Investigation of Freckle Formation under Various Solidification Conditions
In the present work the influence of process parameters on freckle formation in superalloy CMSX-4 is demonstrated. A series of experiments were carried out using a laboratory furnace of Bridgman-type in which the temperature gradient G and solidification velocity V can be precisely controlled and individually varied over a wide range. On the etched surface and longitudinal sections of the quenched samples the formation and evolution of freckles were investigated. The initiation position of the freckles within the mushy zone was then determined. Based on the experimental observations a complete diagram was plotted to indicate the probability of freckle occurrence which is related to solidification parameters G and V. In this freckling map the freckle region is delimited by different criteria. Freckles arise only within a certain G/V-range for columnar dendrite growth; otherwise the solidification structure changes into cellular or equiaxed structure, depending on whether G/V-value increases or decreases. In comparison with the well-known freckling map of Copley et al., some new freckle-free regions are proposed. It is interesting to note that the freckle formation is also suppressed at very low cooling rates. In addition, the initiation position of the freckle formation in the mushy region was determined to be less than 2 mm below the dendrite tip of the solidification front.
D. X. Ma, B. Zhou, A. Bührig-Polaczek, "Investigation of Freckle Formation under Various Solidification Conditions", Advanced Materials Research, Vol. 278, pp. 428-433, 2011