Numerical Simulation of Transport Phenomena for a Double-Layer Laser Powder Deposition of Single-Crystal Superalloy
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-CRYSTAL (SX) superalloys have been commonly used in the turbine engine industries due to their superior performance in high-temperature strength and creep resistance.[1] The repair of turbine blades made of SX superalloy has drawn attention due to the great value (up to 80 pct) it saves compared to costly blade replacement.[2] Laser powder deposition technology has shown to be an effective means to restore the worn blade squealer tips in a near-net shape and to be able to produce sound material properties that meet the repair requirements.[3] However, this technology has only been used to deposit polycrystalline alloys in turbine blade repair applications. The polycrystalline material once deposited on SX turbine blade causes degraded material mechanical properties and reduced creep-fatigue life of parts due to the appearance of grain
ZHAOYANG LIU, Ph.D. Candidate, and HUAN QI, Assistant Professor, are with the University of Michigan—Shanghai Jiao Tong University Joint Institute, Shanghai 200240, P.R. China. Contact e-mail: [email protected] Manuscript submitted July 12, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
boundaries. Misoriented grains or stray grains in SX, once formed, produce grain boundaries with low melting temperature elements, which weaken the material strength and cause strain age crack in the repaired material. The ideal SX blade repair technology should produce a similar crystalline structure to the base material, which requires the SX or directionalsolidified (DS) crystalline orientation of the base material to be retained and to grow continuously during the solidification of laser powder deposition process. Previous studies[4,5] have shown that misoriented or stray grains are often formed near the top surface of the laser-deposited SX alloy, which leads to the formation of grain boundaries along the crystalline orientation transition front. Thus, for a multilayer laser powder deposition of SX superalloy, the remelting of the top portion of the misoriented or stray grains plays an important role to enable the continuous epitaxial growth of the [001] crystalline orientation from the previously deposited layer for successful SX blade tip repair. Many works have shown that the distribution of microstructure in deposit is affected by many parameters of the laser-deposition process. Ga¨umann et al.[6] identified the necessary columnar-to-equiaxed transition (CET) condition based on SX superalloy CMSX-4,
which can also be used for other multicomponent nickelbased SX superalloys. The solidification microstructure in the deposit is directly influenced by the solidification conditions such as temperature gradient G and solidification velocity V prevailing at the solid/ liquid transformation front of molten pool. Liu and DuPont[7,8] studied the effects of molten pool geometry and substrate crystallographic orientations on the crystalline growth direction and microstructure development during laser remelting processing of SX material. They proposed a predefined three-dimensional molten pool geometry model a
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