鎳基單晶高溫合金DD11雜晶形成的過冷度時間判定Determination of Undercooling Time for Stray Grain Formation of DD11 Single Crystal Superalloy

以第二代鎳基單晶高溫合金DD11為研究對象,參照工程單晶鑄件橫向突變結(jié)構(gòu)設(shè)計平臺試樣進(jìn)行澆注及定向凝固實驗。采用光學(xué)顯微鏡(OM)觀察平臺晶體組成及其形貌特征,并借助差熱分析法(DSC)研究DD11合金的過冷效應(yīng)。以過冷度時間(ΔT/v)判定雜晶形成條件,探討平臺尺寸、空間位置及抽拉速率對雜晶形成傾向性的影響機制,并研究定向凝固過程中的雜晶形成規(guī)律。結(jié)果表明,當(dāng)抽拉速率為2mm/min時,外側(cè)平臺長度從8mm到26mm均未形成雜晶,而內(nèi)側(cè)平臺尺寸超過18mm時開始形成雜晶,表明較大的平臺試樣長度和內(nèi)側(cè)位置會增加雜晶形成的傾向性。隨著抽拉速率提高到6mm/min,不同尺寸的外側(cè)平臺均形成雜晶,內(nèi)側(cè)平臺則在尺寸大于12mm后出現(xiàn)雜晶,表明增加抽拉速率均提高內(nèi)外側(cè)平臺試樣的雜晶形成傾向。與內(nèi)側(cè)平臺試樣相比,抽拉速率對外側(cè)平臺試樣形成雜晶的影響更顯著。平臺尺寸、空間位置及抽拉速率影響了定向凝固過程中的環(huán)境溫度和冷卻速率,其通過產(chǎn)生較小過冷度時間(ΔT/v)從而增加了雜晶形成的傾向性。

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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