# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 2 Laser Irradiation and Material Addition Mechanisms
### Powder Flow  
In order to optimize the LP-DED process, several studies have been conducted to obtain an in-depth characterization of the powder flow. In general, it has been observed that the powder flow is influenced by particle size, powder surface morphology, and rheological properties [49]. Optimizing and understanding the powder flow could improve the capture efficiency, that is, the fraction of powder particles entering the melt pool [50]. In the LP-DED process, the capture efficiency is relatively low, less than 30%, with a huge amount of scattered powder [51]. Consequently, increasing the capture value results in \begin{table}
\begin{tabular}{c c c} \hline \hline
**Configuration** & **Advantages** & **Disadvantages** \\ \hline Lateral & Part accessibility & Directional deposition \\  & Width of deposited track: 0.5–25 mm & Less powder efficiency \\  & Unidirectional deposition & Alignment between powder and laser beam \\  & & No integrated protective gas feeding \\  & & Unidirectional deposition & Restricted part accessibility \\  & Width of deposited track: 2–7 mm & Low powder capture efficiency \\  & Applied laser power up to 5 kW & (diameter of the powder-gas jet in focus: minimum 400 mm) & minimum 2.5 mm) \\  & Integrated protective gas feeding & \\  & Unidirectional deposition & \\  & Width of deposited track: 0.3–5 mm & Restricted part accessibility \\  & Applied laser power: up to 3 kW & Gravity influence, no deposition for tilting \\  & Powder capture efficiency: maximum 90\% & angles higher than 20\({}^{\circ}\) because of the \\  & (diameter of the powder-gas jet in focus: minimum 400 mm) & inhomogeneous powder density distribution \\  & Integrated protective gas feeding & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Advantages and disadvantages of deposition head configurations [48]. Figure 4: Deposition head configurations: (**a**) lateral configuration, (**b**) discrete coaxial configuration, and (**c**) continuous coaxial configuration (adapted from [48]). improving the overall process efficiency. Among other parameters, such as powder velocity and surface tension, the capture efficiency is mainly influenced by the distribution of the powder flow [50; 52]. As a result, several studies have been conducted to understand the factors that significantly affect the powder flow. [PERSON] [52] performed one of the first experimental studies on coaxial flow during the LP-DED process. In his work, optical sensors and image analysis were used to measure the distribution of the powder stream. The results showed a quasi-Gaussian distribution of the powder flow in the radial direction. Later, [PERSON] and [PERSON] [53] developed a mathematical model to predict the spatial distribution of powder flow using a continuous coaxial nozzle. The results were experimentally validated using optical and image analysis techniques, which showed that it is possible to highlight two regions in the powder stream, one before merging into a single jet and another after this point. Before the merging point, the powder flow is characterized by an annular distribution, while after the merging point, a Gaussian distribution is observed. In addition, [PERSON] and [PERSON] [54] showed that the powder distribution near the nozzle outlet, above the focal point of the powder stream, does not assume a perfect uniform annular distribution, but four concentration zones can be identified that correspond to the location of the nozzle inlets (Figure 5). Between the nozzle outlet and the merging point, [PERSON], et al. [55] showed the presence of a transition zone (Figure 6). Figure 5: Modeled particles distribution and velocity obtained at different planes: (plane a) above the flow focal point, (plane b) at the flow focal point, and (plane c) below the flow focal point (reproduced with permission from [54]; Copyright © 2011 Taylor & Francis). Figure 6: Evolution of powder flow distribution at different planes (reproduced with permission from [55]; Copyright © 2010 Elsevier).