## 2 Microstructure
Previous research has established that epitaxial grain growth and consequently columnar grains is the common mechanism in most of the titanium alloys such as Ti-6 Al-4V, Ti-6.5 Al-3.5 Mo-1.5 Zr-0.3 Si and so on, and even in titanium aluminides alloys such as Ti-48 Al-2 Cr-2 Nb [75; 76; 77]. However, in various direct laser deposited parts, different morphologies of prior beta grains were reported in near-alpha, \(\alpha+\beta\), and near-\(\beta\) titanium alloys. For instance, in laser deposited near-\(\beta\) titanium alloys such as Ti-5 Al-5 Mo-4V-1 Cr-1 Fe a \"bamboo-like\" grain morphology developed in the building direction, including olive-like arrays (coarse short column) and fine equiaxed grains inside the short columns. On the other hand, it was demonstrated that in DED Ti-6.5 Al-3.5 Mo-1.5 Zr-0.3 Si by tailoring the parameters it is possible to obtain a mix of columnar and equiaxed grains perpendicular to the building direction [28; 78]. However, it seems there is a lack of experiment and proof to explain the diversity of prior beta grain morphology. [PERSON] et al. observed that the microstructure of layer bands of Ti-5 Al-5 Mo-4V-1 Cr-1 Fe near beta titanium alloy produced by direct laser deposition is duplex with low hardness, whereas between the layers the microstructure is ultra-fine basket-weave with very high hardness [28]. Indeed, they used a 8 kW continuous wave CO\({}_{2}\) laser material system with a four axis Computer Numerical Controlled (CNC) work station and the pre-alloyed powder was fed into the melt pool through a coaxial nozzle. An argon purged processing chamber with oxygen content less than 50 ppm was employed to avoid oxidation. They showed the typical microstructure of beta grain morphology which includes three sorts of grains: large columnar grains, small columnar grains, and equiaxed grains (Figure 1). The mixture of small columnar and equiaxed grains is defined as \"bamboo-like\" grain morphology, in which the equiaxed grains are revealed between the small columnar grains in the building direction. This kind of grain morphology evolution during the DED process is in line with the other works [79; 80; 81]. In this study, it was found that the large columnar and bamboo-like grains are arranged alternatively in the front and top view. It was also shown that at the opposite direction of laser scanning, small columnar grains with a zigzag pattern and 20\({}^{\circ}\) orientation with respect to the building direction are grown. This specific orientation is explained by the direction of heat conduction during the direct laser deposition [28]. It was found that the different processing and post-process conditions were expected to result in various microstructures and a wide range of mechanical properties [82; 83; 84]. For instance, the microstructure of components produced by laser or electron beam based DED in as deposited condition exhibited a lamellar \(\alpha+\beta\) structure. Despite a similar microstructure in both laser and electron beam DED, the scale of these microstructure characteristics varied depending on the AM process. It was reported that the microstructure of electron beam based DED is coarser with respect to the laser based one [59; 82]. [PERSON] et al. suggested the development of grain morphology during the direct laser deposition of different geometries (Figure 2). As can be seen in Figure 2a, when a single scan track is deposited, columnar grains at the bottom of the melt pool are formed as a consequence of high thermal gradient and rapid cooling undergone by the substrate. By increasing the height of the melt pool, the thermal gradient and cooling rate decreased up to the top of the melt pool. These variations in thermal gradient and cooling rate result in the formation of equiaxed grains on top of the melt pool. In the case of multi tracks (Figure 2b), in the overlapped areas between consecutive scans the equiaxed grains remelt and then solidify in the form of columnar grains.