Effect of Gravity on Directionally Solidified Structure of Superalloys

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 gamma/gamma' 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 gamma/gamma' eutectic on the upper surface of the single crystal turbine blade castings. Stray grains also formed in the re-melting 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 gamma/gamma' 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.