Fig. 9. SDAS vs. solidification time in aluminum casting alloys is graphed.
Many of the early papers
reported cell size in their studies. However, it is now known that a better measure is the secondary dendrite arm spacing (SDAS). Te easiest way to measure SDAS is to use the linear intercept method. Tis is illustrated in Fig. 8 for a modified Al-7% Si alloy. Lines are drawn on a micrograph where well defined dendrite arms can be observed, and the average spacing between the centers of adjoining arms is measured. Typically, a number of measurements are made and the results averaged. Te SDAS can be used to determine the local solidifi-
cation time at any point in a casting. Te results of many commercial and laboratory measurements on Al-Cu alloys have been reviewed. Results from castings made from 356 and 319 alloys are also shown in Fig. 9, where measurements of SDAS are plotted versus the local solidification time (as measured by thermocouples in the casting). It can be seen that, for a given freezing rate, the copper-
containing 319 alloy has a somewhat smaller SDAS than the 356 alloy. Te correlation for most other foundry alloys would probably lie somewhere between these two curves. Te ability to measure SDAS, and the correlations shown in Fig. 9, represents a useful tool. It can help in learning about the thermal history of a sample from an “unknown” casting (e.g., a competitor’s product) or from one’s own castings. It may not always be convenient to place thermocouples in the mold, but the solidification time at various points in the casting can be estimated from the SDAS. Te dendritic structure is often visible if you look carefully
into pores on the fracture surface of tensile bars. An example is shown in Fig. 10. Te rounded ends of the secondary dendrite arms are sticking out from the left hand-side of this picture. Te SDAS in the sample appears to be between 40 and 50 microns, which cor- responds to a local solidification time of about two minutes (for an A356 alloy).
Te paper this article is based on and was originally presented at the 117th American Foundry Society Metalcasting Congress.
局部凝固时间
他们的研究中的晶核大 小。然而，现在公认的 更好的测量对象是二次 树枝晶臂间距（SDAS） 。最早测量SDAS的方 法是使用线性截取方 法。对于改良的Al-7%
Si，见图8。在显微镜下划线，能观察到清晰形态的 树枝晶臂，可测量临近的树枝晶臂的中心的平均距 离。通常，得到大量的测量结果，然后取测量结果 的平均值。 SDAS 被用于确定铸件任何点的局部凝固时间。 许多商业以及实验室关于Al-Cu测量结果得以重 现。356 和 319合金铸件的结果如图9所示，SDAS 的尺寸都与局部凝固时间相对应（根据铸件中热电 偶测量结果）。
能够看出，对于给定的凝固速度，含铜的319合 金尺寸比356合金稍微小点。对于大部分其他的铸造 合金，位于这两曲线之间。SDAS的测量以及图9所 示的相互关系，是很重要的工具。这能够帮助我们 了解来自“未知”铸件（比如竞争对手的产品）的 样品的热历史，或者自己的铸件。在铸型中放置热 电偶并不总是很方便，但是铸件中各个部位的凝固 时间可以通过SDAS来估算。
如果仔细查看拉力试棒的断面的气孔，经常能看 到树枝晶结构。如图10例子所示。二次枝晶臂圆端 从图中的左手边突出。样 品中的SDAS在40-50微 米，相当于对应局部凝固 时间约2分钟（对于A356 合金而言）。
本文基于第117届美国铸造 大会上发表的论文编制。
图9：铝合金铸件中SDAS与凝 固时间的对照表
Fig. 10. This is a SEM micrograph of secondary dendrite arms in a large pore (A356 alloy.)
图10：这是一个大孔内的二次 树枝晶臂的扫描电子显微照片
June 2014
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