Microstructure refinement of single crystal Ni-based superalloy by improvement of thermal radiation shielding in the industrial-scale Bridgman solidification process

Abstract

Refinement of dendritic microstructure in CMSX-4 Ni-based single crystal casting blade, manufactured using standard adjusted (ARB) or novel inner radiation baffles (IRBs) in the industrial-scale Bridgman solidification process, was investigated in the paper. Temperature gradient in the blade was mainly controlled by baffle geometry and by changing local solidification rate along casting height. Results showed that the use of relatively simple modification of mold, by introducing IRBs, resulted in increase of temperature gradient and noticeable refinement of dendritic microstructure. Temperature gradient in airfoil of blade increased by 30 and 95%, while primary dendrite arm spacing (PDAS) was favorably reduced by 7.8 and 21% for ARB and IRBs, respectively, compared with standard ring radiation baffle. The use of IRBs and ARB resulted in increase of tendency to form sliver defect in airfoil and low angle grain boundary in blade platforms. However, length of freckle chains decreased at root edge of blades when IRBs were used. New design and a proposal of the implementation of IRBs technique for the production of large IGT blades using Bridgman method were also discussed in this article.