Laser-based powder bed fusion of nickel-based superalloy designed specifically for turbine blades using high-power flat-top laser: Towards high-speed manufacturing and directional solidification

In this paper, high-power flat-top laser is used during the laser-based powder bed fusion of metals (PBF-LB/M) additive manufacturing process of the ZGH451 alloy, which is a new-type directionally solidified nickel-based superalloy designed specifically for turbine blades. The phase composition, metallurgical defects, build rate, microstructure, grain morphology, crystal orientation, and microhardness of the as-built ZGH451 alloy are studied and compared with the corresponding results of the conventional PBF-LB/M technology using low-power Gaussian laser. By using appropriate processing parameters, both the specimen built using the high-power flat-top laser (hereafter called as HF-PBF-LB/M specimen) and that built using the low-power Gaussian laser (hereafter called as LG-PBF-LB/M specimen) exhibit acceptable relative density that is higher than 99.5%. However, some hot cracks are observed in the HF-PBF-LB/M specimen. The build rate of the LG-PBF-LB/M specimen is only 5.2 cm³/h, while the build rate of the HF-PBF-LB/M specimen is as high as 103.0 cm³/h, which is 19.8 times of the former. The phase composition and microstructure of as-built ZGH451 change with the employed laser type. The phase composition of the LG-PBF-LB/M specimen is mainly composed of γ-Ni matrix phase, γ' precipitate, and MC carbide, while the HF-PBF-LB/M specimen also contains some γ/γ' eutectics in addition to γ-Ni matrix phase, γ' precipitate, and MC carbide. In comparison with the LG-PBF-LB/M specimen, the HF-PBF-LB/M specimen exhibits coarser cellular dendrite, γ' precipitate, and MC carbide. The solidification microstructure of both the LG-PBF-LB/M specimen and the HF-PBF-LB/M specimen presents a mixture of columnar grains and equiaxed grains. The proportion of the columnar grains with "<001>//building direction" crystal orientation of the LG-PBF-LB/M specimen is about 41.2%. In contrast, the proportion of the columnar grains with "<001>//building direction" crystal orientation of the HF-PBF-LB/M specimen increases significantly to about 98.3%, which means the HF-PBF-LB/M specimen exhibits stronger directional solidification trend than the LG-PBF-LB/M specimen. There is microhardness anisotropy in both kinds of specimens and the microhardness value of HF-PBF-LB/M specimen is slightly higher than that of LG-PBF-LB/M specimen. In summary, using high-power flat-top laser is helpful to increase the build rate and also enhance the directional solidification trend of the ZGH451 alloy during PBF-LB/M processes, but it also leads to some new issues such as hot cracks, the coarsening of microstructure, and the formation of harmful γ/γ' eutectics, therefore requiring further research.