Stray grain formation in the seed region of single-crystal turbine blades

Seed crystals are frequently used to provide an off-axial 〈001〉 crystallographic orientation to investment cast single-crystal, nickel-based superalloy turbine blades. However, stray grain defects can form during the melt-back of the seed crystal, requiring the use of a helical grain selector between the seed and the blade to remove them. Using meso-scale numerical simulations, the formation mechanisms of these stray grain defects have been investigated. Also investigated was the influence of the seed’s crystallographic orientation relative to blade axis. The model is first validated by comparison to experimental observations and then by its application to a range of casting situations. The results show that initiation of these defects is difficult to avoid. Instead, the impact of stray grains should be controlled during their growth.