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Laying a Foundation for Solidification


Phase diagrams of aluminum alloys provide a picture of castability and expected properties before you pour the casting.


GEOFFREY SIGWORTH, GKS ENGINEERING SERVICES, DUNEDIN, FLORIDA 构建凝固基础


在浇注铸件之前,铝合金相图描绘了铸造性能和预期的机械性能。 佛罗里达州的GKS工程服务公司Geoffrey SiGworth


Tis article is the first in a two-part series on solidification in alu- minum castings. While the series focuses on solidification principles in aluminum alloys, many can be applied to other metals, as well.


T


he metalcasting industry is concerned primarily with the solidification process, which essentially is a phase transformation from the hot, liquid state to a colder, solid state. Phase diagrams tell


us a great deal about how this transformation occurs. Tis gives us clues about castability as well as the properties in the finished product. For example, they tell us about: • What phases form. • At what temperatures the phases form. • The composition of phases and how solute elements are distributed between the phases.


• How difficult it will be to place a specific alloying element into aluminum. If pure aluminum is slowly heated, it remains solid until it


reaches 1,220F (660C). Ten, it starts to melt but remains at 1,220F until all the metal is molten. Once it is fully liquid, it can be heated to higher temperatures. Tis situation is akin to melting ice or placing ice cubes in a glass of water. Ice and liquid water coexist only at a single temperature: the melting point. Te liquid temperature always is above this point, and the solid temperature always is below. One way to describe the situation is the phase rule (p +


f = n + 2); where p is the number of phases present, f is the number of degrees of freedom, and n is the number of com- ponents present). For a pure metal, the number of components (n) is equal


to one. When both solid and liquid are present, the number of phases (p) is equal to 2. Terefore, the number of degrees of freedom (f) must be equal to 1. However, in practice, the pressure is fixed by the prevailing atmospheric pressure, which uses up the one degree of freedom. In other words, the melting temperature is not free to vary or change, as long as two phases are present in a pure material. If a pure metal was melted in a high pressure furnace in


a lab, the melting point would increase. Aluminum exhibits about a 7% volume increase when it melts. Higher pressures would make it more difficult to melt metal by opposing this volume increase. Te single degree of freedom means as long


这篇文章首次分两部分系列内容来介绍铝合金铸件的凝固。 这两部分内容主要关注铝合金的凝固原理,其中很多原理也 适用于其他金属。





属铸造业主要关注凝固过程,其实质是从 热的、液态到冷的、固态的相变过程。相 图告诉我们许多关于相变是如何发生的信 息。这给我们提供了铸造性能和最终产品的力学性能 的线索,比如,相图告诉我们: • 形成什么相 • 相在什么温度下形成 • 相的组成以及溶质在两相中的分布情况 • Al中加入一种特定的合金元素的难度 如果纯铝缓慢加热,在660℃之前会一直保持固 态。之后,开始熔化但温度维持在660℃,直到金属 完全熔化。一旦全部成为液态,纯铝可以被加热到更 高的温度。


这种情况与融化的冰或者是把冰块放到一杯水中的 情形相似。只有在单一温度——熔点时,冰与液态水 同时存在。液态温度经常是在熔点之上,同时固态温 度经常是低于熔点。


描述这种情形的一种方法是相律(p + f = n + 2) ;这里的p 是当前平衡相数,f 是自由度数,n 是当前 组份数。


对于纯金属,n=1。当固态和液态共存时,p=2。 因此,f一定等于1。而实际上,压力一般固定为大气 压力,用去了这个自由度。换句话说,只要在纯材料 中存在2相,熔点是不会任意变动或改变。 在实验室里如果纯金属在一个很大压力的炉体内熔 化,熔点将提高。Al在熔化时体积增大约7%。由于 对体积膨胀有反作用,压力越高,金属越难熔化。单


56 | FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION June 2014


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