Mechanism of microstructure evolution of DD5 single-crystal superalloy fabricated by dual-wavelength laser powder bed fusion

In this paper, the microstructural characteristics of the DD5 nickel-based single-crystal superalloy manufactured using dual-wavelength Laser Powder Bed Fusion (LPBF) technology and its evolution mechanism were investigated. Initially, single-track specimens were deposited on a DD5 substrate to examine the microstructure and characteristic dimensions of the molten pool. Quantitative analysis was conducted to assess the influence of each process parameter on epitaxial growth and the proportion of stray grains, aiming to determine the optimal process parameter range conducive to the epitaxial growth of single crystals via LPBF. DD5 single-crystal samples with a height of 4 mm was successfully fabricated by LPBF. The resulting structure exhibited a fraction of low angle grain boundaries (LAGB) exceeding 96.1 %, with misorientation angle differences along the deposition direction of less than 10 degrees. This suggests that the use of a dual-wavelength laser beam can mitigate residual stresses induced by the LPBF process, thereby preventing the formation of high angular grain boundaries and stray grains. Finally, dendrites in LPBF-prepared DD5 single-crystal superalloys exhibited fine characteristics, with primary dendrite arm spacing (PDAS) ranging approximately between 1 and 3 mu m, significantly smaller than as-cast counterparts. PDAS increased proportionally with laser energy density (LED) augmentation. The gamma '-phase presented heterogeneous spherical shapes with an average size ranging between 60.3 and 191 nm and a volume fraction ranging from 55.9 % to 64.5 %. The segregation of refractory elements Re, W, and Ta escalated with increasing LED. LPBF-prepared DD5 single-crystal samples exhibited lower elemental segregation which is beneficial to reduce the heat-treatment process and avoid the formation of stray grains due to recrystallization during heat-treatment.