Stray Grain Formation in Welds of Single-Crystal Ni-Base Superalloy CMSX-4
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OYS with an Ni base have long been used in the manufacture of gas turbine engine components because of their high melting temperatures and excellent high-temperature mechanical properties. Advanced single-crystal (SX) casting techniques have been developed that extend the maximum service temperatures within the engine for improved efficiency, and the resulting improvements in creep resistance have also extended their service lifetimes.[1] However, replacement of SX components is still necessary due to the effects of thermal fatigue and erosion. Owing to a high manufacturing cost (~$30,000 for a single turbine blade[2]), the development of an effective weld repair strategy for failed or miscast SX components has been widely pursued due to the potential cost savings in the engine industry. The primary goal has been to produce weld fusion zones that are free of equiaxed grains that compromise the integrity of the SX component by introducing grain boundaries. Because these equiaxed grains have crystal orientations that differ from that of the base metal, they are often termed ‘‘stray’’ grains (SGs). Early SX welding research performed by Rappaz et al.[3–5] described the major features of an SX weld T.D. ANDERSON, Research Engineer, is with the Upstream Research Division, ExxonMobil, Houston, TX 77002. J.N. DUPONT, R.D. Stout Distinguished Professor, is with the Materials Science and Engineering Department, Lehigh University, Bethlehem, PA 18052. Contact e-mail: [email protected] T. DEBROY, Professor, is with the Materials Science and Engineering Department, Pennsylvania State University, College Park, PA 16802. Manuscript submitted March 3, 2009. Article published online November 6, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
zone using electron-beam (EB) welds conducted on a high-purity Fe-Ni-Cr stainless-steel-type alloy. Solidification of the weld metal was observed to proceed via epitaxial growth from the base metal along the h100i (i.e., ‘‘easy’’) growth directions. They developed geometric models that demonstrate that the active h100i growth direction will be that which is most closely aligned to the maximum heat-flow direction. Knowledge of the growth direction permits calculation of the dendrite tip velocity, which is a function of the heatsource travel speed and the weld-pool geometry. Experimental weld trials also showed that the weld-pool shapes were independent of orientation with respect to the SX substrate, indicating that heat flow was isotropic in the SX base metal. The root cause for equiaxed grain formation was not discussed, because these formations were rarely observed in these high-purity alloys. Subsequent studies indicated that the simplified stainless steel composition did not induce the necessary extent of constitutional supercooling required for stray grain (SG) formation. Vitek et al.[6,7] expanded upon the work of Rappaz, and showed that all of the central features of SX solidification were present in SX Ni-base superalloy welds. The theory of constitutional supercooling was used to describe the form
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