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Fig. 6. This microstructural image shows the progression of hot tearing.


图6:微观结构图显示了热裂的形 成过程。


solid phases in the solidifying metal, ranged from 75-90%. Te defects were reduced or eliminated when the fraction solid was reduced to 65-75%, due to the presence of liquid metal that fed the solidification shrinkage. Shrinkage underneath the top risers and the junctions


were eliminated by redesigning the risers by either using cylindrical insulating risers with increased contact or by insu- lating the current top risers.


Successfully Filling the Mold Te bracket component was selected because of its fairly


complex geometry that could be typical of structural compo- nents. Given its dimensions and geometry, the bracket is difficult to produce by traditional permanent mold casting techniques, especially when using alloys that are prone to hot tearing. Te mold was preheated by gas torch, and two thermo-


couples (TC) were placed in the mold to monitor the mold temperature (Fig. 5). Te thermocouples were located at the top (TC1) and bottom (TC2) of the moving half of the mold. Infrared (IR) pyrometers also measured mold surface temperatures at two locations. Te mold surface temperature between the two risers was measured by IR3, and IR4 mea- sured the temperature of the surface near the flange under the hoop section of the casting. Te thermocouples and pyrometers were placed 0.75 in. (19mm) from the surface of the die cavity. Because TC1 and IR3 were closer to the pour- ing cups than TC2 and IR4, the mold temperatures at TC1 and IR3 usually were higher. When the mold temperature in certain sections of the


mold rose above 932F (500C), some castings broke during ejection. To lower the mold temperature and reduce break- age, engineers turned the cooling water flow rate to its highest level.


些问题部位与热裂的产生有关。


热裂一般发生在固相占比(即处于凝固过程中的 固相所占的份额)为75-90%时。当固相占比降低到 65-75%时,由于有补给凝固收缩的金属液,铸件缺 陷将减少或消失。


通过改进冒口设计,采用圆柱形保温冒口增强接触 或对现行的顶冒口加以保温,可消除顶部冒口下方和 热节处的缩松。


成功充型


选择支架铸件的原因是:该铸件的几何形状相当复 杂,具有结构铸件的特点。对于这样的尺寸和几何形 状的铸件,用传统的金属型铸造工艺很难生产,尤其 是使用易于产生热裂的合金。


金属型由煤气喷枪预热,金属型中放置2个热电偶


(TC)以监控温度(图5)。热电偶位于铸型可移动部 分的顶部(TC1)和底部(TC2)。红外高温计也能监 测这两个点的铸型表面温度。两个冒口之间金属型的 表面温度由红外高温计IR3监测,红外高温计IR4监测环 部下方凸缘的表面温度。热电偶和高温计与型腔表面的 距离为19mm。由于TC1、IR3距离浇口杯比TC2、IR4 近,通常情况下,TC1、IR3的铸型温度更高。 当某些部位的铸型温度升至932F (500C)以上时, 一些铸件就会在顶压离型过程中破裂。降低铸型的温


Fig. 7. The distribution of shrinkage in an un- etched microstructure sample is shown in a 1.3-in. section (left) and a 0.51-in section (right).


图7:1.3-in.


(3.3cm)部位(左) 和0.51英寸(1.3cm) 部位(右)未浸蚀 的微观结构显示缩 孔的分布状况。


44 | FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION March 2015


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