本的目的。 图1为复合工艺制备的半固态A356合金铸锭不同 部位心部的组织形貌。由图可见，在铸锭的心部， 无论其位置高低，其半固态组织形貌基本都是由 颗粒状或球状的初生α相构成，而且尺寸较常规的 低过热度浇注和弱电磁搅拌所制备的初生α相更细 小。沿着铸锭的高度方向上，初生相的形貌没有明 显的变化。说明复合工艺制备的半固态A356铝合金 浆料，可以保证铸锭心部的组织由颗粒状或球状的 初生相组成，且尺寸细小。
May 2013
FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION | 43
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