# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 4 Solidification Mechanisms
### Residual Stress  
The dynamic nature of thermal phenomena in LP-DED processes induces a high level of residual stresses. These residual stresses develop due to the reiteration of heating/cooling cycles and the high-temperature gradient during the process. According to [PERSON] and [PERSON] [139], two mechanisms are primarily responsible for the generation of residual stresses: the Temperature Gradient Mechanism (TGM) and the cooling-down phase. The TGM is caused by the high-temperature gradient near the laser spot, whereas in the cooling-down phase model, the molten material shrinks during the solidification due to thermal contraction. However, this contraction is limited by the underneath material [139; 140; 141]. The magnitude of residual stresses is influenced by the stress-strain relationship of the material and by the strain misfit between two adjacent regions during the cooling phase [142]. In particular, higher residual stresses have been observed in materials characterized by a higher modulus of elasticity (\(E\)) and higher yield strength (\(Y\)s). For example, [PERSON], et al. [143] showed that residual stresses in the IN718 sample (\(E=205\) GPa, \(Y\)s = 1100 MPa) were approximately 1.5 times higher than residual stresses in the 316L sample (\(E=196\) GPa, \(Y\)s = 450 MPa). Residual stresses in some areas can reach very high values, even 75% of the yield strength [143]. The strain misfit between adjacent regions is crucial during the production of Functionally Graded Material (FGM), where materials with very different thermal expansion coefficients (CTE) are used. [PERSON], et al. [144] studied the residual stress distribution obtained in five FGM samples produced with different chemical compositions. They observed that as the chemical composition of the samples changed, there was an abrupt variation from tension to compression. The presence of residual stresses affects important characteristics of the produced components, such as tensile and fatigue strength, thus influencing component integrity and service life [138]. Moreover, the residual stresses induce distortion [145; 146] and loss of geometrical tolerances [147]. Due to complex heating and cooling cycles, stresses in the deposited material assume a highly non-uniform distribution. [PERSON], et al. [148] measured the residual stresses of two stainless steel samples using the neutron diffraction method. They analyzed samples with different geometries, i.e., a thin wall and a pillar with a square cross-section. They showed that a compression state is observed at the centre of each sample, while a tension state is obtained near the edge. Later, [PERSON], et al. [143] measured the residual stress distribution of two samples with rectangular and square cross-sections using neutron diffraction and the contour method. They observed that the stresses were almost uniaxial along the building direction. [PERSON], et al. [149] analyzed the stress distribution on parallelepiped samples using the contour method. The results, illustrated in Figure 33, showed that near the building platform, the stresses along the building direction were almost zero; the longitudinal stresses, on the other hand, were highly compressive. On the contrary, the stress distribution on top layers showed tensile stresses along the building direction, but longitudinal stresses had almost zero values. This behavior was observed by analyzing the macroscale residual stress distribution and confirmed by several authors [150; 151; 152; 153; 154]. Nevertheless, the high non-uniformity of residual stresses can also be observed in the mesoscale and the microscale levels, which analyze the stress distribution on the layer track length scale, respectively [155; 137; 156]. [PERSON], et al. [155] measured the residual stresses at the mesoscale level of Ti-6 Al-4V samples produced by LP-DED using hole drilling and slitting methods. They proved that the samples were characterized by a tensile state on the external surface that was balanced by a compression state in the centre.