Formation Mechanism of Fusion Zone in Growth of Single Crystal Superalloy with Low-Segregated Heterogeneous Seed

Seeding technique is a promising method for growing single crystal superalloy blade. However, sometimes stray grains nucleate in the transformation process of single crystal structure from a seed, which always cause failure of single crystal growth. In order to obtain single crystal with high perfection structure, Ni-based single crystal superalloy was prepared with low-segregated seeds by high rate solidification (HRS) method in the dual heating zone furnace. The melt-back zones of seeds were investigated systematically, and the results showed that a fusion zone without microsegregation exists in front of the melt-back equilibrium interface of seeds, in which solidification interface transited from planar to cellular. Further experiments showed that increasing the W content of seeds or the solidification rate can both accelerate the whole non-steady transition process and make fusion zone shrink. Compared with the traditional seeding method, the low segregated heterogeneous seeding technique can increase the casting yield by avoiding the nucleation of stray grains in the fusion zone, which caused by the pinched-off secondary dendrites and constitutional undercooling.