Some New Observations on Freckle Formation in Directionally Solidified Superalloy Components
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FRECKLES have attracted the attention of researchers for more than 40 years since the pioneering works by Giamei et al.[1,2] Freckles are a detrimental grain defect observed in directional solidification (DS) and singlecrystal (SC) castings of superalloys leading to a high rejection rate. They appear as long trails of equiaxed (EQ) grains, aligning roughly parallel to the direction of gravity. It is commonly believed that freckles are formed as a result of the thermosolutal convection induced by inverse density in the mushy zone.[2] During upward directional solidification, the interdendritic melt can become gravitationally unstable because of the chemical segregation of alloying elements. The density difference between the less dense solute in the mushy zone and the bulk liquid ahead of the dendrite tip can cause thermosolutal convection when the driving force for the fluid flow exceeds the surrounding frictional forces. Because of the remelting and fragmenting of the dendrite arms, freckles may appear. Once a freckle chain has formed in a fully solidified crystal, it is impossible to remove such an undesired defect through thermal treatments. Generally, the occurrence of freckles is known to be dependent mainly on three factors: alloy chemistry, solidification parameters, and casting size.[1,3] First, the probability of freckle occurrence has been found to be strongly dependent on the composition of the alloys.[1,2,4,5] As the level of refractory alloying additions to SC superalloys has increased to improve hightemperature mechanical properties, freckle formation during DS has increasingly become a serious problem. Although freckles can be reduced potentially by optimizing the levels of Ta, W, and Re, this is unlikely to DEXIN MA, Senior Scientist, QIANG WU, Ph.D. Student, and ANDREAS BU¨HRIG-POLACZEK, Professor, are with the Foundry Institute, RWTH University, Aachen 52072, Germany. Contact e-mail: [email protected] Manuscript submitted May 5, 2011. Article published online November 29, 2011. 344—VOLUME 43B, APRIL 2012
benefit phase stability, corrosion, and creep fatigue properties simultaneously. Therefore, alloying alternatives that improve solidification characteristics without significantly compromising the physical and mechanical properties of the alloys are highly desirable.[4] Another factor playing an important part in freckle formation is the solidification condition. The process parameters affect directly the dendrite spacing and the permeability of the mushy zone. A lower temperature gradient G and solidification velocity V result in the larger dendrite spacing and higher permeability. This result promotes convective flows in the mushy zone and assists the formation of freckles. Finally, the extent of freckling becomes more pronounced as the casting size increases.[1,4,6,7] When a casting is of significant crosssectional area, the wide mushy zone can provide a sufficient reservoir to support the interdendritic convection and, hence, the freckling onset. Therefore, for a defined alloy system and casting process
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