# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 3. Melt Pool Generation Mechanisms  
The laser power available on the substrate, i.e., that which is not attenuated by the powder flow, is focused into a small area and causes a local increase in the temperature of the building platform, generating a melt pool [86]. Figure 17a schematically illustrates the melt pool generation and the addition of material. When the powder enters the melt pool, it melts very quickly, and a raised track is obtained due to the material addition [22]. Typically, the melt pool generated during the LP-DED process is characterized by dimensions between 0.25 to 1 mm in width, 0.25 to 0.5 in height, and 0.1 to 0.5 mm in depth [20,87]. During the melt pool generation, thousands of degrees can be reached in a few milliseconds, and the values and distribution of temperature depend on several factors, such as laser power, laser  
Figure 16. Resulting laser power intensity at different values of powder flow rate (reproduced with permission from [82]; Copyright © 2007 AIP Publishing). \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline  & & **Powder Flow Distribution and Velocity at the Substrate** & & **Laser Power Attenuation** & **Powder Temperature Increment** \\ \hline  & Govern the spatial distribution of powder particles during in-flight time & Influence the shape of the powder stream and velocity at the substrate & Influence the location of the plane of maximum powder concentration & Cause a reduction of the useful power due to laser-power due to laser-power interaction & Determine the absorption of thermal energy by powder particles \\ \hline Deposition head design Stand-off distance & [52, 56, 57] & [61, 62, 63, 64, 65, 66, 67, 68] & [63, 64, 65, 66, 67, 63, 68] & [50] & \\ Powder morphology Powder flow rate & & [67, 70, 71] & [77, 82, 83, 85] & [82, 83] \\ Material properties Gas Row Laser power & & [67, 70, 71] & [66, 73] & [80, 81, 84] & [82] \\ \hline \hline \end{tabular}
\end{table} 
Table 3. Summary of the factors influencing laser irradiation and material addition mechanisms. beam diameter, building platform material, and travel speed [88]. Figure 17b shows the thermal image acquired by a 12-bit digital charge-couple device camera of a moving melt pool and the corresponding temperature obtained during the deposition of a thin wall of 316L stainless steel. Since melt pool generation is the starting point of the solid track formation, its study is of fundamental relevance to optimizing building conditions [23; 74; 90]. It is, therefore, important to understand which parameters have the most significant impact on the melt pool and how the melt pool dynamics affect the heat transfer. The equations governing the melt pool generation and the thermal behavior are summarized in this paragraph. The energy available at the substrate, which generates the melt pool and overheads the molten material, can be estimated using the following equation [74; 91; 92]:  
\[I^{\prime\prime}=I_{L}-I_{loss} \tag{5}\]  
where \(I^{\prime\prime}\) is the useful heat flux at the substrate, \(I_{L}\) is the heat flux at the substrate provided by the laser beam, and \(I_{loss}\) is the heat loss at the melt pool-vapor interface. Assuming a Gaussian distribution of the heat source, \(I_{L}\) can be expressed as [74]:  
\[I_{L}=\frac{2\times\eta\times P}{\pi r_{L}^{2}}exp\left(\frac{-2\times r^{2}}{ r_{L}^{2}}\right) \tag{6}\]  
where \(P\) is the laser power, \(r_{L}\) is the laser beam radius, \(\eta\) is the absorption coefficient, and \(r\) is the radial distance from the laser beam centre.