## 2 Materials and Methods  
In this work, Ti-6 Al-4V ELI (extra-low interstitials) gas-atomized powder, provided by LPW Technology Ltd (Runcorn, United Kingdom), was used. The particle size distribution of the powder, obtained through image analysis of SEM images, shown in Figure 1a, was quite homogeneous and symmetrical and led to a D (10), D (50) and D (90), respectively, of 49.08 \(\upmu\)m, 79.92 \(\upmu\)m and 119.92 \(\upmu\)m. In Figure 1b a representative Scanning Electron Microscope (SEM) image of the powder was provided to show the spherical morphology of the employed particles. The cross-section of the particles, reported in Figure 1c, highlights the high quality of the powder due to the almost complete lack of pores and defects. The samples analysed were built in SUPSI (University of Applied Sciences and Arts of Southern Switzerland) laboratory (ARM lab-DTI). A Laserdyne(r)430 system was used. This consists of a DED customised three axis laser processing system that employs a Convergent Photonics CF1000 fibre laser, characterised by a maximum laser power (P) of 1000 W, a wavelength (\(\lambda\)) of 1070 nm and a laser spot diameter (D) of 1 mm. A multi-nozzle deposition head, provided by Optomec Inc. (Albuquerque, United States), was employed. Further details on the system are available elsewhere [31]. Argon (purity 4.6) was used as a carrier and shielding gas, using flow rates of 4 L/min and 15 L/min respectively. All the specimens were built deploying the same set of building parameters, described in Table 1.  
Figure 1: Particle size distribution of the Ti-6 Al-4V ELI powder used for deposition (**a**), SEM image of the powder (**b**) and cross-section of the particles (**c**). Cubic samples (\(15\times 15\times 15\) mm) and horizontal bars (\(10\times 10\times 100\) mm) were built in three different conditions, according to the process environment used, with the aim of evaluating the influence of the process atmosphere and temperature on the final component quality. The horizontal bars were used to investigate the influence of the process on larger pieces, while the cubic specimens were used for metallographic investigation of smaller parts. The specimens will be addressed as:  
* SE-CB (Sealed Environment, Cold Base), if built in a glovebox-like chamber using a non-heated platform;
* SE-HB (Sealed Environment, Hot Base), if built in glovebox-like chamber using a heated platform (220 \({}^{\circ}\)C);
* ArS (Argon Shielding), if built in an open environment, using a shielding gas (Ar) as only mean of protection from oxidation and contamination. The use of a sealed process chamber allowed to ensure the oxygen level was kept below 0.2% throughout the whole process. All the cubic samples produced were crack-free and had densities above 99.97% by image analysis. These specimens were cut, mounted, then polished and etched using a Kroll reagent (93% H\({}_{2}\)O, 5% HNO\({}_{3}\), 2% HF), to be prepared to be metallographic samples. HNO\({}_{3}\) and HF were provided by Sigma-Aldrich (St. Louis, United States) and were characterized by a purity of 65% and 40% respectively. The samples were mechanically polished using silicon carbide papers (#400-#2400) and colloidal silica (0.04 \(\upmu\)m) both provided by Struers (Ballerup, Denmark). Phase identification and crystallographic parameters calculations were evaluated by X-ray Diffraction (XRD) using a X-Pert PRO diffractometer (PANalytical, Almelo, Netherlands) in a Bragg Brentano configuration, the measurements were conducted using a Cu K\({}_{\alpha}\) radiation, operating the instrument at 40 kV and 40 mA. The step size was 0.013\({}^{\circ}\) and a 20 range between 30\({}^{\circ}\)and 90\({}^{\circ}\) was considered. Subsequently microstructure observations were performed using a DM ILM optical microscope (Leica, Wetzlar, Germany). High-magnification imaging was performed using a PhenomXL SEM (Thermo Fisher Scientific, Waltham, United States).