Study of the Influence of Shielding Gases on Laser Metal Deposition Of

materials

Article Study of the Inﬂuence of Shielding Gases on Laser Metal Deposition of Inconel 718 Superalloy

Jose Exequiel Ruiz *, Magdalena Cortina ID , Jon Iñaki Arrizubieta ID and Aitzol Lamikiz ID Department of Mechanical Engineering, University of the Basque Country, Plaza Torres Quevedo 1, 48013 Bilbao, Spain; [email protected] (M.C.); [email protected] (J.I.A.); [email protected] (A.L.) * Correspondence: [email protected]; Tel.: +34-946-017-347

Received: 28 June 2018; Accepted: 6 August 2018; Published: 9 August 2018

Abstract: The use of the Laser Metal Deposition (LMD) technology as a manufacturing and repairing technique in industrial sectors like the die and mold and aerospace is increasing within the last decades. Research carried out in the field of LMD process situates argon as the most usual inert gas, followed by nitrogen. Some leading companies have started to use helium and argon as carrier and shielding gas, respectively. There is therefore a pressing need to know how the use of different gases may affect the LMD process due there being a lack of knowledge with regard to gas mixtures. The aim of the present work is to evaluate the influence of a mixture of argon and helium on the LMD process by analyzing single tracks of deposited material. For this purpose, special attention is paid to the melt pool temperature, as well as to the characterization of the deposited clads. The increment of helium concentration in the gases of the LMD processes based on argon will have three effects. The first one is a slight reduction of the height of the clads. Second, an increase of the temperature of the melt pool. Last, smaller wet angles are obtained for higher helium concentrations.

Keywords: LMD; laser metal deposition; shielding gas; argon; helium; additive manufacturing; Inconel 718; melt pool temperature

1. Introduction Laser Metal Deposition (LMD) is an Additive Manufacturing (AM) technology that consists on the deposition of material layers that are melted by a laser source. This process belongs to the Directed Energy Deposition (DED) technology group and enables working with a very wide range of metal materials. The ﬁller material to be deposited is usually supplied in the form of powder and is conducted through a nozzle into the melt pool, which has been created on a substrate surface by a laser beam. In this process, the powder is melted and deposited by creating a new layer of material. Subsequent layers generate geometries, enabling the addition of features to existing parts or the manufacturing of new ones [1]. The entire process is carried out employing two different gas flows: a shielding gas, whose function is the generation of a protective atmosphere so that oxidation reactions are avoided, and a carrier gas that is used to transport the powder through the entire circuit and nozzle to the melt pool. The use of these two different gas flows has direct consequences on the process. Firstly, similar to other welding processes, the shielding gas is necessary to protect the deposited material from oxidation [2]. It represents the largest gas flow used and it has a direct impact on the quality of the deposited material, mainly in the porosity generation, but this phenomenon can be easily avoided by parameters modification [3,4]. Secondly, the carrier gas drags the powder from the powder feeder to the nozzle and into the melt pool. This gas flow accelerates and injects the powder particles at the nozzle exit. The interaction of both, carrier and shielding gas, with the powder and the atmosphere determines the powder distribution in the focal point of the nozzle. This powder distribution is a key factor for the LMD process efficiency [5,6].

Materials 2018, 11, 1388; doi:10.3390/ma11081388 www.mdpi.com/journal/materials Materials 2018, 11, 1388 2 of 10

Bibliography of LMD process situates argon as the most common gas (both for protective and drag gas), followed by nitrogen. This is due to the fact that nitrogen is mostly unreactive and more economical than argon, but does not always protect the process from chemical reactions, since nitrogen reacts with Ti, Nb, and V [7]. The use of the LMD technology as a manufacturing and repairing technique in industrial fields, like the die and mold and aerospace industry, is increasing within the last years. In these industrial sectors, the process is specially focused on different steel alloys, nickel superalloys [8,9] and titanium alloys. Steels are usually alloyed with vanadium, nickel superalloys, like Inconel 718 or Inconel 625, have titanium and niobium in its chemical composition and one of most typical titanium alloys is Ti6Al4V, which contains both titanium and vanadium. Thus, in these cases nitrogen should not be used as shielding or dragging gas on LMD process. There are many similarities between the LMD process and Laser Beam Welding in terms of shielding gases. In fact, both use the same heat source and materials must be protected from chemical reactions with a shielding gas. Different research work carried out in the field of Laser Beam Welding states that the use of argon with high densities of energy can lead to plasma formation due to the lower ionization energy of argon compared to other gases, like helium [10]. This plasma formation creates a shield that blocks the laser beam [11], thus reducing the energy that finally reaches the workpiece. On the one hand, the depth of the welds using an argon-helium mixture is comparable to those that are achieved with a pure argon flow. On the other hand, the width is bigger when the argon-helium mixture is used [10]. As previously mentioned, argon and not nitrogen is usually employed for LMD. In fact, machine tool builders working with LMD process, like DMG MORI (DMG MORI, Bielefeld, Germany), Trumpf (TRUMPF GmbH + Co., KG, Ditzingen, Germany), or MAZAK (Yamazaki Mazak Corporation, Oguchi, Japan), among others, usually recommend the use of argon despite its high price in order to avoid the risk of unwanted reactions in the process. However, some of these leading companies have started to use helium and argon as carrier and shielding gas, respectively. This is due to a fluid dynamical issue: helium’s density is lower than argon’s, so when the helium flow crosses the argon one, turbulences are reduced, and a more stable gas flow is achieved at the substrate. This phenomenon helps to improve the powder concentration and protective gas concentration at the melt-pool, hence simplifying the nozzle design. Therefore, there is a necessity to know how the use of different gases may affect the LMD process. From the literature review, it is noted that some research has been realized focusing on the use of argon or nitrogen individually, with the argon case the most documented. However, there is a lack of information about the influence that helium or gas mixtures may have on the LMD process. The aim of the present work is to evaluate the effect of a mixture of argon and helium on the LMD process by analyzing single tracks of Ni based alloy Inconel 718. The height, width, and depth of the clads, along with the temperature of the process, are measured for the different gas mixtures employed.

