Mathematical Modeling of Transport Phenomena in Multi-track and Multi-layer Laser Powder Deposition of Single-Crystal Su
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IN recent years, in order to increase the fuel-use efficiency of gas turbine engines, nickel-base single-crystal (SX) superalloys have been used as the hot-section components such as blades, blisks, and vane seal segments.[1–4] The main advantage of SX over polycrystalline nickel-based superalloy is their improved high-temperature creep and thermal fatigue resistance, due to the suppression of grain boundary strengthening elements and the elimination of grain boundaries.[5–8] In order to reduce the manufacturing cost and develop the potential of single-crystal turbine blades, researchers are trying to
ZHAOYANG LIU is with the College of Innovation and Entrepreneurship, Southern University of Science and Technology, Shenzhen 518055, China and also with Shenzhen Key Laboratory for Additive Manufacturing of High-Performance Materials, Southern University of Science and Technology, Shenzhen 518055, China. Contact e-mail: [email protected] LIANG JIANG and ZI WANG are with the State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China. LIJUN SONG is with the Hunan Provincial Key Laboratory of Intelligent Laser Manufacturing, Hunan University, Changsha 410082, China. Manuscript submitted November 20, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
manufacture three-dimensional (3D) SX parts directly by laser powder deposition (LPD) process.[9] LPD process, which allows rapid and accurate addition of controlled amounts of material at the required locations with minimal heat input, has been proven to be a very promising technology in restoring the worn tip of SX components,[10–12] and can manufacture metallic parts track by track and layer by layer with computer-aided designing (CAD) and computer-aided manufacturing (CAM) methods.[13] The major challenge of directly manufacturing SX parts by LPD technology is to keep the continuity of SX microstructure between tracks and layers, and avoid the columnar-to-equiaxed transition (CET). Stray grains (mainly equiaxed grains), once form, destroy the original columnar dendritic growth and, furthermore, cause the formation low-melting grain boundaries which act as the easy path for crack initiation and propagation.[14] During the low-penetration LPD process of SX superalloy on a substrate with similar material, the crystal growth behaviors are determined by the constitutional supercooling (CS) at the solidification interface. Hunt showed that the CS can be described by the G/V ratio, where G is the thermal gradient and V is solidification velocity at the crystal tip.[15] Ga¨umann et al. developed Hunt’s analytical model for the LPD process of CMSX-4 superalloy on a substrate with [001]/
crystallographic orientation normal to the depositing surface.[10] It was demonstrated that a full columnar structure can be obtained in the deposited bead when the ratio G3.4/V exceeds a critical value KCET. High thermal gradient G and low growth velocity V tend to benefit the epitaxial growth of columnar dendrites. Recently, Liu et al. developed a 3D mathematical
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