Nickel-based single crystal superalloys have excellent high-temperature properties and are mainly used in the manufacture of aero-engine turbine blades. A high temperature gradient is the key to single crystal formation. Selective laser melting (SLM) is a kind of metal additive manufacturing, which uses a laser as the heat source to form the parts with fine metal powders layer by layer, especially the complex structures. The small laser spot (80-100 μm) has extremely high thermal density, which promotes a temperature gradient of 10³-10⁵ K/cm and a solidification rate of 10⁵-10⁷ K/s in the molten pool. In addition, a laser has obvious directional heat transfer characteristics, and the solidification heat can be directionally dissipated along the path from the molten pool to the substrate. Therefore, to explore the feasibility of single crystals fabricated by SLM, thirty-five single track samples are fabricated under different laser powers and scanning speeds. The geometric features, crystal orientation, and microstructures are first analyzed systematically. Then, the formation mechanisms of cracks and stray grains are discussed, which provides theoretical and technical reference for the preparation of large-scale single crystal structures by SLM.

The particle size distribution of the gas-atomized DD91 nickel-based single crystal superalloy powder is in the range of 8.6-37.5 μm with an average of 20.4 μm. The SRR99 single crystal substrate prepared by directional solidification is tested by electron backscattered diffraction (EBSD) and it is found that the grain grows along the direction of [001], which meets the experimental needs. Before the experiment, the powder is placed in an oven at 80 ℃ for 10 h to remove moisture. The single crystal substrate is polished with sandpaper, and impurities such as abrasive debris are removed in an ultrasonic cleaning machine. DD91 alloy powder with a layer thickness of 30 μm is spread on the (001) crystal surface of the SRR99 single crystal substrate. A self-developed SLM equipment with a laser spot diameter of ~100 μm is used for the printing experiments. Thirty-five single-track samples with different parameters are fabricated on the substrate with a laser power of 245-305 W and a scanning speed of 500-1100 mm/s. Optical microscope (OM) is used to observe the surface morphologies of the single-tracks and evaluate the forming quality. Scanning electron microscope (SEM) is used to observe the microscopic morphologies and microstructures of powder and molten pools. ImageJ software is used to measure and analyze the geometric feature sizes of molten pools. EBSD is used to analyze the crystal orientation with a scanning step of 0.25 μm, and AztecCrystal software is used to process the collected data.

When the line energy density is in the range of 414-580 J·m^-1, excellent single-tracks could be obtained with clear surface contours, continuous flatness, and obvious metallic luster on the surface (Fig. 3). The shape of the molen pool is more regular and stable at a higher power and a lower speed, which could provide good geometric conditions for the epitaxial growth of single crystals. At a laser power of 290 W and scanning speeds of 1000 mm/s and 1100 mm/s, cracks perpendicular to the scanning direction are produced, running through the entire single-track. The dendrites are exposed to the crack section, leaving a wide irregular crack, which is the characteristic of a typical solidification crack, probably due to the excessive Al content of the alloy and the tearing of the liquid film between the dendrites under stress (Fig. 4). Most of the grains in the molten pool exhibit a very clear meritocratic orientation, i.e., they can continue the orientation of the single crystal substrate and grow along [001] epitaxially. However, there are still crystal defects in some molten pools (Fig. 7). They can be divided into three categories: first, poor metallurgy between the molten pool and the single crystal substrate, resulting in disordered grain growth; second, during the growth of the crystal, its growth direction is shifted by a small angle, producing an orientation deviation due to the flow field in the molten pool; third, the columnar to equiaxed transition (CET) occurs at the top and sides of the molten pool due to the reduced temperature gradient and the accelerated solidification rate at the late solidification stage, resulting in equiaxed stray grains (Fig. 8). At the bottom of the molten pool, near-parallel columnar dendrites grow along [001], continuing the orientation of the substrate without secondary dendrite arms. In the middle of the molten pool, the grains also grow and continue the [001] crystal orientation, however, the grain morphology changes to a spindle shape. At the top of the molten pool, the grain growth direction changes from [001] to [100] (Fig. 9). Due to the larger height of this molten pool, the distance of heat transfer to the substrate from top to bottom at the end of solidification becomes farther and the heat is mainly transferred along the scanning direction (parallel to the substrate). Suitable process parameters should be selected to minimize the height of the molten pool, thus to reduce the heat transfer distance from the top to the substrate and reduce the generation of such defects.

Continuous, smooth, and straight single-tracks could be obtained within a fixed layer thickness of 30 μm, a laser power of 290-305 W and a scanning speed of 500-700 mm/s. The direction of the temperature gradient in the flat and regular molten pool is closer to[001], which is more conducive to the unidirectional heat transfer from top to bottom in the molten pool, and provides a guarantee for the stable growth of the single crystal. A good metallurgical bond between the molten pool and the single crystal substrate is the key to the epitaxial growth of crystals. It is difficult to obtain the initial orientation of the substrate for the molten pool under poor metallurgical conditions, resulting in disordered crystal growth. Small angular orientation deviations occur due to the internal flow field. A small number of stray grains occur at the top and sides of individual molten pools, caused by the lower temperature gradient and the increased solidification rate in the later stages of solidification. The grains in different regions within the molten pools have different characteristics. Grains at the bottom and both sides of the molten pool can continue the substrate [001] epitaxial growth, and the primary dendrite arm spacing is only 0.6-0.8 μm, much lower than that of the directional solidification. Spindle-shaped grains occurring in the middle of the molten pool also continue the [001] orientation, but the solidification shrinkage holes with a width of 0.1 μm generate between the dendrites. At the top of the molten pool, the grains show a fine cellular structure, growing along the [100] direction, which is caused by the heat transfer along the scanning direction (parallel to the substrate). This study provides a process basis for the fabrication of large-size single crystals by SLM.