重力对高温合金定向凝固组织的影响

Effect of Gravity on Directionally Solidified Structure of Superalloys

分别利用常规下抽拉法与新型上提拉法进行不同方向的高温合金定向凝固实验,对比研究重力对单晶铸件凝固组织的影响。结果表明,在常规下抽拉法实验的向上凝固过程中,容易出现雀斑、γ/γ’共晶上聚和籽晶回熔紊乱等问题。原因是糊状区内液体由于元素偏析引起密度减小,在重力作用下形成了上重下轻的失稳状态并引起对流。而通过新型上提拉法实现的顺重力凝固过程中,密度减小的液体处于糊状区上端,形成上轻下重的稳定状态,使重力的作用由失稳因素转化为维持稳定的因素,抑制了液体对流的产生与发展。采用新型上提拉法制备的单晶铸件中彻底消除了雀斑缺陷,抑制了γ/γ’共晶组织的向上聚集,也保证了低密度籽晶稳定的回熔和外延生长。顺重力定向凝固技术从根本上消除了重力对高温合金定向凝固的不良影响,有希望发展成为新一代的先进单晶叶片成型技术。

While using traditional methods of directionally solidifying superalloy castings,the liquid density at the lower region of the mushy zone gradually lowers than the top.This is due to a strong segregation of alloying elements.The gravitational force then exacerbates this density inversion,leading to upward convection from the mushy zone to the liquid ahead of the solidification front.This process,known as solutal convection,results in several solidification defects such as freckle defects,an upward accumulation of γ/γ' eutectic,and seeding process issues.As higher-generation single crystal superalloys continue to develop,the problems of element segregation and solutal convection become more pronounced.Traditional measures,such as adjusting process parameters,struggle to effectively alleviate these issues.Given that these problems largely arise from gravity-induced fluid flow,this work aims to investigate the role of gravity on solidification structure and propose appropriate solutions.To achieve this,the conventional pull-down and novel pull-up methods were adopted to perform directional solidification experiments with superalloys.The influence of gravity on solidification behavior is starkly different in these two experiments.In the pull-down process,dendrites grow upward,against gravity,leading to a variety of solidification defects such as freckles on the casting's lateral surface and an upward accumulation of γ/γ' eutectic on the upper surface of the single crystal turbine blade castings.Stray grains also formed in the remelting region during seeding.These phenomena are caused by the density inversion of the remaining liquid between dendrites,resulting in a top-heavy and bottom-light hydrodynamic state.Liquid convection in the mushy zone was then unavoidable under gravity in the pull-down process.In contrast,the pull-up process had dendrites growing downwards,in line with gravity,leaving the least dense liquid at the top of the mushy zone.In this top-light and bottom-heavy state,gravity stabilizes the segregated residual liquid in the mushy zone,thereby preventing solutal convection.Consequently,freckle defects were eliminated,and the γ/γ' eutectic structure was evenly distributed,not accumulated,on the upper surface of the single crystal blade's platform.Additionally,the stability of remelting and epitaxial growth of seed crystals was ensured by eliminating liquid convection.By using this pull-up process,the negative effects of gravity on the directional solidification of superalloys were removed,and all gravity-related solidification defects consequently disappeared.This novel pull-up process could potentially be developed into a new production process for single crystal superalloy castings,significantly improving casting quality.However,it should be noted that this new pull-up process is more complex in comparison to the conventional method.Although this work lays the groundwork for this process,further technological enhancements are required before this method can be applied to industrial production.