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tion of the results, only the phase boundaries are shown. Also a detailed section from Fig. 5 is taken. Te result is shown in Fig. 6 for alloy iron contents of 0.3 and 0.6%. Two segregation curves are given for each case. Te lower (red) curve is for a solidification time of 10 seconds. Te upper (blue curve) is for a longer freezing time of 1,000 seconds. From this result, it can be seen that 0.3% iron is a


form. At higher iron contents (e.g., 0.6%) primary β forms before any Al-Si eutectic, regardless of the freezing rate. Tis alloy was studied by the Center for Advanced


borderline case for this alloy. In rapidly solidified parts of a casting there should be no primary β phase, only a ternary eutectic according to this reaction: Liquid → Al(solid) + Si(solid) + β At slower solidification rates, however, primary β should


Solidification Technologies (CAST) researchers in Aus- tralia. Tey found that casting defects were associated with iron contents that produced primary β. However, when they switched to a higher silicon version of the same alloy, the defects went away. Te reason for this behavior may be seen by considering Fig. 7. Similar calculations are made for the same two iron contents. In this alloy the higher iron content (0.6%) becomes the borderline case. Terefore, 9% silicon alloy can tolerate twice the iron content of the 5% silicon alloy. In exactly this manner, the CAST researchers calculated


solidification paths for numerous silicon contents. In this way they derived a map of safe iron contents for their cast- ing (Fig. 8). Te results of related research, conducted by Caceres


and co-workers, should also be considered. Tey produced castings in a number of alloy compositions and measured mechanical properties. Some of their results are shown in Fig. 9, which illustrates the importance of microsegregation during segregation, and how higher silicon contents may be used to advantage in Al-Si based alloy castings. Te tensile strengths for castings heat treated to the T6 temper are shown on a quality plot (Ultimate Tensile Strength [UTS] versus the log of elongation). Te red lines show constant values of quality index (in


MPa). Te blue arrows indicate the change in alloy compo- sition. For example, iron was added to alloy 1 to obtain alloy 2. Te result was a significant loss in casting quality—about 120 MPa according to the quality index. Silicon was added to alloy 2 to obtain alloy 3. Nearly


all of the lost quality was regained by increasing the silicon content from 4.5 to 9%. A similar result was found going from alloys 1 → 6 → 7,


alloys 1 → 4 → 5, only a small loss of quality was found in alloy 4. ■


except in this case copper was added along with the iron. Te loss in quality with the combined addition of iron and copper was larger—about 200 Mpa—but that loss was regained by increasing the silicon content. By contrast, when copper was added by itself; in the


Tis article was based on Paper 13-1224, which was presented at the 117th Metalcasting Congress.


58 | FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION September 2014


1000秒的较长的凝固时间。


从这个结果,可以看出含铁量0.3%的合金处于边 界情况。根据以下反应,铸件快速凝固部分不应该有 初生β相,只有三元共晶: 液态→ Al(固态) + Si(固态) + β


但在较低的凝固速率时会形成初生β相。含铁量较 高时(如0.6%)无论凝固速率如何,初生β相先于 Al-Si共晶出现。


澳大利亚先进凝固技术研究中心(CAST)的研究 人员研究了这种合金。他们发现,铸造缺陷与生成初 生β相的铁含量有关。然而,当他们在相同的合金中 提高硅的含量,缺陷消失了。这种现象产生的原因通 过图7可以看出,类似的计算都是在相同的两个铁含 量下进行。在该合金中较高的铁含量(0.6%)变成 边界情况。因此,含9%的合金可容许的铁含量是含 硅的5%合金的两倍。


以这种方式,CAST研究人员精确计算了许多硅含 量情况的凝固路径。他们由此得出了铸件的安全铁含 量图(如图8所示)。 Cascers及其合作者开展的相关研究结果理应受到 关注。他们生产了有很多合金成分的铸件并测试了力 学性能,其中一些结果如图9所示。图示指出偏析过 程中微观偏析的重要性,以及到底多高的硅含量对 Al-Si基铸造合金有利。


铸件进行T6处理后的拉伸强度如质量曲线所示(极 限抗拉强度vs.对数延伸率)。 红线为质量指数(单位为MPa)。蓝色箭头示出了 合金成分的改变,如将铁加入到合金1中得到合金2。 结果显示出铸件质量有很大的损失——根据质量指数 大约损失120MPa。


将硅键入合金2得到合金3,随着硅含量从4.5%提 高到9%,几乎所有损失的质量重新回归。 除了将铜含量和铁共同加入这种情况,从合金1→


合金6→合金7发现了类似的结果。 铁和铜共同加入时质量损失较大——约


200MPa——但会由硅含量的增加重新获得。 作为比较,铜加入时,合金1→合金4→合金5,之 后合金4有较小的质量损失。 ■


本文发表于117届金属铸造大会,文章代号13-1224.


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