Freckle formation and freckle criterion in superalloy castings
- PDF / 895,996 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 37 Downloads / 205 Views
I. INTRODUCTION A. Background Freckles are a common defect found in nickel-based superalloy castings. They appear as a long trail of equiaxed grains with a composition shift consistent with alloy segregation. Some porosity and feeding shrinkage may also be present in and adjacent to the freckle line. Freckles are highly undesirable in critical applications because of their deleterious effect on mechanical performance. Moreover, castings containing freckles must be scrapped, causing considerable economic loss because they cannot be removed by postcast, thermomechanical treatments. Development in large aircraft engines and also in large land-based gas turbines for power generation requires a considerable scale-up in the diameter of turbine disks and in the size of turbine blades. One of the main problems encountered in scaling-up has been the extensive freckling observed in these large castings, which has led to unacceptable rejection rates. Traditionally, for example, freckle formation in ingots can be avoided by keeping ingot diameters and melting rates below critical values, whereas in directional solidified (DS) and single crystal (SX) turbine blades, their occurrence has been avoided by maintaining high thermal gradients at the solidification front. Two general approaches appear in the literature for predicting the onset of freckling: one, based on the use of
P. AUBURTIN, formerly Postdoctoral Candidate, Department of Metals and Materials Engineering, University of British Columbia, is Process Modeling Research Engineer with PSA-Peugeot Citroen, Bievres, France 91570. T. WANG, Postdoctoral Candidate, S.L. COCKCROFT, Associate Professor, and A. MITCHELL, Professor, are with the Department of Metals and Materials Engineering, University of British Columbia, Canada V6T 1Z4. Manuscript submitted August 23, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
mathematical criteria;[1–4] the other, based on solution of mass, momentum, energy, and species conservation equations.[5–10] While the latter approach is more fundamental, and may ultimately be applicable to a broader range of conditions, the computational severity of this approach together with the absence of high-temperature data presently precludes its practical application to industrial processes. Thus, given the present limitations, the best approach seems to be to adopt mathematical criteria for use with more straightforward “heat-flow” mathematical models.
B. Literature Review It is now generally agreed that freckles arise due to channels associated with thermosolutal convection in the mushy zone and that they are driven by a variation in density originating from interdendritic segregation (Figure 1). A variety of straightforward mathematical criteria based on the local thermal gradient G, local solidification rate R, and local solidification time have been suggested in the literature.[1,3,11,12] Unfortunately, these simple criteria generally perform poorly against experimental results.[2] A mechanistically more correct criterion based on the Rayleigh numbe
Data Loading...
