Investigation on freckles in directionally solidified CMSX-4 superalloy specimens with abrupt cross section variation

摘要

The freckle formation was investigated in directionally solidified CMSX-4 superalloy specimen with abrupt cross section variation to better understand that in the real hollow blade. Freckles were usually observed on the specimen surface within a range of 2-15 mm above the platform, and the freckles on both sides were slanted to respective edge. The freckling number was reduced with increasing withdrawal rate. Combining with the simulation, freckle formation was analysed. The deceleration of dendrites growth above the platform caused by the abrupt variation in cross section leads to an increase in the tendency of the fact that the interdendritic fluid-flow velocity is larger than the growth rate, and thus increasing the freckling tendency. The pressure difference in the horizontal direction induced by the concaved solidification interface forces the interdendritic segregated melt to flow toward the surfaces and edges, and slants the freckles. The freckle formation can be suppressed with increasing withdrawal rate for the refined dendrite microstructure. (C) 2016 Elsevier B.V. All rights reserved.