## 1 Introduction  
Additive manufacturing (AM) is a group of near-net-shape technologies which generally deploy pre-alloyed powders as feedstock material to build 3D objects layer by layer, directly from a computer-designed 3D model. For metal alloys, the available processes that involve the use of powder as raw material can be divided into powder-bed fusion (PBF) processes, such as laser powder-bed fusion (LPBF) and electron beam melting (EBM), and directed energy deposition (DED) processes. In DED systems, the powder is delivered directly onto the building area through nozzles, usually mounted on a multi-axis deposition head, and then melted through a heating source, commonly a laser. The absence of a powder bed and the movement freedom granted by the motion system make DED technologies particularly well suited for component repairing, large component manufacturingand compositionally graded materials production [1; 2; 3; 4; 5; 6]. Laser DED systems were successfully adopted for producing components by using different materials, such as stainless steels [7], Ni alloys [8] and Ti alloys [9]. Amongst titanium alloys for the DED technology, the most frequently used is Ti-6 Al-4V [10; 11; 12], making it possible to produce parts with mechanical properties similar or even higher than the traditionally manufactured counterparts [13]. The Ti-6 Al-4V alloy is a well-established material for aerospace applications, although it finds usage in other industrial sectors, such as automotive, biomedical, energy production and the chemical field [14; 15; 16]. Among all the AM technologies, DED is currently the least investigated for Ti-6 Al-4V. Further details on this technology are available elsewhere [1]. Currently most works available in the literature focus on analysing the process parameters optimization in order to improve the deposited component quality [17; 18; 19]. For example, [PERSON] et al. [19] investigated the influence of scanning speed and laser power on the microhardness of Ti-6 Al-4V produced via DED. Notwithstanding this, a deep knowledge to correlate the final component properties and quality to the process parameters and environment is still lacking. For example, atmosphere control is a key aspect to investigate, especially concerning its influence on oxygen, nitrogen and hydrogen content in the alloy, particularly critical for Ti-6 Al-4V. It is well-known in fact that oxygen and nitrogen are \(\alpha\)-stabilizing interstitial elements in titanium alloys, while hydrogen is a \(\beta\)-stabilizer (\(\beta\)-eutectoid) [14]. Oxygen indeed fits well in the octahedral vacancy of \(\alpha\)-Ti (h.c.p.) due to the fact that it shares a similar radius (\(\approx\)0.60 A). Interstitials concentrations require strict monitoring due to their direct impact on mechanical properties [14]. For instance, ductility is severely reduced even by very low amounts of O and N, which might also change the deformation mode of the material from twinning to slip [20]. Interstitials influence was proved to be more critical for \(\alpha\)-Ti alloys than \(\beta\)-Ti alloys and it is even more impacting for AM components, especially if characterized by a martensitic \(\alpha^{\prime}\) microstructure. [PERSON] et al. [20] estimated a threshold value of 0.22% wt. for oxygen content in AM Ti-6 Al-4V, after which the material shows complete brittleness [20]. Nitrogen shows a similar behaviour in Ti alloys and its influence on this class of materials is also comparable. In general, in DED processes, using a sealed processing chamber should ensure improved atmosphere and process control, however it limits the maximum component size, which must be smaller than the chamber itself, and makes repairing operations more complex and restricted. In fact, several studies are available in the literature in which the deposition process was conducted using a sealed environment [21; 22; 23]. Among these, [PERSON] et al.