Abstract
Laser powder deposition is one of the most promising methods for the repairing of the single crystal Ni-based superalloys components used in the hot-section of gas turbine engines in order to extend their lifetime and reduce their overall cost. The microstructure of Ni-based superalloys deposited on single crystal substrates of similar materials depends mainly on the materials involved, on the orientation of the deposited tracks in relation to the substrate and on the deposition parameters. In the present paper these relations are discussed and illustrated for the case of single and multiple layer depositions of NiCrAlY and Rene N4 on (100) single crystal substrates of SRR99 and CMSX-4 Ni-based superalloys. On the other hand, when the aging treatment is applied directly to the solidification microstructure resulting from laser deposition, abnormal γ/γ′ microstructures may result, due to the inhomogeneity created by alloying elements partition during solidification. Performing a homogenization annealing before aging circumvents this difficulty. The homogenizing annealing also eliminates undesirable brittle phases present in the solidification microstructure, such as carbides and topologically close-packed compounds.
Highlights
Nickel-based superalloys used in the hot-section of gas turbine engines are exposed to extremely severe operating conditions, requiring ductility, high-temperature strength, high fatigue, creep, thermal fatigue, and oxidation resistance.1 In order to achieve this combination of properties, these alloys are designed with a c-Ni matrix, strengthened by a high concentration of alloying elements in solid solution and a large volume fraction of Ni3(Al,Ti) precipitates (c0-phase) with a cuboid shape, coherent with the matrix.2The deposition of Ni-superalloys such as Inconel 625,3–5 Waspaloy,6 Inconel 718,7 and Inconel 738 (Ref. 8) on polycrystalline substrates has been extensively investigated
Laser powder deposition is one of the most promising methods for the repairing of the single crystal Ni-based superalloys components used in the hot-section of gas turbine engines in order to extend their lifetime and reduce their overall cost
The tensile strength and the fatigue resistance at room temperature and at 650 C of IN-625 alloy produced by laser powder deposition (LPD), casting, and hot working were compared by Theriault et al
Summary
Nickel-based superalloys used in the hot-section of gas turbine engines are exposed to extremely severe operating conditions, requiring ductility, high-temperature strength, high fatigue, creep, thermal fatigue, and oxidation resistance. In order to achieve this combination of properties, these alloys are designed with a c-Ni matrix, strengthened by a high concentration of alloying elements in solid solution and a large volume fraction of Ni3(Al,Ti) precipitates (c0-phase) with a cuboid shape, coherent with the matrix.. Santos et al. carried out a similar observation in what concerns the topologically close-packed compounds formed during solidification of Rene N4 alloy These authors showed that when ageing is directly applied to the material in the as-deposited condition, the precipitates present an abnormal shape due to alloying elements segregation within the dendrites and the normal structure can only be achieved after an homogenizing annealing.. G€aumann et al. showed that clad layers of CMSX-4 Ni-based superalloy deposited under appropriate processing conditions on a (100) single crystalline substrate of the same material by laser cladding were single crystalline, with a crystallographic orientation similar to the substrate They explained this result by the fact that, usually, in laser deposition solidification starts by epitaxial growth on the substrate and growth is columnar dendritic. Epitaxial growth is frequently observed in welding, laser melting, and laser cladding because in these
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