Microstructural evolution of nickel-based single crystal superalloy fabricated by directed energy deposition during heat treatment

Directed energy deposition (DED), a typical additive manufacturing technology, is now regarded as a promising technique to further improve the performance of single-crystal (SX) superalloys by minimizing chemical segregation. The multi-track overlapping deposition is an effective way to increase the three-dimensional size of the component, which provides the opportunity to fabricate large-size SX parts. However, more serious uneven thermal processes and stress concentration are also introduced by the track overlapping, which may pose challenges to the SX structural homogenization and stability during heat treatment. This investigation aims to reveal the microstructural evolution of multi-track overlapped nickel-based SX by DED during heat treatment. In this study, the effects of solution annealing (1290 °C /4 h), primary aging annealing (1150 °C /4 h), and secondary aging annealing (870 °C /8 and 14 h) on the microstructural evolution of a multi-track SX block fabricated via DED are investigated. Compared with conventional solution annealing, direct primary aging is more appropriate for the first-step treatment of DED multi-track SXs. Both the eutectic γ–γ' and strain concentration disappeared, whereas no excess carbides appeared after the primary aging process. The recrystallization is accompanied by the reduction of strain concentration during both solution annealing and primary aging, as detected via electron back-scattered diffraction. After 14 hours of secondary aging annealing, relatively uniform size distribution of γ' precipitates is obtained. The present study demonstrates the microstructural evolution during the heat treatment of multi-track SX components and provides a basis for the further optimization of the heat treatment process.