镍基单晶高温合金DD11杂晶形成的过冷度时间判定Determination of Undercooling Time for Stray Grain Formation of DD11 Single Crystal Superalloy

以第二代镍基单晶高温合金DD11为研究对象,参照工程单晶铸件横向突变结构设计平台试样进行浇注及定向凝固实验。采用光学显微镜(OM)观察平台晶体组成及其形貌特征,并借助差热分析法(DSC)研究DD11合金的过冷效应。以过冷度时间(ΔT/v)判定杂晶形成条件,探讨平台尺寸、空间位置及抽拉速率对杂晶形成倾向性的影响机制,并研究定向凝固过程中的杂晶形成规律。结果表明,当抽拉速率为2mm/min时,外侧平台长度从8mm到26mm均未形成杂晶,而内侧平台尺寸超过18mm时开始形成杂晶,表明较大的平台试样长度和内侧位置会增加杂晶形成的倾向性。随着抽拉速率提高到6mm/min,不同尺寸的外侧平台均形成杂晶,内侧平台则在尺寸大于12mm后出现杂晶,表明增加抽拉速率均提高内外侧平台试样的杂晶形成倾向。与内侧平台试样相比,抽拉速率对外侧平台试样形成杂晶的影响更显著。平台尺寸、空间位置及抽拉速率影响了定向凝固过程中的环境温度和冷却速率,其通过产生较小过冷度时间(ΔT/v)从而增加了杂晶形成的倾向性。

The effects of platform size, site and withdraw rate on the formation of stray grain during the directional solidification of DD11 single crystal superalloy were investigated. The platform structure was designed to simulate the sudden change of plan area in single crystal blades. Based on the composition and microstructure observed with optical microscopy (OM) and the undercooling effect studied with differential thermal analysis (DSC), the critical undercooling time(ΔT/v) for stray grain formation was discussed. Results show that when the withdraw rate is 2mm/min, no stray grain is observed as the outer platform length ranges from 8mm to 26mm, whilst stray grain is formed when the length exceeds 18mm in inner platform. This suggests that larger platform size and inner position lead to stray grain formation. As the withdraw rate gradually increases to 6mm/min, stray grains are revealed in outer platform with various length, and only presented in inner platform with length more than 12mm. It is indicated that the increase of withdraw rate promotes the tendency of forming stray grain in both outer and inner platforms, while it shows more influence in outer ones. During directional solidification, the furnace temperature and cooling rate are influenced by platform size, position and withdraw rate, which lead to higher tendency of stray grain formation associated with small value of undercooling time.

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