## 2 Microstructure
On the other hand, in the peripheral areas the grains grow at a slant angle with respect to the building direction owing to the participation of radiation in the heat dissipation. After HIP, the transformation of martensite into lamellar \(\alpha+\beta\) was suggested [88]. Figure 8: Schematic illustration for the formation of fine near-equiaxed grains for titanium alloy components during the layer-by-layer laser melting in longitudinal (**a**) and transverse (**b**) cross sections (reproduced with permission from [87] Copyright Elsevier, 2015). Several studies have shown that in general a different thermal gradient and cooling rate which result in different microstructures have influence on the microhardness of Ti-6 Al-4V components [18]. In the case of DEDed Ti-6 Al-4V components, as discussed earlier the microstructure of the part in both the top and bottom includes the Widmanstatten structure, although the morphology varies in each area. In fact, at the top of the sample the colonies of parallel, very fine lamellae, and larger latths is revealed whereas in the bottom region the lamellae become thicker (Figure 10). This discrepancy can be related to the thermal gradients, cooling rates, and cycling thermal treatments during the laser deposition step [42; 89]. Figure 9: (a–c) OM X–Z sectional micrographs showing grain structure in as-DED samples; (d,e) back scattered electron SEM micrographs showing microstructure of as-DED and DED + HIPed samples, respectively. The arrows show the grain growth directions in different regions of the samples (reproduced with permission from [88] Copyright Elsevier, 2015). A residual stress is a stress produced in a body in a mechanical and thermal equilibrium condition without external forces [90]. The main sources of residual stress after DED are the dynamic temperature distribution and cooling/heating rate within the component that contains high thermal gradients and repetitious/rapid local heat transfer rates [91; 92]. Therefore, it is very important to study the thermal history of a component during the DED process because of its significant effects on the anisotropy of microstructure and level of residual stress inside the part. Another significant effect of tensile residual stress inside the component is on the tensile and fatigue resistance. A reduction of mechanical performance and dimensional inaccuracies are the other consequences of the presence of residual stress after the process. Thermal conductivity, coefficient of thermal expansion, Young's modulus, and yield stress together with phase transformation, part geometry, process parameters during fabrication, and scanning pattern all affect the level and type of the residual stress inside the sample [91; 93]. It was found that materials with higher strain mismatch during the cooling step, high Young's modulus, and yield stress usually show the formation of higher level of residual stresses [91]. Several lines of evidence suggest that in Ti-6 Al-4V alloy there is a correlation between residual stresses within the building plane and the laser scanning direction in which there is a compressive residual stress at the center and tensile stress at the edges of the component. Moreover, it was reported that at the starting point of laser scanning the level of residual stresses is lower and finally reach their maximum at the end of the laser scanning path [94]. On the other hand, it was detected that in the first deposited layers the level of compressive stress was very high and by increasing the number of layers this compressive stress transit to tensile residual stress. According to previous works, it would be possible to reduce the residual stresses through the optimization of the process parameters, using an appropriate melt pool size and morphology, employing a proper scan strategy, and preheating of the substrate or last deposited layer [91; 92; 95; 96; 97; 98].