2. Materials and Methods The selected material for this work is a nickel-based superalloy, Inconel 718. This material is commonly used in the aerospace industry for turbine and other structures where temperatures can be higher than 600 ◦C. It is probably one of the most typical materials used in LMD process. Thus, substrate material is an Inconel 718 alloy and powder material is an Oerlikon MetcoClad 718 (Oerlikon, Freienbach, Switzerland), which has been used as the ﬁller material. Both share the same chemical composition within certain compositional limits, as shown in Tables1 and2. Powder particle size distribution is shown in Table3, in order to enable other fellow researchers to replicate the same tests. Materials 2018, 11, 1388 3 of 10

Table 1. Chemical composition (wt %) of the Inconel 718 substrate [12].

Cb + Ta Ni Cr Cb (Nb) Mo Ti Al Co Mn (Nb + Ta) 52.50 18.40 5.08 5.08 3.04 1.03 0.54 0.33 0.24 Si C Cu B Ta P S Fe 0.11 0.052 0.05 0.005 <0.05 0.006 <0.002 BAL

Table 2. Chemical composition (wt %) of the MetcoClad 718 powder [13].

Cr Mo Nb Fe Ti Si Mn C B Ni 19.00 3.00 5.00 18.00 1.00 0.20 0.08 0.05 0.005 BAL

Table 3. Particle size, which follows a Rosin-Rammler distribution.

Minimum Size (µm) Mean Size (µm) Maximum Size (µm) Spread Parameter Number of Diameters 50 90 135 4.2 10

All of the tests that are described within this work were carried out on a laser processing cell, Kondia Aktinos 500 (Kondia, Elgoibar, Spain), rebuilt from a conventional milling center. It has three linear plus two rotary axes for a total of five-axis cinematics, and a work volume of 700 × 360 × 380 mm3. The laser source is a Rofin FL010 (ROFIN-SINAR, Bergkirchen, Germany), a Yb:YAG fiber laser of 1 kW of maximum power and 1070 nm wavelength. An optical multi-mode fiber is used to guide the laser beam to the processing cell and an optical lens focuses it at 200 mm, thus creating a circular spot of 1.6 mm of diameter, approximately. In addition, the powder feeder employed is a Sulzer Metco Twin 10-C (Oerlikon Metco, Pfäffikon, Switzerland), which can be used with argon, nitrogen, and other gases, like helium. The powder is injected by means of a self-developed coaxial nozzle EHU/Coax 2015 (UPV/EHU, Bilbao, Spain) [5]. The gases that were used during the experimental tests, supplied by Praxair (Praxair, Inc., Danbury, USA), are (1) Argon, with a purity of 99.998%, (2) Helistar 25, whose composition is argon 75% and helium 25%, and (3) Helistar 50, which is a half argon and half helium mixture [14–16]. The same gases were supplied both as shielding and as carrier gas, in order to avoid mixing and knowing the precise composition. Two different experiments were designed. One was intended to measure the melt pool temperature and its cooling time. The second one aimed to characterize the deposited tracks by measuring their height, width, dilution depth, and wet angle, along with the temperature of the process. The temperature was measured by means of a digital two-color pyrometer with an IGAR 12-LO (LumaSense Technologies, Inc., Santa Clara, CA, USA) optic fiber [17], which was focused on the same point as the laser beam. The use of this kind of technique instead of a standard one-color pyrometer is because the measurement of a two-color pyrometer is independent of the emissivity in a wide range of temperature and it is unaffected by fume or powder in this case.

2.1. Melt Pool Temperature Measurement and Cooling Time The tests were realized on a 10 mm thickness substrate in order to avoid considerable thermal affection. Previously, the test specimens were cleaned and prepared so that a homogeneous surface was attained. The k-factor (also known as emissivity slope) of the two-color pyrometer was calibrated with a similar substrate of equal dimensions and characteristics heated in a furnace (Helmut ROHDE GmbH, Prutting, Germany) up to 1423 K and then measured with a thermocouple type K. The laser was set at the focal distance of the nozzle with the dragging and shielding gas ﬂows on, while a laser power of 250 W was used to heat the surface for 1 s, so that the melting point was reached. Materials 2018, 11, 1388 4 of 10

Once the laser power was off, the measurement of the temperature continued until the pyrometer Materialsstopped 2018 registering, 11, x FOR anyPEER signal REVIEW intensity from the substrate. The time between measurements was4 of set10 to 16 ms, as it showed stability and compromise with immediate measurements. The three different measurements.gases (Argon, Helistar The three 25 anddifferent Helistar gases 50) (Argon, were tested Helistar with 25 four andrepetitions Helistar 50) of were the measurement tested with four for repetitioneach of them.s of the The measurement Figure1 shows for the each experimental of them. The setup. Figure 1 shows the experimental setup.

