Laser Repair of Superalloy Single Crystals with Varying Substrate Orientations
- PDF / 712,413 Bytes
- 11 Pages / 594 x 792 pts Page_size
- 7 Downloads / 224 Views
Laser Repair of Superalloy Single Crystals with Varying Substrate Orientations S. MOKADEM, C. BEZENC xON, A. HAUERT, A. JACOT, and W. KURZ The casting and repair of single-crystal gas turbine blades require specific solidification conditions that prevent the formation of new grains, equiaxed or columnar, ahead of the epitaxial columnar dendrites. These conditions are best determined by microstructure modeling. Present day analytical models of the columnar-to-equiaxed transition (CET) relate the microstructure to local solidification conditions (temperature gradient and interface velocity) without taking into account the effects of (1) a preferred growth direction of the columnar dendrites and (2) a growth competition between columnar grains of different orientations. In this article, the influence of these effects on the grain structure of nickel-base superalloy single crystals, which have been resolidified after laser treatment or directionally cast, is determined by experiment and by analytical and numerical modeling. It is shown that two effects arise for the case of a nonzero angle between the local heat flux direction and the preferred dendrite growth axis: (1) the regime of equiaxed growth is extended and (2) a loss of the crystal orientation of the substrate often occurs by growth competition of columnar grains leading to an ‘‘oriented-to-misoriented transition’’ (OMT). The results are essential for the definition of the single-crystal processing window and are important for the service life extension of expensive components in land-based or aircraft gas turbines. I. INTRODUCTION
DUE to their outstanding high-temperature properties, nickel-base superalloys are widely used for hot section components in gas turbines. To obtain longer service life of aircraft engines or land-based energy generation systems, single crystals are used for the most solicited highpressure/high-temperature turbine blades. The high cost of these components makes the repair of damaged blades highly desirable, using special welding, brazing, or deposition processes. In all of these processes, it is essential that the single-crystal microstructure is retained and that hot cracks are avoided. Although the concepts presented here are useful for both single-crystal casting and repair, this article will concentrate mainly on repair aspects. Results presented will provide a sound basis for upscaling to an industrial process. During the last decade, it has been shown that singlecrystal repair is feasible by epitaxial laser metal forming. In this process, alloy powder of the same[1–4] or of different composition to that of the substrate[5,6] is injected into the molten pool formed by a moving high intensity laser beam (Figure 1). By depositing successive layers on top of each other, single-crystal parts such as walls or of more complex geometries have been built onto a single-crystal substrate S. MOKADEM, formerly Ph.D. Student, Institute of Materials, Ecole Polytechnique Fe´de´rale de Lausanne, 1015 Lausanne EPFL, Switzerland, is with Siemens Power G
Data Loading...
