Temperature Field Evolution of Seeding during Directional Solidification of Single-Crystal Ni-Based Superalloy Castings

Abstract

By combined numerical simulations and unidirectional solidification experiments, the temperature field evolution of seeding for nickel-based single-crystal superalloys was studied. At the steady state, the position of the mushy zone in the seed moves down as the melt is poured into the mold. The holding time required for the temperature field of the seed segment to reach the steady state is less for seeds with casting than for those without casting. The holding time required to completely eliminate randomly oriented broken dendrites in the melt-back zone is much longer than that required for the temperature field to achieve a steady state. A short incubation stage is required before the temperature field evolution process; then, the migration rate of the isotherm gradually increases until it is the vicinity of the withdrawal rate. Finally, the effect of temperature field evolution on the formation of stray grains in seed segments is discussed.

Orientation Control for Nickel-Based Single Crystal Superalloys: Grain Selection Method Assisted by Directional Columnar Grains

“…Using this novel grain selection method, the misorientation of single crystal casting is limited to a small angle. Similar to the seeding method, the upper part of the directional columnar grains was re-melted, whereas the lower part remained un-melted due to the temperature field characteristics during the heating and holding stage [30]. During the directional solidification process, the melt epitaxially grows on the un-melted directional columnar grains so that the <001> direction of the grains at the top of the starter block deviates from the directional solidification within a small range.…”