Laser-Aided Direct Writing of Nickel-Based Single-Crystal Super Alloy (N5)
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ingle-crystal (SX) nickel-based super alloys that are widely used in high-temperature high-pressure gas turbine blades exhibit superior creep strength, thermal fatigue life, and corrosion resistance when compared to equiaxed conventional cast (CC) or columnar directionally solidified (DS) counterparts.[1–3] SX super alloys are mostly produced by casting, which requires a high-cost vacuum furnace with specially designed molds and a complex cooling system. Laser-aided direct metal deposition (DMD) is a rapid manufacturing technique, which can form 3-dimensional geometries directly from metal powder using a high-power laser beam without the assistance of tooling.[4,5] It offers one of the most promising alternatives to single-crystal airfoil manufacturing and repairing, providing its enormous design flexibility along with potential cost savings. As first demonstrated by Kurz et al. in 1999,[6] the process of depositing layers epitaxial on single-crystal substrate using lasers has been frequently designed,
YICHEN WANG, Graduate Student, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2125. JEONGYOUNG CHOI, Graduate Student, is with the Department of Mechanical Engineering, University of Michigan. JYOTI MAZUMDER, Robert H Lurie Professor of Engineering, is with the Department of Materials Science and Engineering, University of Michigan, and also with the Department of Mechanical Engineering, University of Michigan. Contact e-mail: [email protected] Manuscript submitted March 9, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
modeled, and simulated. Most research published had verified the feasibility of repairing turbine airfoil using this process in small volume, with single crystal grown up to 12 layers or approximately 3 mm in height.[6–11] However, no laser-deposited nickel-based single crystal had reached the scale of practical turbine blades, that is, few inches in height. Due to the size limitation of single-crystal deposit, very few mechanical properties were reported.[12] Based on theory developed by Hunt[13] and Ga¨umann et al.,[6,14] the current investigation was undertaken to form a large-sized nickel-based single crystal by depositing Rene´ N5 powder on a Rene´ N5 (100) single-crystal substrate layer by layer. The identical crystallographic structure and lattice parameter of the feeding and substrate materials promote the formation of a homoepitaxial interface[12] between the liquid and solid with a near-zero energy barrier to solidification. Following a kinetically favored epitaxial growth, the deposit would inherit the (100) crystallographic orientation of the substrate at very low undercooling rate. However, the SX nature of the deposit could be lost when equiaxed grains nucleate and grow ahead of the moving solidification interface and eventually leads the dendritic morphology to undertake a columnar to equiaxed transition (CET). For an uninterrupted SX growth at the solidification front, as proposed by Ga¨umann et al.,[6] it is critical to keep combina
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