Gases entrances

2-Color Nozzle Pyrometer

Me lt pool Generated a tmos phere S ubs tra te

Figure 1. Experimental setup.

2.2. Laser Metal Deposition Experiments 2.2. Laser Metal Deposition Experiments The same substrate preparation for these experiments were carried out. For the data record and The same substrate preparation for these experiments were carried out. For the data record and subsequent analysis, three tracks were deposited with pure argon, so that they could be used as the subsequent analysis, three tracks were deposited with pure argon, so that they could be used as the reference tracks. The process parameters were also set with reference values, in order to test the reference tracks. The process parameters were also set with reference values, in order to test the different gases with the same conditions. The parameters were selected from previous works in order different gases with the same conditions. The parameters were selected from previous works in order to avoid cracks and porosity, and they are shown in Table 4. to avoid cracks and porosity, and they are shown in Table4.

Table 4. Laser Metal Deposition (LMD) experiments process parameters. Table 4. Laser Metal Deposition (LMD) experiments process parameters. Dragging Gas Power Feed Rate Spot Diameter Powder Mass Flow Shielding Gas Flow Reference Power Feed Rate Spot Diameter Powder Mass Flow Shielding Gas Flow DraggingFlow Gas Flow Reference (W) (mm·min· −1) −1 (mm) (g·min· −1−1) · (L·min−1 −1) · −1 (W) (mm min ) (mm) (g min ) (L min ) (L min(L·min) −1) [1[]1] 400400 500 5001.6 1.6 8.08.0 12.012.0 4.54.5 [2] 600 500 1.6 8.0 12.0 4.5 [3[]2] 800600 500 5001.6 1.6 8.08.0 12.012.0 4.54.5 [3] 800 500 1.6 8.0 12.0 4.5

Once the reference tests were carried out, the same process parameters were used for testing the two gasgas mixtures, mixtures, Helistar Helistar 25 and25 and Helistar Helistar 50, with 50, 25% with and 25% 50% and of helium 50% concentration,of helium concentration respectively., Therespectively measurement. The measure of the temperaturement of the temperature was taken by was following taken by the following laser spot, the solaser that spot the, pyrometerso that the waspyrometer always was coincident always coincident with the melt with pool. the melt Once pool. the experimentalOnce the experimental tests were tests ﬁnished, were thefinished different, the differenttracks were tracks cut inwere a wet cut abrasive in a wet cut-off abrasive machine cut-off with machine a corundum with a cut-offcorundum wheel cut bonded-off wheel with bonded rubber withand thenrubber etched and then with e Kalling’stched with reagent Kalling’s 2. The reagent cross 2. sections The cross were sections analyzed were by analyzed means ofby a means confocal of amicroscope confocal microscope Leica DCM Leica 3D (LeicaDCM 3D Microsystems (Leica Microsystems GmbH, Wetzlar, GmbH, Wetzlar, Germany). Germany) For the. crossFor the section cross sectioncharacterization, characterization, four main four parameters main parameters were measured were (See measured Figure2 ),(See including Figure height, 2), includin width,g depth height, of width,dilution, depth and wetof dilution angle [,18 and,19 ].wet angle [18,19].

Materials 2018, 11, 1388 5 of 10 Materials 2018,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 5 of 10

width

Clad Bead height wet angle depth of material dilution Substrate

Figure 2 2.. Clad cross section parameters.

3. Results

3.1. Analysis of the Melt Pool Temperature andand CoolingCooling TimeTime A preliminary preliminary analysis analysis of of the the temperature temperature indicate indicatess that that measurements measurements taken taken before before the thefirstﬁrst 500 to500 800 to ms 800 are ms very are veryunstable unstable.. This is This due is to due the to different the different phenomena phenomena occurred occurred during during the rapid the rise rapid of therise temperature of the temperature that mislead that mislead the pyrometer the pyrometer sensor. sensor. For this For reason this reason,, the time the while time while the laser the laserwas wasradiating radiating the substrate the substrate was was set setto one to one seco second.nd. The The results results show show the the same same order order of of magnitude magnitude of temperature forfor thethe threethree studiedstudied gases.gases. Figure 33 showsshows thethe measurementmeasurement results.results. TheThe pyrometerpyrometer hashas aa minimumminimum temperaturetemperature rangerange ofof 823 K on display, andand aa maximummaximum ofof 27732773 K,K, andand itit waswas calibratedcalibrated beforebefore thethe experiments.experiments.

2600

2300

2000

1700

1400 Argon 99.998% - 10 l/min Temperature [K] Temperature [K] 1100 Helistar 25 - 10 l/min 800 Helistar 50 - 10 l/min 500 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 Time [s]

Figure 3 3.. Temperature [K] vs.vs. time [s].[s].

