# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 4 Solidification Mechanisms
### Surface Quality  
Components produced by the LP-DED process suffer from low surface quality. Hence, post-machining operations are required in order to achieve the dimensional and geometrical tolerances defined by the specific application. The surface quality of a component is mainly characterized by its surface roughness and dimensional accuracy [170]. Surface roughness is one of the most challenging issues in LP-DED [20; 171; 172]. [PERSON], et al. [173] performed one of the first experimental investigations on the surface finish of samples produced by the LP-DED process. Their analysis showed that the average surface roughness (R\({}_{\text{a}}\)) ranges between 8 and 20 \(\upmu\)m, and the surface roughness value is mainly influenced by powder particle size. In particular, the lowest value of surface roughness was obtained when the smallest powder particle size was used. This behavior was attributed to the presence of unmelted powder particles on the analyzed surface [171]. In addition to the particle size, the literature shows that the surface roughness is highly influenced by the powder stream [174]. Figure 34: (**a**) Bi-directional and (**b**) offset-out deposition strategies and (**c,d**) the corresponding modeled deformation caused by residual stress (reproduced with permission from [161]; Copyright © 2002 Emerald Publishing Limited). Based on the powder flow rate value, [PERSON], et al. [175] distinguished the surface roughness into three categories, which are related to the low, medium, and high powder flow rate values. In detail, using a low value of powder flow rate resulted in smoother surfaces and the lowest values of surface roughness, but the build rate was very low. Using a higher value of powder flow rate, the build rate increased, but the surface roughness increased significantly. Using a medium value of powder flow rate, a good building rate with reasonable surface roughness was observed. The surface roughness is also influenced by other process parameters such as layer thickness, laser power, and travel speed. [PERSON], et al. [176] noticed that if a thin additive layer is used in combination with high laser power and high travel speed, an improvement in the surface finish is observed. This was confirmed by [PERSON], et al. [171], that showed that increasing the laser power results in a reduction of the maximum surface roughness value; however, the average surface roughness remains almost constant. In contrast, [PERSON], et al. [177] and [PERSON], et al. [172] observed that average surface roughness decreases if the laser power value increases. [PERSON], et al. [176] identified two types of roughness along the building direction (z-axis): micro-roughness and macro-waviness. The former was attributed to the particle agglomeration in inter-layer areas and the solidification line and was mainly influenced by the powder stream, while the macroscopic contribution, which was mainly influenced by the process parameters, was related to the formation of periodic menisci, associated with the stability of the melt pool. In general, surface roughness can be measured on the top surface (building plane) and the lateral surfaces (along the building direction) [178]. Despite the fact that most works have analyzed the surface roughness on the lateral surfaces [173; 175; 176; 179], it is observed that the surface roughness on the top surfaces is slightly higher than that measured on the lateral surfaces [168; 180]. In particular, [PERSON], et al. [168] showed that the morphology of the lateral and the top surface is mainly influenced by the layer thickness and the hatching distance, respectively. In addition, by analyzing the morphology of the surface described by the kurtosis and the skewness parameters, they showed that the morphology of an LP-DED sample is similar to that obtained in a milling operation. In addition to varying process parameters, different methods have been developed to improve surface roughness. [PERSON], et al.