# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 2 Laser Irradiation and Material Addition Mechanisms
### Powder Flow
As discussed above, the distribution of powder varies along the axial direction (z-axis). The comparison of numerical and experimental results of the powder distribution evaluated in different planes along the axial direction is shown in Figure 7. Before the merging point, the powder concentration increases with the axial distance from the nozzle exit [56]. Furthermore, the peak of powder concentration shifts close to the nozzle axis as the axial distance increases. After the focal point, the powder flow is characterized by a Gaussian distribution of powder, with a sharp decrease in peak as the axial distance increases. The point at which the maximum concentration is reached corresponds to the focal point located at some distance from the nozzle exit [57]. Focal point location and distribution strongly influence the capture efficiency and the attenuation of the laser energy. Therefore, studies focus on evaluating the effect of variables affecting distribution, such as nozzle design, powder and gas flow rate, and powder properties [58]. In detail, the geometrical features of the nozzle that mainly influence the powder flow behavior are the nozzle configuration, the injection angle, and the outlet channel dimension [52; 53; 58; 59; 60; 61; 62]. [PERSON] [60] developed a two-dimensional numerical model in order to analyze the effect of the deposition nozzle configuration [60]. The results showed that using a configuration with an inner nozzle positioned outward (Figure 8a), the powder concentration value increases by about 50% compared with that obtained with the inner nozzle in the inner position. In addition, the author observed that the concentration peak was strongly influenced by the velocity of the outer shielding gas, but the axial position of the powder stream focus plane remained almost unchanged. Figure 8: Coaxial deposition head configurations: (**a**) outward and (**b**) inward nozzle position (reproduced with permission from [60]; Copyright © 2000 Elsevier). Figure 7: (**a**) Experimental and (**b**) numerical powder distribution evaluated at different planes below the nozzle exit (adapted from [53]). In general, it is observed that a smaller outlet size leads to a smaller powder flow diameter at the focal point [61; 62]. In addition to the outlet diameter, [PERSON], et al. [62] showed that the internal shape of the nozzles and their length also influence the powder distribution and focal point position, with the best results obtained using straight and long channels. It should also be considered that a free and undisturbed powder flow differs significantly from a flow of powder impinging on a surface [63; 64]. Hence, the substrate and its distance from the deposition head play an important role [63]. The interaction between the substrate and the powder flow was studied by [PERSON] and [PERSON] [63]. They found that the position of the substrate strongly influences the powder distribution. In particular, a higher concentration was obtained when the substrate was located very close to the nozzle (Figure 9). Another factor that affects the distribution of the powder stream is the velocity of powder particles [65; 66]. [PERSON] and [PERSON] [54] showed that collisions within the nozzle walls cause a reduction in the powder particle's velocity. Then, during the in-flight time, the powders increase their momentum due to the effect of gases, and an increase in velocity is observed. Below the nozzle, the powder particle's velocity (\(u_{p}\)) slightly increases. However, a wide range of velocities was observed, with the fastest particles having twice the velocity of the slowest particles [67]. The powder particle's velocity was also influenced by the interaction between powders and laser radiation. In fact, it was observed that when the laser turned on, the particles were characterized by higher speed [68]. This acceleration was related to the light propulsion caused by the reaction between the material-vapor recoil and the irradiated part of the particle [69]. In addition, [PERSON], et al.