Papers by Keyword: Freckles

Abstract: 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.

Abstract: Freckle occurrence is known to be dependent on the casting size and the superalloy components with large cross section are more prone to freckle formation. In our directional solidification experiment of superalloys freckles are observed in some thin samples, while some significantly thicker samples in the same shell mould cluster remain freckle free. The EBSD analysis shows that the samples with freckles have a good <001> axial orientation, while in freckle free samples the primary dendrite stems diverge from the sample axis. In an analogue system the flow behaviour through a mushy zone was simulated, in which the dendrite network was placed in different orientation. The <001> dendrite array shows the lowest resistance to the vertical melt flow. As the dendrite alignment diverges from <001> orientation, the flow speed slows down and reaches the minimum at the <111> orientation. This orientation effect of the interdendritic permeability explains well why the crystal orientation plays such an important role in the occurrence of freckles in superalloy castings.

Abstract: The alloys required for fossil fuel power systems are transitioning from stainless steels that operate below 600oC to nickel-based alloys that can operate up to 760oC in advanced ultra-super critical steam turbines. This transition brings with it major metallurgical as well as economic challenges related to alloy design, melt processing, and fabrication of these large size components. The alloys, in general, must maintain creep resistance over 100,000 hours of service life while at the same time maintaining resistance to severe steam oxidation and general oxidation. A need exists for nickel-based alloys that are not only highly alloyed, but are also impervious to phase instabilities during initial processing and service. The potential exists for severe segregation when casting large ingots. This possibility must be dealt with during thermo-mechanical processing to obtain the wrought structures that can be inspected to design defect levels. This paper will detail these challenges as they have been addressed in smaller aerospace turbines and discuss strategies to overcome these problems. New computational modeling tools will play a critical role in engineering solutions for alloy design, solidification, forging, and heat treatment. Since any solution also needs to be economically viable, the paper also discusses processing cost issues in terms of the process yields and operational strategies. The use of powder metallurgy will also be reviewed as a cost effective alternative to alloys that have traditionally been cast-wrought processed.