## 2 Microstructure
wavelength of 980-1020 nm, a 4-axis mechanical work station, an argon-purged processing chamber, a coaxial powder feeding was employed to fabricate the component. To avoid any possible oxidation the oxygen content was kept less than 80 ppm. It was reported that by increasing the m values the area fraction of equiaxed grains increases whereas the area fraction of columnar grains decreases significantly (Figure 5). In fact, when the mass deposition rate is increased to a very high value, the mechanism of grain evolution changes: in these conditions the epitaxial growth of parent grains is prevented and new nucleated equiaxed grains cover more than 80% of the whole deposited layer. However, a short distance from the pool fusion line some short-columnar or olive-like morphology can be detected. Moreover, it was stated that by increasing the mass deposition rate the penetration melting depth significantly decreases and after a critical point no penetration melting occurs, making layer by layer additive manufacturing impossible [87]. It was found that there is a critical threshold for the \"m\" values, below which a full columnar grain structure can be achieved (Figures 6 and 7). Beyond the upper value of this threshold, the penetration melting depth is lower than mass deposition rate and a fully equiaxed grain structure will be obtained. Indeed, in this region the excessive laser power is very low to melt all the delivered  
Figure 5: Optical micrographs showing the as-deposited prior beta grain morphologies on transverse cross section of single-track deposited specimens as a function of mass deposition rate, (**a**) 6 g/min, (**b**) 11 g/min, (**c**) 15 g/min, (**d**) 25 g/min, (**e**) 31 g/min, (**f**) 36 g/min, (**g**) 44 g/min, (**h**) 55 g/min and (**i**) 59 g/min (reproduced with permission from [87] Copyright Elsevier, 2015). powder, thus these partially melted particles either in the melt pool or on the surface of the melt pool promote a heterogeneous nucleation and growth mechanism that results in the formation of a fully equiaxed grain structure (Figure 8) [87]. On the other hand, in titanium materials deposited with the mass deposition rate between the lower and upper values of threshold (m\({}_{\mathrm{CL}}\)! m! m! m! m! m! m! m! m! m! m! m! m! m! m! m! m! m! m  
layer. This evolution occurs because in this zone, grains start to grow epitaxially owing to the different thermal gradient with respect to the center of the melt pool. In general, the authors concluded that a low specific mass deposition rate results in high melt superheating, large penetration re-melting, and a high thermal gradient, promoting the epitaxial growth. As a consequence, columnar grain structure is observed in the final microstructure. On the other hand, a high specific mass deposition rate implies low melting temperature, insufficient powder melting, and high superficial and endogenous heterogeneous nucleation sites, leading to the formation of a near equiaxed grain structure [87]. In the case of fabrication of large Ti-6 Al-4V structures produced by the DED process, [PERSON] et al. found that it was feasible to produce a porosity free component by using a combination of high laser power together with a reasonably low powder feed rate [88]. According to their studies, the height of deposition is correlated to the Z steps: under-build phenomena or excessive building can occur with high or low values, respectively, and columnar grain structure with martensitic needles is observed. In addition, the authors also evaluated the effect of Hot Isostatic Pressing (HIP) on the microstructures of DED specimens, as presented in Figure 9. First, it is possible to underline that columnar grains and martensitic are the dominant structure in the as-DED condition. The formation of columnar grains can be explained by the heat dissipation phenomenon during the DED process. In particular, the columnar grains start to grow epitaxially in the middle section in the opposite direction of heat dissipation.