## 4 Results Analysis and Discussion
### Double-Channel Overlapping Deposition  
#### 4.3.1 Calculation and Comparison of Double-Channel Overlapping Deposition Shapes  
Under the parameters of a powder feeding rate of 12.0 g/min, a laser power of 500 W, and a laser scanning speed of 10 mm/s, simulations of a two-channel overlap with overlapping rates of 20%, 30%, and 40% were performed, and the morphologies of the deposited layers were produced as shown in Figure 12. Comparing Figure 12a-c, the distance between the two deposition channels decreases as the overlapping rate rises, from a clear depression in the center of the two deposition channels at an overlapping rate of 20% to a prominent bulge higher in the overlapping deposition channel at an overlapping rate of 40%. The simulation findings of the dual-overlapping deposition morphology indicate that a 30% overlapping rate is suitable for the coaxial powder feeding laser directional energy deposition of the IN718 procedure. Figure 11: Three dimensional molten pool morphology of single path forming at different moments. (**a**) 0.05 s; (**b**) 0.15 s; (**c**) 0.30 s; (**d**) 0.40 s.  
3.2 Analysis of the Temperature Field and Melt Pool Characteristics of Double-Channel Overlapping Deposition  
For a single-layer, two-channel deposition process with a powder feeding rate of 0.2 g/s, laser power of 500 W, laser scanning speed of 10 mm/s, and overlapping rate of 30%, numerical calculations were undertaken. Using the parallel unidirectional overlapping deposition method, the intermediate section with stable shape of the deposited layer obtained from the overlapping experiment under the same parameters was selected, and the width and fusion depth of the deposited layer after overlap were measured by ultra-field depth microscopy and compared with the simulation results, as shown in Figures 13 and 14, respectively. The measured and calculated widths W\({}_{1}\) and W\({}_{2}\) of the deposited layer are 3196.78 m and 2953.15 m, respectively, and the relative error between the simulation and measurement results is 8.24%; the measured fusion depth D\({}_{3}\) and calculated fusion depth D\({}_{4}\) are 288.55 m and 303.63 m, respectively, and the relative error between the simulation and measurement results is 5.23%. Figure 12: Shapes of deposited layers with varying overlapping rates. (**a**) Overlapping rate of 20%; (**b**) Overlapping rate of 30%; (**c**) Overlapping rate of 40%. During the overlapping process, the temperature effects of the first deposition channel and the overlapping deposition channel are mutual: the thermal accumulation of the first deposition channel has an effect on the overlapping channel melt pool and deposition temperature; simultaneously, the high temperature state of the overlapping deposition channel has an effect on the temperature and cooling rate of the first deposition channel, which may even further affect the solid-state phase change. Figure 15 depicts the temperature field distribution of the deposition at various instants, with the overlap start instant serving as the zero instant. From Figure 15a-d, it can be seen that due to the effect of heat accumulation, the temperature distribution on the surface of the overlapping deposition channel is no longer symmetrical along the central axis of the overlapping channel, and the distribution of the same temperature range on the scanning side of the first channel becomes significantly larger; Concurrently, it can be seen that the isotherm spacing on the left and right sides of the central axis of the surface of the overlapping deposition channel is distinct. The temperature gradient on the side of the first deposition channel is much less than that on the side of the overlapping deposition channel, and the cooling conditions on the two sides of each deposition are clearly distinct. Figure 14: Comparison of experimental and simulated overlapping molten pool morphology. Figure 13: Comparison of simulated and experimental overlapping deposition morphology.