In all cases, temperature reaches a mean value of approximately 2420 K at the time of 1 s, and it In all cases, temperature reaches a mean value of approximately 2420 K at the time of 1 s, and it takes approximately 130 ms (131 ms for Argon 99.998%, 130 ms for Helistar 25, and 138 ms for takes approximately 130 ms (131 ms for Argon 99.998%, 130 ms for Helistar 25, and 138 ms for Helistar Helistar 50) from that point to reach a temperature below 823 K approximately. These results imply 50) from that point to reach a temperature below 823 K approximately. These results imply that, that, when working with low laser power (such as 250 W), there is no significant difference between when working with low laser power (such as 250 W), there is no signiﬁcant difference between the the mixtures that were studied. mixtures that were studied. 3.2. Analysis of the Laser Metal Deposition Experiments 3.2. Analysis of the Laser Metal Deposition Experiments Along these tests, the influence of different gas mixtures on the LMD process was analyzed for Along these tests, the influence of different gas mixtures on the LMD process was analyzed for three three different laser powers: 400, 600, and 800 W. The following figures illustrate the experimental different laser powers: 400, 600, and 800 W. The following figures illustrate the experimental tests and the tests and the resulting tracks for the different gas mixture. The morphology of the clads is represented and measured, followed by a table with average values of each of the three repetitions of the experimental tests.

Materials 2018, 11, 1388 6 of 10 resulting tracks for the different gas mixture. The morphology of the clads is represented and measured, Materials 2018, 11, x FOR PEER REVIEW 6 of 10 followedMaterials 2018 by, a11 table, x FOR with PEER average REVIEW values of each of the three repetitions of the experimental tests. 6 of 10 3.2.1. Experimental Tests with 400 W 3.2.1.3.2.1. ExperimentalExperimental TestsTests withwith 400400 WW Table 5 shows that, for the same energy density value, width, dilution depth and height of the TableTable5 5 shows shows that, that, for for the the same same energy energy density density value, value, width, width, dilution dilution depth depth and and height height of of the the track decrease as the concentration of helium grows. The results show very similar height, width, and tracktrack decreasedecrease as the the concentration concentration of of helium helium grows grows.. The The results results show show very very similar similar height height,, width width,, and dilution in all cases. Wet angle presents also differences between the tests, but the main variation is anddilution dilution in all in cases all cases.. Wet Wet angle angle presents presents also also differences differences between between the the tests, tests, but but the the main main variation variation is for the Helistar 50. isfor for the the Helistar Helistar 50. 50.

Table 5.5. MeasurementsMeasurements forfor differentdifferent gasesgases atat 400400 W.W. Table 5. Measurements for different gases at 400 W. Argon 99.998% Ar 75% He 25% Ar 50% He 50% ArgonArgon 99.998%99.998% Ar Ar 75% HeHe25% 25% Ar Ar 50% HeHe50% 50%

Regarding the wet angles, the optimal values should not exceed 65◦°. In this case, the parameter RegardingRegarding thethe wetwet angles,angles, thethe optimaloptimal valuesvalues shouldshould notnot exceedexceed 6565°.. InIn thisthis case,case, thethe parameterparameter combination of 400 W laser power and selected powder mass flow is not adequate to obtain optimum combinationcombination ofof 400400 WW laserlaser powerpower andand selectedselected powderpowder massmass ﬂowflow isis notnot adequateadequate toto obtainobtain optimumoptimum wet angles. wetwet angles.angles. Due to possible variations in the measurements of the clads, the experimental tests were DueDue toto possible possible variations variations in thein measurementsthe measurements of the of clads, the theclads experimental, the experimental tests were tests repeated were repeated three times. Average values of the measurements are presented in Table 6 with average threerepeated times. three Average times. valuesAverage of values the measurements of the measure arement presenteds are presented in Table6 inwith Table average 6 with absolute average absolute deviation around the mean (MAD) value in brackets. deviationabsolute deviation around the around mean the (MAD) mean value (MAD) in brackets. value in brackets. Table 6. Summary of results for experimental tests at 400 W. (absolute deviation around TableTable 6. 6Summary. Summary of of results results for experimentalfor experimental tests attests 400 at W. 400 (absolute W. (absolute deviation deviation around the around mean (MAD)the mean in brackets). (MAD) in brackets). the mean (MAD) in brackets). Height Width Depth Wet Angle Gas Mixture HeightHeight Width DepthDepth WetWet Angle Angle GasGas Mixture Mixture (mm) (mm) (mm) (°) ◦ (mm)(mm) ((mm)mm) (mm(mm)) (°) ( ) Argon 99.998% 0.69 (0.040) 1.15 (0.018) 0.16 (0.013) 89 (1.0) ArgonArgon 99.998% 99.998% 0.690.69 (0.040) (0.040) 1.15 1.15 (0.018)(0.018) 0.16 0.16 (0.013) (0.013) 89 (1.0) 89 (1.0) Ar 75% He 25% 0.63 (0.020) 1.10 (0.014) 0.13 (0.023) 88 (1.5) ArAr 75% 75% He 25%He 25% 0.630.63 (0.020) (0.020) 1.10 1.10 (0.014)(0.014) 0.13 0.13 (0.023) (0.023) 88 (1.5) 88 (1.5) ArAr 50% 50% He 50%He 50% 0.600.60 (0.029) (0.029) 1.06 1.06 (0.009)(0.009) 0.10 0.10 (0.009) (0.009) 78 (1.0) 78 (1.0) Ar 50% He 50% 0.60 (0.029) 1.06 (0.009) 0.10 (0.009) 78 (1.0)

