3.2.2. Experimental Tests at 600 W

Continuing with the same methodology, tests with a laser power of 600 W are presented in Table 7. There are significant increments in width due to the higher power, but not in height, which mainly depend on the powder mass flow. For this laser power, the width of the different clads hardly variates among gas mixtures as well as the depth of material dilution. However, the variation of the height as a result of the use of different gases is more significant, and, again, a greater variation is observed for the highest helium concentration tests (Helistar 50).

**Table 7.** Measurements for different gases at 600 W.

Table 7 shows also a reduction of wet angles of the clads due to the higher laser power. However, significant differences of wet angle are observed for clads made with each gas mixture. The higher the helium concentration, the smaller the wet angle of the clad.

Again, a repetition of the tests was made in order to consider the possible variations. The results with average values are presented in Table 8. As it can be observed, height, width, and dilution depth values are practically constant, regardless of the gas composition. However, the wet angles vary considerably for the different gas mixtures.


**Table 8.** Summary of results for experimental tests at 600 W.

#### 3.2.3. Experimental Tests at 800 W

Tests results for 800 W laser power are shown in Table 9. Because of power increment, the width and dilution are higher than those obtained in the 400 W and 600 W tests.

Similarly to the previous experiments, the highest variation between the clads is in height, whereas the dilution depth and width remain almost invariable.

Because of a higher laser power, the wet angle is smaller than previous tests with lower laser power. In addition, the wet angle changes for different gas compositions, as it can be observed in Table 9. The wet angle reduction is the most significant variation when using different gases, while the rest of measurements present similar values.

The experimental tests once again are repeated to obtain more information and to analyze the possible variation on the geometry of the clad. Table 10 shows these results as mean values of the measurements. As it was stated before, the more significant effect of the different gases use is the wet angle variation, and this variation is more considerable for higher helium concentrations.


**Table 10.** Summary of results for experimental tests at 800 W.

#### 3.2.4. Temperature Analysis

In addition to the geometry analysis, the temperature of the process was registered via pyrometry and Table 11 shows the outputs of these measurements.

In all cases, the utilization of helium in the process results in higher temperature. When pure argon was used, mean temperatures of 1938 K, 1991 K, and 2121 K were reached for laser powers of 400 W, 600 W, and 800 W, respectively. Meanwhile, when Helistar 25 (Ar 75% and He 25%) and Helistar 50 (Ar 50% and He 50%) was employed, temperatures with average values of 2093 K and 2097 K where measured, respectively, at 400 W. However, increasing the amount of helium in the mixture did not result in a higher temperature.

**Table 11.** Summary of results for experimental tests at 400 W, 600 W, and 800 W.

#### **4. Discussion**

As it has been observed in the previous tests, the helium content of the gas affects the geometry of the deposited clad. The comparison between the tests that were realized with argon and those with Helistar 50, shows height value differences of 70 μm, 90 μm, and 140 μm for laser power values of 400 W, 600 W, and 800 W, respectively. These height variations are slightly lower when Helistar 25 is used. With regard to width and depth values, the differences do not exceed 30 μm for dilution depth and 50 μm for width, and no clear tendency is appreciated. Moreover, the wet angle seems to have a correlation with the helium concentration of the gas, changing the shape of the clad geometry by smoothing the slope. The wet angle decreases when the helium proportion is increased and variation values up to 21 degrees are registered.

The melt pool temperature variation is other factor, which is strongly influenced by the gas mixture. With argon, variations of almost 200 K were registered from a 400 W to 800 W laser power. However, the same differences of laser power do not show significant variations of the temperature when gases with presence of helium are used, merely of 20 K approximately.

As Andreas Patschger and Rolf Wester et al. state [6,7], the ionization energy of the gas is important due to the formation of plasma in the laser beam way. In this case, argon is more susceptible of forming ionized gas because of its lower ionization energy value, which is near to the 64% of the helium's one. In addition, the heat conductivity of these two gases are very different, being helium's (0.151 W·m−1·K<sup>−</sup>1) near 40 times higher than argon's thermal conductivity (0.018 W·m−1·K<sup>−</sup>1).

Plasma works as an isolation for the laser beam and higher heat conductivities allow for the heat that is radiated to be fed back to the process. The variation of the temperature for the different gases can be explained with these two phenomena. For a high energy density process, the use of argon contributes to plasma formation, and thus, the isolation of part of the energy provided by the laser. On the other hand, helium is able to work with higher energy densities without promoting plasma formation and contributing to feeding back heat to the process due to its greater thermal conductivity.

#### **5. Conclusions**

The present work studies the influence of the gas composition on the LMD process. Three different gas compositions have been tested: Ar 99.99%, Ar 75%-He 25%, and Ar 50%-He 50%. Up to 150 microns, differences in height are observed between Ar 99.99% and Ar 50%-He 50% concentration gases. The higher the helium presence in the mixture, the smaller the height of the clad. The rest of the geometry characteristics remain virtually stable with a variation of less than 60 microns for extreme cases.

The most significant variation on the shape of clads is the wet angle. This parameter variates within a wide range with the helium concentration, decreasing its value while the presence of this gas goes in augment. Variations of 10 to 20 degrees are observed between the use of pure argon and a mixture with 50% of helium. In addition, the temperature of the melt pool is also influenced by the presence of helium when it is combined with high energy densities.

Conclusions of the present research work can be summarized in the following way:


**Author Contributions:** J.I.A. and J.E. conceived and designed the experiments; M.C. and J.E.R. performed the experiments and analyzed the data, helped by J.I.A.; A.L. supervised the whole research work; J.E.R., M.C. and A.L. wrote the paper.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was supported by the DPI 2016-79889-R INTEGRAddi Project and the POCTEFA 90/15 Transfron3D Project. Special thanks are addressed to Praxair (Praxair, Inc., Barakaldo 48903, Spain) for their technical and supply supports in this work.

**Conflicts of Interest:** The authors declare no conflict of interest.
