The formation mechanism, influencing factors and processing control of stray grains in nickel-based single crystal superalloys

The thermal profile of a directionally solidifying nickel-based single crystal (SX) turbine blade was simulated using a Procast finite element technique. The result showed that the local thermal profile in the platform is wave-shaped rather than a planar or consistent concave. Such a distributed thermal profile forms an isolated thermal undercooling zone in the platform end and affects the dendritic branching process into the platform region. Based on the thermal and grain growth analyses, the criterion of stray grain formation was proposed as a competition process on the time of the dendritic branching from the airfoil to the platform end and the cooling of the platform end to the heterogeneous nucleation temperature. The influencing factors, namely the withdrawal rate, the directional solidification method, the platform dimension, the alloy composition and the crystal orientation on stray grain formation were then systematically investigated according to the criterion. Several stray grain (SG) controlling techniques with different mechanisms were proposed and analyzed based on the thermal evolution process.