抽拉速率对单晶铸件中共晶组织不均匀分布的影响

Effect of Withdrawal Rate on Non-uniform Distribution of Eutectic in Ni-based Single Crystal Superalloy Castings

摘要:

单晶高温合金的微观结构相对简单，主要由γ枝晶和γ/γ′共晶组成。在镍基单晶高温合金的定向凝固过程中，抽拉速率是影响 γ/γ′共晶沿重力方向分布的关键参数。结果表明，无论抽拉速率如何，铸件平台上表面的γ/γ′共晶比例始终高于平台下表面。且随着抽拉速率的降低，上表面中γ/γ′共晶的比例显著增加，而下表面则减少。在抽拉速率降低时， γ/γ′共晶向上聚集变得更为严重。同时，样品中Al+Ta的百分比与γ/γ′共晶的比例呈正相关。Al和Ta元素在凝固前沿中的热-溶质对流是导致共晶非均匀分布的主要原因。即使经过后续的固溶热处理，也不能消除γ/γ′共晶的非均匀分布，导致上表面共晶超标并最终致使叶片报废。

Abstract:

The microstructure of single crystal superalloy is relatively simple, consisting primarily of γ dendrites and γ/γ′ eutectics. During the directional solidification process of Ni-based single crystal superalloys, withdrawal rate is a critical parameter affecting the spatial distribution of γ/γ′ eutectic along gravity direction. The results show that the γ/γ′ eutectic fraction of the upper platform surface is always higher than that of the lower one, regardless of withdrawal rate. As the withdrawal rate decreases, there is a significant increase in γ/γ′ eutectic fraction on the upper surface, while it decreases on the lower surface. The upward accumulation of γ/γ′ eutectic becomes more severe as the withdrawal rate decreases. It is also found that the percentage of Al+Ta is positively correlated with the γ/γ′ eutectic fraction. Thermo-solute convection of Al and Ta solutes in the solidification front is the prime reason for the non-uniform distribution of eutectic. The non-uniform distribution of γ/γ′ eutectic cannot be eliminated even after subsequent solution heat treatment, resulting in excess eutectic on the upper surface and thus leading to the scrapping of the blade.