## 2 Microstructure
This change in the grain morphology in the overlapped zones was reported as a consequence of the lower depth of melt pool in these regions which results in higher thermal gradient and cooling rate that promote the epitaxial grain growth. Regarding multi layer deposition (Figure 2c), it was reported that the partially remelted, equiaxed grains act as nuclei for the epitaxial grain growth, resulting in the formation of columnar grain morphology. Thereafter, the grain structure of the new deposited layer becomes similar to the previous layer and this grain structure repeats in every layer up to the last deposited layer. However, it was concluded that the variation of direct laser deposition parameters changes the evolution of grain morphology, thus further investigations about the effect of LMD process parameters on the development of grain morphology are still required. Figure 1: Typical \(\beta\) grain morphology of laser melting deposited Ti-5 Al-5 Mo-5V-1 Cr-1 Fe, at the center of the plate-like sample (reproduced with permission from [28] Copyright Elsevier, 2003). Figure 3 shows the formation of a fine basket-weave microstructure in the area between the bands and a duplex microstructure including ultra-fine and coarse alpha latths within the layer bands. It was thus concluded that high cooling rates involved in DED result in the formation of ultra-fine alpha latths that are much finer than the Widmansattein microstructure which forms in the \(\alpha\) or \(\alpha+\beta\) alloys [28; 85]. [PERSON] et al. investigated the microstructure and mechanical properties of laser metal deposited Ti-6.5 Al-3.5 Mo-1.5 Zr-0.3 Si, an \(\alpha+\beta\) titanium alloy [25]. In this work, to fabricate the specimens a 6 kW fiber laser melting deposit manufacturing system with an argon purged processing chamber was used. The oxygen content was kept less than 50 ppm to prevent any oxidation. In this research, laser power 6 kW, laser beam diameter 5 mm, scan speed 800 mm/m, powder feed rate 500 g/h were chosen as optimum parameters and the starting powder was fed by means of a coaxial nozzle. In these materials  
Figure 3: Scanning electron microscopy micrographs showing the typical microstructures of laser melting deposited Ti-5 Al-5 Mo-5V-1 Cr-1 Fe alloy: (**a**) between the layer bands and (**b**) within the layer bands (reproduced with permission from [28] Copyright Elsevier, 2003). Figure 2: Schematic illustration of the formation process of \(\beta\) grains during the DED process, (**a**) one track, (**b**) multi-tracks, (**c**) multi-layers (reproduced with permission from [28] Copyright Elsevier, 2003). the columnar grains are presented in the overlapping zones whereas the equiaxed grain structures are located in the body zones. [PERSON] et al. correlated this kind of microstructure with the temperature gradient (G) and solidification rate (V). Indeed, they observed that at high G and low V, columnar grains are predominant, while at low G and high V equiaxed grains form [86]. Thus, they concluded that the values of G and V at various locations of a melt pool should be different, although the geometry and size of melt pool is dynamically stable while it keeps moving [25]. They also demonstrated schematically the 3D grain morphology of Ti-6.5 Al-3.5 Mo-1.5 Zr-0.3 Si titanium alloys obtained after direct laser deposition (Figure 4), with a particular microstructure consisting of alternative arrays of columnar and equiaxed grains [25]. [PERSON] et al. investigated the solidification nucleation and growth mechanisms of the local melt pool over direct laser deposition, evaluating the effect of mass deposition rate (m) on the area fraction of equiaxed and columnar grains [87]. In this research a 10 kW fiber coupled diode laser with a beam  
Figure 4: (**a**) Optical micrograph of the YOZ section, (**b**) Schematic illustration of grain morphology in three dimensions of the LMD sample (reproduced with permission from [25] Copyright Elsevier, 2014).