Laser directed energy deposition (L-DED) uses a laser as the energy source, which has higher temperature gradient and cooling rate, making it easier to alleviate metallurgical defects and promote the directional growth of crystals. However, during the preparation of multi-layer single crystal alloys, the heat input inside the molten pool is constantly changing, which increases the complexities of heat transfer and solidification in the molten pool. This is not conducive to the directional growth of crystals, and increases the risk of formations of stray grains and cracks. Therefore, this study reports the effects of the laser power, powder feeding rate, and scanning speed on the microstructure and defects of single-channel single-layer samples. The effects of single-channel multi-layer and multi-channel multi-layer deposition strategies on the crystal growth and micro-defects are also presented. The results of this study can be used as a reference for the L-DED forming of single crystal components.

First, single-channel single-layer DD6 nickel-based single crystal superalloy samples are prepared using L-DED at different laser power values, and the microstructures of the samples are studied using optical microscope (OM). Then, samples are prepared using different laser power values, powder feeding rates, and scanning speeds in an orthogonal experiment. The microstructure of each sample is observed using OM, and the directional growth height of the crystals (H_E), molten pool depth (H_R), deposition height (H_D), and crystal directional growth ratio H_E/(H_R+H_D) are measured. A range analysis is also performed. Then, the microstructures of single-channel multi-layer and multi-channel multi-layer samples prepared using a continuous deposition strategy are examined using OM and scanning electron microscope (SEM), and the precipitates are identified using energy dispersive spectrometer (EDS). Finally, the microstructures and grain orientations of single-channel multi-layer and multi-channel multi-layer samples prepared using an intermittent deposition strategy are characterized using OM, SEM, and electron back scatter diffraction (EBSD).

When the laser power is too low, the heat input is insufficient, and defects such as lack-of-fusion holes, pores, and cracks are observed inside the sample. As the laser power gradually increases, the intensity of the Marangoni convection inside the molten pool increases. This results in a wavy molten pool morphology and increased tendency to generate stray grains. When the laser power is too high, the heat accumulation further increases, and cracks form under the combined actions of the thermal stress and liquid film (Fig. 2). The samples formed under different laser power values, powder feeding rates, and scanning speeds have large areas of stray grains on both sides of the molten pool, and the growth of columnar crystals in the middle area is relatively good. When the heat input per unit time is too large, the directional growth of crystals is poor, and the deposition height is insufficient. The internal stress of a sample also increases, thereby increasing the tendency to produce cracks (Figs. 3?5). The H_E, H_D, and H_E/(H_R+H_D) values of the samples produced under different process parameters are listed in Table 4, with the results of range analyses listed in Tables 5?7. The optimal process parameters are a laser power of 1000 W, a scanning speed of 15 mm/s, and a powder feeding rate of 10 g/min. Under these process parameters, H_E/(H_R+H_D) is 73.02%, H_E is 474.55 μm , and H_D is 257.30 μm . The crystal shows good directional growth and no defects such as cracks and pores. The H_E/(H_R+H_D) value for single-channel 30-layer samples reaches 83.05% when using the continuous deposition strategy. The primary dendrite arm spacing at the bottom of the molten pool is approximately 3.8 μm . When the deposition height is gradually increased, the heat accumulation increases, which decreases the temperature gradient in the molten pool and increases the dendrite arm spacing of the high deposition layer to 19.5 μm . The direction of heat dissipation is also easy to change, resulting in the formation of a secondary dendrite arm with an arm spacing of 2.2 μm (Fig. 6). In the multi-channel 10-layer sample produced using the continuous deposition strategy, the bottom of the molten pool has a wavy morphology, and the H_E/(H_R+H_D) value is only 69.53% (Fig. 7), with Re segregated inside the columnar crystal and Ta segregated between the columnar crystals (Fig. 8). When single-channel multi-layer samples and multi-channel multi-layer samples with a height of approximately 4.6 mm are deposited using the intermittent deposition strategy, which reduces the heat input per unit time, improves the morphology of the molten pool, and alleviates the generation of deflecting dendrites, the H_E/(H_R+H_D) values are 83.73% and 88.63%, respectively. However, there is still a small amount of deflecting dendrites in the single-channel multi-layer samples produced using the continuous deposition strategy, is along with an overlap rate in the multi-layer samples, which can remelt the wavy molten pool and primary deflecting dendrites, thereby reducing the internal deflecting dendrites (Figs. 9 and 10). The crystal orientation differences at the bottoms of the single-channel multi-layer and multi-channel multi-layer deposition areas of samples produced using the intermittent deposition strategy are basically within 10°, and an overall single-crystal structure is produced (Fig. 11).

This study reports the effects of different L-DED molding process parameters and deposition strategies on the micro-defects and directional crystal growth of samples. When the laser power is too low, it is easy to produce a large number of lack-of-fusion holes and pores. When the laser power is too high, the tendency to form stray grains and cracks increases. When using a laser power of 1000 W, a scanning speed of 15 mm/s, and a powder feeding rate of 10 g/min, taking into account the forming quality and deposition efficiency, the H_E/(H_R+H_D), H_E, and H_D values are 73.02%, 474.55 μm, and 257.30 μm, respectively.