# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 3. Melt Pool Generation Mechanisms
### Forces within the Melt Pool  
In general, it has been reported that the morphology and shape of a melt pool are mainly determined by the forces and the flows within the melt pool itself [111; 112; 113]. Two main types of forces that can be recognized are the buoyancy forces and the Marangoni forces [114; 37; 115]. The flow of the metal liquid caused by the buoyancy forces is illustrated in Figure 23a. Differences in material density cause the buoyancy forces due to the temperature gradient in the melt pool. Indeed, the temperature of the melt pool is higher in the centre and gradually decreases near the boundary. As a consequence, the density of the metal melt pool increases from the centre (point a) to the border (point b) [114]. The Marangoni forces cause the Marangoni flows, also named surface tension-driven or thermocapillary convection. These flows are caused by surface tension gradients. The Marangoni number (\(Ma\)) is used to measure the magnitude of Marangoni flows as:  
\[Ma=-\frac{\partial\gamma}{\partial T}\frac{1}{\mu\kappa}t_{mp}\,\
abla T \tag{9}\]  
where \(\partial\gamma/\partial T\) is the surface tension temperature coefficient, \(\mu\) is the dynamic viscosity, \(\kappa\) is the thermal diffusivity, \(t_{mp}\) is the melt pool thickness on the surface, and \(\
abla\)T is the temperature gradient [111]. Figure 22: Variation of track width with respect to the travel speed varying the laser power (reproduced with permission from [101]; Copyright © 1995 Elsevier). By comparing the magnitude of the flows acting in the melt pool, it is possible to observe that Marangoni flows dominate the melt pool behavior [37; 111; 112], which is why most works focus on this type of flow. The effect of Marangoni flows on the melt pool shape has been investigated numerically by several authors [111; 115; 117; 118], and the effect of surface tension gradient on melt pool flows is illustrated in Figure 23. It has been shown that with a negative surface tension temperature coefficient value (Figure 23b), melt pool flows are directed from the centre to the boundary, resulting in a wide and shallow melt pool morphology. On the other hand, the melt pool becomes deep and narrow with a positive value of surface tension temperature coefficient (Figure 23c). Usually, the value of surface tension decreases with increasing temperature, so a negative value of the surface tension temperature coefficient is obtained [114]. However, the presence of surfactant elements such as sulphur and oxygen changes the value of the surface tension temperature coefficient from negative to positive [37]. Nevertheless, Marangoni flows only occur below a critical travel speed [119]. Above this critical speed, the interaction time is too low, and the surface-tension gradients are avoided. For a 316L stainless steel, the critical velocity is of the order of 90 mm/s. The main characteristic of the melt pool generation process and the most important factors are summarized in Table 4. \begin{table}
\begin{tabular}{c c c c} \hline  & **Temperature** & **Melt Pool** & **Melt Pool** \\  & **Distribution** & **Dimension** & **Morphology** \\ \hline  & Influence the distribution and the peak of temperature into the melt pool & Determine the height, the width and the penetration depth & Determine the shape of the melt pool \\ \hline Laser power attenuated & [88; 94; 104] & [45; 85; 97; 98; 99; 100; 101] & \\ Travel speed & [88; 101] & [98; 100; 101; 108] & \\ Powder flow distribution and & [97] & [45; 97; 100; 105; 106; 107; 108; 109] & \\ velocity & [89; 93] & [97] & [111; 112; 113; 114] \\ Marangoni flows and buoyancy & & & [115; 116; 117; 118] \\ forces & & & \\ \hline \end{tabular}
\end{table} 
Table 4: Summary of the factors influencing the melt pool generation mechanisms. Figure 23: (**a**) Buoyancy forces and flows caused by the material density differences due to the temperature gradient (adapted from [114]).