Fig. 6. Solidification sequence for conven- tional gating is illustrated (time in minutes).
thick end of the casting solidified in 20 minutes. Microstructures of the castings were compared at loca-
tions L, C and E (indicated in Fig. 6). Dendrite arm spacing was virtually the same at the three locations; however: In the thick section (L) of the casting, the level of micro-
porosity is less in the conventionally gated blade (0.44% vs. 0.88%).
In the thin section of the casting, the level of microporos-
ity is less in the direct poured blade (0.19% vs. 0.25%). No significant differences in the metallurgical quality of the gated and direct poured blades were noticed in spite of the much lower pouring temperature used with the direct pour technology. Te blades were given a standard T6 heat treatment
consisting of solutionizing for 12 hours at 1,000F (538C), followed by quenching in 149F (65C) water and aging for six hours at 320F (160C). Tirteen tensile test bars were excised from the blades at
locations shown in Fig. 7. Te yield strength, ultimate tensile strength and elonga-
tion at break appear in Table 1, along with the Quality Index (Q), which represents the metallurgical quality of the alloy. Te yield strength does not vary much around a value of
200 MPa. However, the ultimate tensile strength and elonga- tion are higher in the thinner parts of the blade where the solidification time is lower. Te quality index is less than 316 MPa, the minimum value of Q required for the ASTM B26 standard tensile specimen. Tis is explained by the fact that the 0.5-in. diameter standard specimen solidifies in about one minute while the solidification time in the blade varies from two to 20 minutes.
Heat Treatment and Modified Heat Treatment Distortion from the quenching process was evaluated by
measuring the shift at certain coordinates. Tis distortion should be reduced as much as possible. Since the mechanical properties in the blades exceeded the requirements (YS ≥140 MPa, El ≥ 2%), the researchers reduced the solutionizing temperature from 1,000F (538C) to 842F (450C). Te modified heat treatment reduced distortion consid-
图6：传统浇注系统的凝固顺序（时 间：分）
在铸件的厚壁部分（L），传统浇注叶片的显微疏 松水平较低（0.44％ vs. 0.88％）。
在铸件的薄壁部分，直接浇注叶片的显微疏松水 平较低（0.19％ vs. 0.25％）。
尽管直接浇注方式的浇注温度低得多，传统浇 注方式或直接浇注方式叶片的冶金质量没有明显差 异。
叶片一般需经过标准的T6热处理，包括在 1000℉（538℃）下12小时的固溶处理，随后在 149F（65℃）的水中淬火和在320℉（160℃）下6 小时的时效处理。
13根拉伸试棒从叶片上取样位置如图7所示。 屈服强度、极限抗拉强度和断裂延伸率，以及代 表合金冶金质量的质量指数（Q），见表1。 屈服强度在约200MPa时变化不大。然而，叶片 较薄部分的凝固时间较短，因此最终的拉伸强度和 延伸率较高。其质量指数低于316MPa，这是ASTM B26标准拉伸试样所需的最小质量指数。事实证明， 直径为0.5英寸的标准试样凝固时间约为1分钟，而 叶片不同部位的凝固时间在2到20分钟。
热处理和改进热处理
淬火过程中的变形通过测量特定坐标的位移来评 估，应尽可能减少这样的变形。由于叶片的机械性 能超过了需求（屈服强度≥140 MPa，延伸率≥2％ ），研究者将固溶温度从1000℉（538℃）降低到 842℉（450℃）。
改进后的热处理工艺极大的减少了变形；然而， 必须对机械性能仍然超过规定的最小值进行验证。 叶片经改进工艺的热处理后，质量指数下降约50 MPa，但是屈服强度和延伸率仍然高于所需的最低
Fig. 7. The sketch shows the location of test bars excised in the blade.
图7：从叶片上取下来 的测试棒的位置图
December 2014
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