Effects of Overlapping Process on Grain Orientation and Microstructure of Nickel-Based Single-Crystal Superalloy DD491 Fabricated by Selective Laser Melting

Aero-engine turbine blades are operated under harsh conditions such as high temperature, pressure, and load. Therefore, weak grain boundaries at high temperatures should be eliminated from the turbine blades, whereas convection channels inside the blades should be added to dissipate heat. Achieving integrated manufacturing of specialized microstructure in complex components has been a long-term research priority in turbine blade manufacturing. Nickel-based single-crystal superalloys are key materials for manufacturing single-crystal turbine blades for aero-engines, and selective laser melting (SLM) is feasible and technically advantageous for manufacturing complex components with single-crystal microstructures. Owing to the extremely high temperature gradient and scanning speed during SLM, the melt pool is unstable, thereby interrupting directional crystal growth. The metallurgical environment of SLM is further complicated by the large number of overlapping tracks and stacking layers. The quality of the overlaps is critical for the integrity of the single-crystal structure during SLM. Herein, the effects of scanning hatch (h = 0.06, 0.09, 0.12, and 0.15 mm) on the melt track morphology, metallurgical defects, crystal orientation, and microstructure of DD491 fourth-generation nickel-based single-crystal superalloy were investigated. Directionally solidified and solution-aged DD6 single-crystal superalloy rods were used as the substrate, and DD491 powder was coated to a thickness of 40 mu m. Electron backscatter diffraction was used to characterize the crystal orientation of the samples. Results show that low power/low speed (S1) and high power/high speed (S4) combinations of laser power and scanning speed provide geometrically and metallurgically stable conditions for directional crystal growth, and the grains at the bottom of the melt track can orient the substrate to achieve [001] directional growth. Different types of crystal orientation defects were observed in different regions, including equiaxed stray grains in the top middle region, [010] and [100] columnar stray grains in the top side regions, and small orientation deviation in the internal region. The scanning hatch affected the crystal orientation in the overlapping regions mainly through the remelted proportion of the old melt pool and the substrate microstructure of the new melt pool during solidification. The higher overlapping ratio with a smaller scanning hatch was beneficial for reducing stray grain defects on both sides of the melt tracks. The role of residual heat on solidification conditions was related to the heat gradient vector of laser input, and multitrack overlapping samples under the S1 process accommodated higher residual heat without causing orientation deviation in the overlapping regions. The multitrack overlapping samples under S1, h = 0.06 and 0.09 mm, had maximum pole densities along the y- z plane as high as 47.66 and 46.85, respectively, exhibiting a typical [001] single- crystal structure.