3.2.2. Experimental Tests at 600 W 3.2.2.3.2.2. ExperimentalExperimental TestsTests atat 600600 WW Continuing with the same methodology, tests with a laser power of 600 W are presented in Table ContinuingContinuing withwith the the same same methodology, methodology, tests tests with with a a laser laser power power of of 600 600 W W are are presented presented in in Table Table7. 7. There are significant increments in width due to the higher power, but not in height, which mainly There7. There are are significant significant increments increments in in width width due due to to the the higher higher power, power, but but not not in in height, height, whichwhich mainlymainly depend on the powder mass flow. For this laser power, the width of the different clads hardly variates dependdepend onon thethe powderpowder massmass flow.flow. ForFor thisthis laser power,power, thethe widthwidth ofof the different cladsclads hardlyhardly variatesvariates among gas mixtures as well as the depth of material dilution. However, the variation of the height as amongamong gasgas mixturesmixtures as as well well as as the the depth depth of of material material dilution. dilution However,. However, the the variation variation of of the the height height as as a a result of the use of different gases is more significant, and, again, a greater variation is observed for resulta result of of the the use use of of different different gases gases is moreis more significant, significant and,, and again,, again a greater, a greater variation variation is observed is observed for thefor the highest helium concentration tests (Helistar 50). highestthe highest helium helium concentration concentration tests tests (Helistar (Helistar 50). 50).

Materials 2018,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 7 of 10 Materials 2018, 11, x FOR PEER REVIEW 7 of 10 Materials 2018, 11, 1388 7 of 10 Materials 2018,, 11,, xx FORFOR PEERPEER REVIEWREVIEWTable 7 .. Measurements for different gases at 600 W.. 7 of 10 Materials 2018, 11, x FOR PEER REVIEWTable 7 . Measurements for different gases at 600 W. 7 of 10 Argon 99.998% Ar 75% He 25% Ar 50% He 50% Argon 99.998% Table 7.7. MeasurementsMeasurementsAr 75% forfor He differentdifferent 25% gasesgases atat 600600 W.W. Ar 50% He 50% Table 7. Measurements for different gases at 600 W. Argon 99.998% Ar 75% He 25% Ar 50% He 50% ArgonArgon 99.998%99.998% Ar Ar 75% HeHe25% 25% Ar Ar 50% HeHe50% 50%

Table 7 shows also a reduction of wet angles of the clads due to the higher laser power. However, 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 significant differences of wet angle are observed for clads made with each gas mixture. The higher the heliumTable 7 concentration shows also a ,,reduction the smaller of wetthe wetangles angle of the of theclads clad due.. to the higher laser power. However, the heliumTableTable7 7 concentrationshows shows also also a a reduction ,reduction the smaller of of wet wetthe angles wetangles angle of of the the of cladstheclads clad duedue. toto thethe higherhigher laserlaser power.power. However,However, significantAgain, differences a repetition of of wet the angletests was are madeobserved in order for clads to consider made withthe pos eachsible gas variations. mixture. The resultshigher significantsignificantAgain, differencesdifferences a repetition of of of wet wet the angle angletests are was are observed madeobserved in for order for clads clads to madeconsider made with withthe each pos each gassible gas mixture. variations. mixture. The The higher resultshigher the withthe helium average concentration values are presented, the smaller in Table the wet 8.. As angle it can of bethe observed, clad. height, width,, and dilution depth heliumwiththe helium average concentration, concentration values are the presented smaller, the smaller the inwet Table the angle wet 8. As angle of it the can of clad. bethe observed, clad. height, width, and dilution depth valuesAgain, are practically a repetition constant, of the tests regardless was made of in the order gas tocomposition. consider the However, possible variations. the wet angles The results vary valuesAgain,Again, are practically aa repetitionrepetition constant, ofof thethe teststests regardless waswas mademade of inin the orderorder gas to tocomposition. considerconsider