# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 4 Solidification Mechanisms
### Residual Stress
High levels of residual stresses were observed near the bottom due to the high value of the cooling rate and the cumulative effect of successive depositions. In particular, using the hole-drilling strain gauge method, it was observed that the residual stresses are characterized by an oscillatory nature, in which the amplitude of the oscillation is correlated to the layer thickness and the hatching distance [155; 157]. Using the Vickers micro-indentation method on Inconel 718 samples, [PERSON], et al. [137] showed that higher stresses are obtained in the overlapping regions between two adjacent tracks. Since residual stresses can adversely affect the mechanical performance and functionality of the part, several efforts have been made to define methods to minimize residual stresses. To this end, three main aspects were investigated: building platform temperature, deposition strategy, and process parameters. Based on the results gained from welding processes, [PERSON], et al. [158] simulated the deposition of a single steel track. They showed that preheating the building platform caused a reduction of the thermal gradient and residual stresses. [PERSON], et al. [159] proposed a thermo-mechanical model and developed a process map to predict residual stresses. They showed that the main contribution to the reduction of residual stresses came from the uniform preheating of the building platform. [PERSON], et al. [160] used a laser sensor to analyze the effect of preheating temperature and building platform thickness on deformation during the deposition of Ti6 Al4V alloy. They showed that the preheating phase reduced distortion when a thin building platform is used; however, when a thick building platform is used, the preheating phase induces more deformation. [PERSON], et al. [145] used a Finite Element (FE) to simulate the deposition of Ti6 Al4V samples. They analyzed the influence of different laser paths used to increase the temperature of the building platform locally. They showed that when local preheating was used, the residual stress value was lower than in the nominal case without preheating. However, the distortion increased by approximately 3 mm. Instead, by increasing the temperature of the entire building platform to 700 \({}^{\circ}\)C, the residual stresses and distortions of the Ti6 Al4V samples were reduced by 80.2% and 90.1%, respectively, compared with the nominal case. Optimization of the deposition strategy is another method used to achieve a reduction in residual stresses. [PERSON], et al. [121] compared two deposition strategies: the 0-90\({}^{\circ}\), characterized by an orthogonal deposition direction between two layers, and the 0-67\({}^{\circ}\), which is characterized by a 67\({}^{\circ}\) rotation for each new layer. They showed that the residual stresses on the top surfaces were independent of the deposition strategy; however, higher residual stresses were observed on lateral surfaces produced by the 0-90\({}^{\circ}\) deposition strategy. [PERSON], et al. [144] measured residual stresses in FGM samples produced with different deposition strategies. They showed that the range of stress, i.e., the difference between the maximum and the minimum value, was reduced from 950 MPa to 680 MPa by rotating the deposition strategy by 90\({}^{\circ}\) at each layer. The lower range value of about 430 MPa was obtained using an island deposition strategy. However, using the island deposition strategy, a higher level of defects was observed. [PERSON] and [PERSON] [161] and [PERSON], et al. [162], using a FE model analysis, showed that residual stresses and related distortions depend significantly on the laser deposition strategy. Using a bi-directional deposition strategy elongated along a direction, the resulting distortion was characterized by a saddle shape (Figure 34a,c). The distortion was reduced by varying the laser deposition pattern.