Simulation of stray grain formation during single crystal seed melt-back and initial withdrawal in the Ni-base superalloy CMSX4

Stray grains are often found to occur in the seed melt-back region of single crystal turbine blade castings. The use of a helical grain selector between the seed and the blade is thus needed to prevent these defects from growing; however, this helix has inherently practical limitations. In this study, mesoscale numerical simulations were conducted to investigate the influence of casting parameters on stray grain formation towards the final goal of precluding the need for a grain selector. The model was first validated by comparison of predicted and experimentally observed microstructural features. Then, the influence of withdrawal velocity, thermal gradient, crystallographic orientation and alloy properties on the maximum solute-adjusted undercooling in the liquid, and hence the potential for stray grain formation was studied during seed melt-back. The predictions show that maximum solute-adjusted undercooling is highly sensitive to the solidification rate and the seed orientation. However, increasing the thermal gradient had little influence on the maximum solutal adjusted undercooling and hence should have less effect on stray grain formation.