Fig. 5. This shows a schematic view of silicon atoms in front of a moving aluminum crystal.


of snowflakes shown in the book is mind boggling. Something similar happens every time metal solidifies in the mold. Te liquid-to-solid transformation involves the formation of many small, individual crystals of solid aluminum. Tis is a fascinating area, one which has received a great deal of study. A brief overview will be given here, touching on the aspects of solidification most important to the casting industry. Earlier, this article described the use


of phase diagrams to see the sequence of phases forming during solidification, which influences final casting properties and can provide insight into castability issues. Even more about alloy properties can be gleaned from delving deeper into how the structure of a metal changes as it freezes. Tis knowledge is an important tool in choosing alloying combinations for your desired result. Te solid aluminum crystals forming during solidifica-


tion are like snowflakes. Te metallurgists first observing these crystals thought they resembled trees and called them dendrites, after the Greek word for tree (δένδρον or dén- dron). Dendrites were first observed by polishing metal samples or by etching the polished surface. More recently, real-time X-ray studies have observed the in situ formation of dendrites in Al-Cu alloys. Because the aluminum crystal contains much less copper than the surrounding liquid, they appear lighter in X-ray images. Examples are shown in Fig. 4. Te formation of dendritic crystals is a curious phenom-


enon, and many scientists have studied them. Te technical literature in this area is extensive; however, a relatively simple explanation will suffice to understand what is happening. One important clue is that pure metals do not form


dendrites. But when silicon or other elements are alloyed to alu- minum, dendrites appear. From the Al-Si phase diagram, only 13% of the silicon in the liquid metal remains in the first solid. Tis means that the silicon atoms pile up in front of the grow- ing solid crystals. Te situation is shown schematically in Fig. 5. In keeping with the snowflake analogy, the growing aluminum grain is represented by a snow plow. Consider the act of shoveling snow.


When a shovel is pushed, the snow quickly piles up in front, so one can go no farther. Years ago, sidewalks in some cities were cleared of snow by a horse-drawn plow. Te plow would use a “V”-shaped


Fig. 6. This schematic view shows silicon atoms in front of a growing dendrite tip.


图6：在树枝晶端前方的硅原子示意图 June 2014 FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION | 61


图5：在一个活动的铝晶体前面的 硅原子示意图


包括许多小的、固态铝的结晶晶体。 这个吸引人的领域得到了大量的研 究。本文就对铸造行业至关重要的凝 固方面进行简要的概括。


使用相图查看整个凝固过程中的相 变顺序，这将影响最终的铸件性能并 能够洞察铸造性问题，本文的前段对 此进行了描述。通过深入研究凝固时 金属的组织变化，甚至能够得到更多 的关于合金特性的知识。这些知识对 于按照所期望的结果选择合金的组分


是个很重要的工具。


凝固中固态铝晶体的形成很像雪花。冶金学家们首先 观察这些晶体认为它们像树，以希腊单词树命名为树枝 晶。树枝晶是在抛光的金属样品或者是腐蚀抛光后的表 面首次观察到。最近，有实时X射线研究实时观察到了 原位自生形成的Al-Cu合金树枝晶。因为对比周围液态 而言，铝晶体包含的铜很少，他们在X光下表现很轻。 详见图4。


树枝晶的形成是一个奇妙的现象，许多科学家都在研 究它们。关于这个领域的技术文献很多，然而，一个相 对简单的解释将更容易理解这一现象的发生。 一个重要的线索就是纯金属不会形成树枝晶。但是当 铝中加入硅或其他合金元素时，树枝晶出现了。从Al-Si 相图可见，液态金属中只有13%的Si保留在初始固态。 这就意味着硅原子在生长的固态晶体前沿堆积。这种情 况在图5有示意。与雪花相类似的情况一致，生长中的 铝颗粒就像是铲雪车。


考虑到铲雪的动作，当推动铲子时，雪很快地在前面 堆积，这样的话，个体就不能走的更 远。很多年前，人行道是由马拖的犁 来打扫。这种犁使用宽度为人行道的 V型叶片来穿过雪，并把雪推到人行


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