The single-channel single-layer and single-channel five-layer depositions are carried out to study the dendritic morphology, crystal orientation, and microstructure. Table 2 lists the DED processing parameters. The evolution of the molten pool geometry and dendritic morphology are observed using a metallographic optical microscope (OM). In addition, the γ/γ′ eutectic band is found using a scanning electron microscope (SEM). The orientations of the epitaxial growth dendritic and stray grains are characterized using an electron backscattered diffraction (EBSD) system with an acceleration voltage of 20 kV, tilt angle of the tested specimen of 70°, and scan step size of 0.5 μm. The ATEX software package is used to conduct the analysis. In addition, the texture and grain misorientation are obtained using the EBSD system to explain the distribution rules of the dendritic morphology and disorientations. The Vickers hardness of the materials in the deposition region is measured using a Vickers microhardness tester with a test pressure of 3 N. To reveal the evolution mechanism of the molten pool geometry, the formation process of stray grains, complex thermal behaviors, and rapid solidification in multilayer DED are investigated by building a 3D transient heat transfer numerical model and solving the conservation equations.

Ni-based single-crystal turbine blades of aeroengines are inevitably damaged during use. Therefore, it is of great significance for commercial aeroengines with high economic requirements to repair single-crystal turbine blades reasonably and continue to realize their value. Directed energy deposition (DED) is a type of metal additive manufacturing technology that uses a laser as the heat source to repair complex structures with fine metal powders, layer-by-layer. In addition, the high temperature gradient and cooling rate of DED are conducive to the epitaxial growth of Ni-based single crystals. However, owing to complex thermal cycles and molten pool convection, stray grains are the most common defects in Ni-based single crystals repaired by DED. Therefore, to reveal the formation mechanism and provide a reference for the inhibition of stray grains, single-channel single-layer and single-channel five-layers are fabricated via DED, and a macroscopic numerical simulation of the single-channel five-layer deposition is carried out. First, the dendrite morphology, crystal orientation, and microstructure are analyzed. Then, the correlation between the columnar-to-equiaxed transition and stray grain formation is studied, and the microscopic mechanism of stray grain formation is revealed, which contributes to the suppression of stray grains in the middle and bottom of the deposition region and promotes the application of DED technology in the repair of commercial aeroengine single-crystal turbine blades.

The substrate is a Ni-based single-crystal superalloy, namely, DD6, that has dimensions of 5 mm × 5 mm × 15 mm. The powder is produced via the vacuum induction-melting gas atomization process based on the DD6 alloy, and the diameter of the powder is 53?150 μm. In addition, the substrate surface is polished using alcohol, and the powder is dried in a vacuum oven for 150 min at (120 ± 5)℃ before use.

According to the dendrite morphology, under the current deposition process parameters, single-channel single-layer DD6 alloy deposition can realize the epitaxial growth of columnar crystals, except for the top stray grain, and there are no obvious porosities, inclusions, or other defects (Fig. 2). Compared with single-channel single-layer deposition, the thermal cycles and cooling conditions during single-channel five-layer deposition are more complicated, which results in a columnar-to-equiaxed transition (CET) in not only the top region but also the middle region (Fig. 3). Simultaneously, the predeposited layer experiences a similar short-term solid solution in the subsequent deposition process, which affects the formation and evolution of the precipitated phase. In addition, the epitaxial growth of the interlayer columnar crystals is difficult to control, and stray grains are inevitable in the deposition area (Fig. 4). The Vickers hardness decreases with an increasing deposition height; however, the stray grains at the fusion line and top of the deposition region significantly decrease the Vickers hardness (Fig. 5). The change in the solidification parameters and molten pool convection during the DED of the DD6 single-crystal alloy result in differences in the element concentration and precipitation time between the dendrite core and interdendrite (Fig. 10). In this case, the directional coarsening of the γ' phase results in the formation of γ/γ' eutectic bands in the region where CET occurs. These eutectic bands appear at the boundaries between the columnar and equiaxed grains, which are shown as grain boundaries between the columnar and stray grains at the mesoscale (Fig. 11).

The single-layer deposition of the DD6 single-crystal alloy can realize the epitaxial growth of columnar crystals, except for the top stray grain. The stray grains in the five-layers are primarily caused by the collapse of the fusion line and CET in the top region, and the presence of stray grain crystals significantly reduces the Vickers hardness of the material. There is a difference in element concentration in the CET region, and similar solid solution treatments under subsequent temperature cycling lead to the directional coarsening of the γ' phase and then produce γ/γ' eutectic bands. The γ/γ' eutectic bands exist in not only the top deposition region but also the middle deposition region. The γ/γ' eutectic bands exist at the boundary between columnar and equiaxed grains, and they penetrate the dendrite core and interdendrite.