thethe However, possiblepossible variations.variations. the wet angles TheThe resultsresults vary considerablywith average forvalues the arediffe presentedrent gas mixtures in Table ..8 . As it can be observed, height, width, and dilution depth withconsiderablywith averageaverage valuesforvalues the arearediffe presentedpresentedrent gas mixtures inin TableTable8 .8 As. As it it can can be be observed, observed, height, height, width, width, and and dilution dilution depth depth values are practically constant, regardless of the gas composition. However, the wet angles vary valuesvalues areare practicallypractically constant,constant, regardlessregardless ofof thethe gasgas composition.composition. However, the wetwet anglesangles varyvary considerably for the diffeTablerent 8 .gas. Summary mixtures of results. for experimental tests at 600 W.. considerablyconsiderably forfor thethe differentdiffeTablerent 8 gas.gas Summary mixtures.mixtures of results. for experimental tests at 600 W. Height Width Depth Wet Angle Gas MixtureTable 8.8 . SummarySummaryHeight ofof results results forforWidth experimentalexperimental Depth teststests atat 600600Wet W.W. Angle Gas MixtureTable 8 . Summary(mm) of results for(mm) experimental (mm) tests at 600 W. (°) (mm) (mm) (mm) (°) Argon 99.998% 0.75Height (0.009) 1.48Width (0.015) 0.18Depth (0.004) Wet81 Angle(1.2) ArgonGas Mixture 99.998% 0.75HeightHeight (0.009) 1.48Width (0.015) 0.18DepthDepth (0.004) Wet81Wet Angle(1.2) Angle GasArGas Mixture 75% Mixture He 25% 0.70(mm) (0.016) 1.49(mm) (0.009) 0.17(mm) (0.008) 73 (°)(1.0) ◦ Ar 75% He 25% 0.70(mm)(mm) (0.016) 1.49(mm) (0.009) 0.17(mm) (mm)(0.008) 73 (°)(1.0) ( ) ArArgon 50% 99.998% He 50% 0.660.75 (0.009) 1.511.48 (0.010)(0.015) 0.150.18 (0.009)(0.004) 6081 (0.8)(1.2) ArgonArArgon 50% 99.998% 99.998% He 50% 0.750.660.75 (0.009) (0.009) 1.511.48 1.48 (0.010)(0.015) 0.150.18 0.18 (0.009)(0.004) (0.004) 6081 (0.8)(1.2) 81 (1.2) Ar 75% He 25% 0.70 (0.016) 1.49 (0.009) 0.17 (0.008) 73 (1.0) ArAr 75% 75% He 25%He 25% 0.700.70 (0.016) (0.016) 1.49 1.49 (0.009) 0.17 0.17 (0.008) (0.008) 73 (1.0) 73 (1.0) 3.2.3. ExperimentalArAr 50% 50% HeTests 50%He at 50% 800 W 0.660.66 (0.009) (0.009) 1.51 1.51 (0.010) 0.15 0.15 (0.009) (0.009) 60 (0.8) 60 (0.8) 3.2.3. ExperimentalAr 50% Tests He at 50% 800 W0.66 (0.009) 1.51 (0.010) 0.15 (0.009) 60 (0.8) Tests results for 800 W laser power are shown in Table 9.. Because of power increment,, thethe width 3.2.3.Tests Experimental results for Tests 800 Wat laser800 W power are shown in Table 9. Because of power increment, the width 3.2.3.and3.2.3. dilution ExperimentalExperimental are higher TestsTests than atat 800800 those WW obtained in the 400 W and 600 W tests.. and dilution are higher than those obtained in the 400 W and 600 W tests. SimilarlyTests results to ftheor 800previous W laser experiments, power are shown the highest in Table variation 9. Because between of power the increment clads is , inthe height, width TestsSimilarlyTests resultsresults to for ftheor 800 800previous WW laserlaser experiments, powerpower areare shownshown the highest inin TableTable variation9 9.. Because Because between of of power power the increment, increment clads is , the inthe height, width width whereasand dilution the dilutionare higher depth than and those width obtained remain in almostthe 400 invariable. W and 600 W tests. andwhereasand dilutiondilution the aredilutionare higherhigher depth thanthan and thosethose width obtainedobtained remain inin thealmostthe 400400 invariable. WW andand 600600 W W tests.tests. Similarly to the previous experiments, the highest variation between the clads is in height, Similarly to the previous experiments, the highest variation between the clads is in height, whereas the dilution depthTable and width9.9.. MeasurementsMeasurements remain almost forfor differentdifferent invariable. gasesgases at at 800800 W. W.. whereas the dilution depthTable and width9. Measurements remain almost for different invariable. gases at 800 W. Argon 99.998% Ar 75% He 25% Ar 50% He 50% ArgonArgon 99.998%99.998% Table 9. MeasurementsAr Ar 75% for HeHe different25% 25% gases at 800 W. Ar Ar 50% HeHe50% 50% Table 9. Measurements for different gases at 800 W. Argon 99.998% Ar 75% He 25% Ar 50% He 50% Argon 99.998% Ar 75% He 25% Ar 50% He 50%

