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Nickel-based superalloy is an indispensable material in the aerospace industry. With the development of metal additive manufacturing theory, the additive manufacturing technology will further expand and accelerate the application of nickel-based superalloy in the aerospace industry. In this study, nickel-base superalloy K447A with high volume fraction γ′ strengthening phase was formed by Powder Bed Fusion Laser Beam (PBF-LB), and the effect of different heat treatments on the microstructure and mechanical properties of K447A alloy was analyzed. In the as-built sample, cellular structures consisting of dense dislocation tangles were formed. Elongated columnar grains grew preferentially along <001> direction. Fine MC precipitates segregated at the boundary of cellular structure. The process of grains recovery (1185 °C), grains recrystallization (1260 °C), and grains rapid growth (1300 °C) of K447A alloy during heat treatment could be observed. Many stable and homogeneous cubic γ′ phases precipitated from the K447A alloy after high-temperature heat treatment. When the heat treatment was increased to 1300 °C, MC precipitates dissolved and the number of carbides in the ST-1300 sample was significantly reduced. The average microhardness and ultimate tensile strength of the as-built sample were 407 HV and 527 MPa, respectively. When the heat temperature reached 1260 °C, the microhardness and ultimate tensile strength reached the maximum. When the heat temperature was raised to 1300 °C, the ultimate tensile strength decreased sharply, but the elongation and the toughness increased significantly. Based on the above results, the relationship between microstructure, mechanical properties, and deformation mechanism was discussed. [ABSTRACT FROM AUTHOR]