# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 2 Laser Irradiation and Material Addition Mechanisms
### Powder Heating and Laser Attenuation  
One of the main characteristics of the LP-DED process is the melting of the powder particles when they enter the melt pool generated by the laser. Actually, the powder particles absorb energy during the in-flight time, and their temperature increases. Depending on specific values of process parameters, powder particles can melt during the in-flight time. Figure 12: Powder distribution at different powder flow rates: (**a**) 0.052 g/s, (**b**) 0.075 g/s and (**c**) 0.088 g/s (reproduced with permission from [70]; Copyright © 2012 Elsevier). Figure 13: Focal plane position for (**a**) Ti6 Al4V and (**b**) 316L powder (reproduced with permission from [73]; Copyright © 2012 COMSOL). However, to improve substrate adhesion and process stability, this condition is typically avoided, and process parameters are selected to melt only the substrate or previously deposited layers [20; 74]. The increase in powder temperature during in-flight time is caused by the interaction between the powder and the laser beam [75]. This interaction depends mainly on the standoff distance, which is the distance between the deposition head and the substrate, and the velocity of the powder particles. The laser-powder interaction is governed by the electromagnetic and radiative properties of the powder particles and the environment. In this interaction, in addition to the rise of powder temperature, the laser power is reduced by the absorption, reflection, and scattering of light by the powder particles, resulting in the attenuation of the laser's useful power [74; 76]. Figure 14 schematically illustrates the phenomena that occur during laser-powder interaction. One of the first analyses of powder heating was performed by [PERSON] [77] using a mono-dimensional analytical model. He showed that the temperature of the powder highly depends on the inner gas velocity and laser power. For example, increasing the gas flow velocity from 2 m/s to 6 m/s, a variation of about 700 \({}^{\circ}\)C in the powder temperature was observed. The model results were compared with experimental results obtained using a pin-hole photodetector, obtaining good agreement. The experimental results showed that the temperature of the powder particles within the stream varied by about 500 \({}^{\circ}\)C due to the particle size distribution in the powder stream. Transient heat transfer phenomena occur when powders interact with the laser beam. These phenomena are essentially conduction, convection, and radiation. The increase in powder temperature is usually calculated by means of an energy balance. An important parameter for studying powder heating is the Biot number [78] defined as \(B_{i}=h_{c}L_{p}/\kappa_{p}\) where \(h_{c}\) is the convection coefficient, \(L_{p}\) is the characteristic length (for a spherical particle with a radius of \(r_{p}\), \(L_{p}=r_{p}/3\)), and \(k_{p}\) is the thermal conductivity of the material. When \(\text{B}_{i}<<1\), as in most cases during the LP-DED process, the temperature gradient within each powder particle is negligible [54; 79]. Under this condition, the lumped capacitance method can be applied, and the energy balance can be described using the following equation:  
\[V_{p}\rho_{p}c_{p}\frac{dT}{dt}=I_{p}\eta_{p}\pi r_{p}^{2}-h_{c}(T-T_{\infty}) 4\pi r_{p}^{2}-\varepsilon_{r}\sigma\Big{(}T^{4}-T_{\infty}^{4}\Big{)}4\pi r_{ p}^{2} \tag{1}\]  
where \(V_{p}\) is the particle volume, \(\rho_{p}\) is the particle density, \(c_{p}\) is the particle-specific heat, \(T\) is the actual temperature of the particle at the time \(t\), \(I_{p}\) is the incident energy on the particle, \(\eta_{p}\) is the particle absorptivity, \(h_{c}\) is the convection coefficient, \(\varepsilon_{r}\) is the radiation coefficient, \(\sigma\) is the Stefan-Boltzmann constant and \(T_{\infty}\) is the temperature of the shielding gas. [PERSON] and [PERSON] [54] analyzed the interaction between the laser beam and the powder particles.