Materials 2018, 11, 1388 8 of 10

Materials 2018, 11, x FOR PEER REVIEW 8 of 10

MaterialsSimilarly 20182018,, 1111 to,, xx FOR theFOR PEERPEER previous REVIEWREVIEW experiments, the highest variation between the clads is in88 height,ofof 1010 whereasBecause the dilution of a higher depth laser and power width,remain the wet almost angle is invariable. smaller than previous tests with lower laser MaterialsMaterials 2018 2018,, ,11 11,, ,xx x FORFOR FOR PEER PEERPEER REVIEWREVIEW REVIEW 88 ofof 1010 power.BecauseBecause In addition, of of a a higher higher the laserwet laser angle power, power changes,, the thethe wetwet for angle angledifferent is is smallersmaller gas compositions, thanthan than previousprevious previous as teststestsit tests can withwith be with observed lowerlower lower laserlaser laserin power.Table 9 .In The addition, wet angle the reduction wet angle is changes the most for significant different variationgas compositions, when using as differentit can be gasesobserved, while in power.power.BecauseBecause In In addition, addition, ofof aa higherhigher the the wet wet laserlaser angle angle powerpower changeschanges,, , thethe wetwet forfor angle angle different isis smallersmaller gas gas compositions, compositions, thanthan previousprevious asas testsittests itcancan withwith be be observed lowerlower observed laserlaser in in Tablethe rest 9. ofThe measurements wet angle reduction present issimilar the most values. significant variation when using different gases, while Tablepower.Tablepower.9. The 9 .. In InThe wet addition,addition, wet angle angle reductionthethe reduction wetwet angleangle is theisis changes changesthethe most mostmost signiﬁcant forfor significant differentdifferent variation variation gasgas compositions,compositions, when when using using asas different differentitit cancan bebe gases, gases observedobserved while,, whilewhile inin the the restThe of experimental measurements tests present once similaragain are values. repeated to obtain more information and to analyze the restTablethetheTable of restrest measurements 99.. . ofof TheThe measurementsmeasurements wetwet angleangle present reductionreduction presentpresent similar issimilarsimilaris thethe values. mostmost values.values. significantsignificant variationvariation whenwhen usingusing differentdifferent gasesgases,, , whilewhilewhile possibleThe variationexperimental on the tests geometry once again of the are clad repeated. Table to 10 obtain shows more these information results as meanand to values analyze of thethe thetheThe rest restTheexperimental of of experimental measurements measurements tests tests present present once onceagain similar againsimilar areare values. values. repeatedrepeated to obtain obtain more more information information and and toto analyze analyze thethe the possiblemeasurements. variation As onit was the statedgeometry before of , thethe cladmore. Tablesignificant 10 shows effect these of the results different as meangases usevalues is the of wetthe possiblepossibleTheThe variation variation experimentalexperimental on on the thethe teststests geometry geometrygeometry onceonce againagain of ooff thethethe areare clad.clad repeatedrepeated.. TableTable toto 1010 obtainobtain showsshows moremore these these informationinformation results results as as mean andand mean to tovalues analyze valuesanalyze of of thethethe the measurements.angle variation, As and it thiswas variationstated before is more, the considerablemore significant for highereffect of helium the different concentration gases uses. is the wet measurements.possiblemeasurements.possible variationvariation As As it oniton was was thethe stated stated geometrygeometry before, before ooff ,, thethethe cladmoremoreclad.. . Table Tablesignificant signiﬁcantsignificantTable 1010 showsshows effecteffect effect these theseofof of thethe the resultsresults differentdifferent different asas meangasesgasesmean gases use usevaluesvalues use isis the isthe ofof the wetwet thethe wet angle variation,, and this variation is more considerable for higher helium concentrations.s. anglemeasurements.measurements. variation, and As As it thisit was wasTable variation stated stated 10. Summary before before is more,, , the the of considerable more resultsmore significant significantfor experimental for highereffect effect tests of of helium the the at different 800different concentrations.W. gases gases use use is is the the wet wet angleangle variation variation,, , and and this this variation variation is is more more considerable considerable for for higher higher helium helium concentration concentrations.s. Table 1010.. SummaryHeight of results Widthfor experimental Depth tests atat 800800Wet W.. Angle Gas MixtureTable 10. Summary of results for experimental tests at 800 W. TableTable 10 10.. . Summary SummaryHeight(mm) of of results results Widthfor (mm)for experimental experimental Depth(mm) tests tests atat 800 800Wet W W.. . (°)Angle Gas Mixture Argon 99.998% 0.79Height(mm)(mm) (0.014) 1.87(mm)Width(mm) (0.013) 0.25(mm)(mm) (0.010)Depth 73 (°)(°)(1.0)Wet Angle Gas Mixture HeightHeight WidthWidth DepthDepth WetWet Angle Angle◦ ArGasGas 75% Mixture Mixture He 25% 0.73(mm) (0.009) 1.82(mm) (0.015) 0.26 (0.008)(mm) 67 (1.2) ( ) Argon 99.998% 0.79(mm)(mm) (0.014)(0.014) 1.87(mm)(mm) (0.013)(0.013) 0.25(mm)(mm) (0.010)(0.010) 73 (°) (1.0)(°)(1.0) ArgonAr99.998% 75%50% He 25%50% 0.790.730.65 (0.014) (0.009)(0.020) 1.821.84 1.87 (0.015)(0.010) (0.013) 0.260.23 0.25 (0.008)(0.006) (0.010) 6753 (1.2)(0.8) 73 (1.0) ArArgonArgon 75% 99.998% 99.998%He 25% 0.790.730.79 (0.014) (0.009)(0.009)(0.014) 1.871.821.87 (0.013) (0.015)(0.015)(0.013) 0.250.260.25 (0.010) (0.008)(0.008)(0.010) 736773 (1.0) (1.2)(1.2)(1.0) Ar 75%Ar 50% He 25% He 50% 0.730.65 (0.009) (0.020)(0.020) 1.84 1.82 (0.010)(0.010) (0.015) 0.23 0.26 (0.006)(0.006) (0.008) 53 (0.8)(0.8) 67 (1.2) 3.2.4. TemperatureAr 50%ArAr 75% He 75%Analysis 