## 2 Materials and Methods
### Description of the Two Tested LMD Machines  
This study evaluates and compares an articulated robot-based LMD machine by BLM GROUP (Cantu, Italy) and a CNC gantry-based LMD machine by SM Systems S.r.l. (Torre Canavese, Italy). The two machines are very similar in terms of laser source, optical, and powder feeding settings; however, they differ significantly in terms of kinematic arrangement, maximum build size, and weight. The technical information on the two systems is summarized in Table 1. Figures 3 and 4 show the two machines. Both machines are equipped with an IPG Photonics (Oxford, MI, USA) high power continuous wave Ytterbium active fiber laser source that generates a laser beam with a wavelength of 1070 nm. The maximum output power of the two sources, however, differs: 3000 W for the robot-based machine and 4000 W for the gantry-based machine. The laser beam is delivered from the laser source to the deposition head in the same way in both machines via a feeding fiber and a process fiber connected by a fiber-to-fiber \begin{table}
\begin{tabular}{c c c c} \hline  & **Machine Manufacturer** & **BLM GROUP** & **SM SYSTEMS** \\ \hline \multirow{3}{*}{Laser source} & Model & IPG YLS-3000 & IPG YLS-4000 \\  & Wavelength, [nm] & 1070 & 1070 \\  & Max power, [W] & 3000 & 4000 \\ \hline Feeding fiber & Core diameter, [μm] & 100 & 100 \\ \hline Process fiber & Core diameter, [μm] & 400 & 600 \\ \hline \multirow{3}{*}{Deposition head} & Model & Kuka MWO-I-Powder & Kuka MWO-I-Powder \\  & Collimating lens focal length, [mm] & 129 & 129 \\  & Focusing lens focal length, [mm] & 200 & 200 \\ \hline \multirow{3}{*}{Laser beam characteristics} & Beam parameter product, \(BPP\) & \multirow{3}{*}{14.1} & 21.3 \\  & [mrad\(\cdot\)mm] & & 41.8 & 62.5 \\ \cline{1-1}  & Quality factor, \(M^{2}\) [–] & & 41.8 & 62.5 \\ \hline Powder nozzle & Model & Fraunhofer ILT COAX-40-F & Fraunhofer ILT COAX-40-F \\ \hline \multirow{3}{*}{Powder feeder} & Model & GTV TWIN PF 2/2-MF & Sulzer TWIN 10 C \\  & Working principle & Rotary disk & Rotary disk \\ \hline \multirow{3}{*}{Motion system} & Kinematics configuration & Articulated robot + positioner & Gantry system + positioner \\  & Number of axes & 6 + 2 (rotation + tilting) & 3 + 2 (rotation + tilting) \\ \cline{1-1}  & Repeatability, [mm] & 0.03 & 0.01 \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of the characteristics and properties of the two machines. Figure 2: Particle size distribution of the used IN718 powder. coupler. The feeding fiber core diameter in both machines is 100 \(\upmu\)m, while the process fiber core diameter in the robot-based and gantry-based machines is 400 \(\upmu\)m and 600 \(\upmu\)m, respectively. Both machines are outfitted with a KUKA AG MWO I Powder deposition head (Augsburg, Germany). The focal lengths of the collimating and focusing lenses of this model of deposition head are 129 mm and 200 mm, respectively. Manipulation of the position of the focal point of the laser along the optical axis is accomplished by adjusting the distance between the collimation lens and the process fiber outlet. Thanks to this feature, the two machines can function with the same laser spot diameter on the working plane even though their process fibers have different core diameters. The beam parameter product BPP and the quality factor M2 at the outlet of the deposition head are 14.1 mrad\(\cdot\)mm and 41.8 mrad\(\cdot\)mm for the robot base system, respectively, and 21.3 mrad\(\cdot\)mm and 62.5 for the gantry-based system. On both machines, a Fraunhofer ILT COAX 40 F powder nozzle (Aachen, Germany) was employed. The two powder feeders mounted on the articulated robot-based and gantry-based LMD systems are a TWIN PF 2/2-MF by GTV Verschelisschutz GmbH (Luckenbach, Germany) and a TWIN 10 C by Sulzer Metco AG (Wohlen, Switzerland). Even though separate companies manufacture these two devices, they both operate on the same rotating disk principle. The powder feeder is also in charge of the process gases, specifically the carrier and shielding gases, both of which were Argon.