50% He He 25% 25% 0.650.730.73 (0.020) (0.009) (0.009) 1.821.82 1.84 (0.015) (0.010)(0.015) 0.260.26 0.23 (0.008) (0.008) (0.006) 6767 (1.2) (1.2) 53 (0.8) ArAr 50% 50% He He 50% 50% 0.650.65 (0.020) (0.020) 1.841.84 (0.010) (0.010) 0.230.23 (0.006) (0.006) 5353 (0.8) (0.8) 3.2.4.In Temperature addition to Analysis the geometry analysis, the temperature of the process was registered via 3.2.4. Temperature Analysis 3.2.4.pyrometry3.2.4.InIn Temperature Temperature additionaddition and Table toto Analysis Analysis the the11 showsgeometrygeometry the outputanalysis,analysis,s of thethesethe temperaturetemperature measurements ofof . thethe processprocess waswas registeredregistered viavia pyrometryInIn addition all cases,and to Table thethe geometryutilization11 shows analysis,t ofhe heliumoutput thes in of temperature the these process measurements results of the processin. higher was temperature. registered viaWhen pyrometry pure pyrometryInIn additionaddition and Table toto the the11 shows geometrygeometry the output analysis,analysis,s of thesethethe temperaturetemperature measurements ofof .. thethe processprocess waswas registeredregistered viavia argonIn was all cases,used, meanthe utilization temperatures of helium of 1938 in K,the 1991 process K, and results 2121 inK werehigher reached temperature. for laser When powers pure of andpyrometrypyrometry TableInIn allall11 cases,cases,showsand and Table Table thethe the utilization utilization11 outputs11 shows shows of t tofheofhe these heliumheliumoutput output measurements.s sinin of of the thethese these processprocess measurements measurements resultsresults inin.. . higherhigher temperature.temperature. WhenWhen purepure a400rgon W, was 600 used, W, and mean 800 temperatures W, respectively. of 1938 Meanwhile, K, 1991 K ,when and 2121 Helistar K were 25 reached(Ar 75% for and laser He powers25%) and of argonInInIn all was allall cases, cases,used,cases, the mean thethe utilization utilizationutilization temperatures of ofof helium heliumhelium of 1938 in inin K, the thethe 1991 processprocessprocess K,, and results resultsresults 2121 inK in were higherhigher reached temperature.temperature. temperature. for laser WhenWhen powers When purepure pureof 400Helistar W, 600 50 (ArW, and50% 800and W, He respectively. 50%) was employed, Meanwhile, temperatures when Helistar with 25average (Ar 75% values and ofHe 209 25%)3 K and argona400argonrgon W, was waswas 600 used, used, used,W,, meanand meanmean 800temperatures temperaturestemperatures W, respectively. of ofof 1938 1938 Meanwhile, K,K, K, 19911991 1991 KK , K,,when , andand and 2121 2121Helistar 2121 KK were Kwere 25 were reached(Arreached reached 75% forforand laser forlaser He laser powers25%)powers powers and ofof Helistar2097 K where 50 (Ar measured 50% and , Herespectively 50%) was, atemployed, 400 W. However, temperatures increasing with average the amount values of ofhelium 2093 Kin and the of400Helistar 400400 W, W,W, 600 60050 (Ar W W,W,, , and50%and 800 and800 W, W,He respectively. respectively.50%) was employed, Meanwhile,Meanwhile, temperatures whenwhen when HelistarHelistar Helistar with 25average25 25 (Ar(Ar (Ar 75%75% values 75% andand and of HeHe 209 He 25%)25%)3 25%) K andand and 2mixture097 K where did not measured result in, arespectively higher temperature., at 400 W . However, increasing the amount of helium in the HelistarHelistar2Helistar097 K 50 where 50 (Ar50 (Ar(Ar 50% measured 50%50% and andand He ,,He Herespectively 50%) 50%)50%) was waswas employed,,, atemployed,employed, 400 W temperatures.. However,However, temperaturestemperatures increasingincreasing with withwith average average average thethe amountamount values valuesvalues ofof of 2093heliumofhelium 209209 K33 andKininK and andthethe 2097 mixture did not result in a higher temperature.temperature. K where22097097 KK measured, wherewhereTable measuredmeasured respectively, 11. Summary,, , respectivelyrespectively of at results 400,, , W. atat for 400 However,400 experimental WW.. . However,However,However, increasing tests increasingincreasing increasingat 400 the W, amount 600 thethethe W amountamount,amount and of helium800 ofofWof . heliumheliumhelium in the inin mixturein thethethe didmixturemixture not result did did innot not a result higherresult in in temperature. a a higher higher temperature. temperature. ArgonTable 99.998% 1111.. Summary of results for experimentalHelistar 25 tests atat 400400 W, 600 W,, andandHelistar 800800 W.. 50 ArgonTableTableTable 99.998% 11. 11 11Summary.. . Summary Summary ofof of resultsresults results for forfor experimentalHelistarexperimental 25 tests tests at at 400 400 400 W, W, W, 600 600 600 W W W,,, , and and andHelistar 800 800 800 W W W... . 50 [K] Argon400 99.998% W 2600 [K] Helistar400 W 25 2600 [K] Helistar400 W 50 2200 ArgonArgon 99.998% 99.998% 2200 HelistarHelistar 25 25 2200 HelistarHelistar 50 50 [K][K] Argon400400 99.998% WW 26002600 [K][K] Helistar400400 WW 25 26002600 [K][K] Helistar400400 WW 50 1800 1800 1800 22002200 22002200 22002200 1400[K][K] 400400 W W 26002600[K][K] 400400 W W 260014002600[K][K] 400400 W W 18001800 1400 1400 22002200 18001800 18001800 1000 220010002200 220010002200 14001400 14001400 14001400 18001800 0,0 1,0 2,0 3,0 4,0 5,0 18001800 0,0 1,0 2,0 3,0 4,0 5,0 18001800 0,0 1,0 2,0 3,0 4,0 5,0 10001000 10001000 10001000 14001400 [s] 14001400 [s] 14001400 [s] 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 10001000 10001000 10001000 [s][s] [s][s] [s][s] 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 [K] 600 W [K] 600 W [K] 600 W [s][s] 2600 [s][s] 2600 [s][s] 2200 2200 2200 [K][K] 660000 WW 26002600 [K][K] 660000 WW 26002600 [K][K] 660000 WW 1800 1800 1800 22002200 22002200 22002200 1400[K][K] 660000 W W 26002600[K][K] 660000 W W 26002600[K][K] 660000 W W 18001800 1400 1400 22002200 18001800 18001800 1000 220010002200 220010002200 14001400 14001400 14001400 18001800 0,0 1,0 2,0 3,0 4,0 5,0 18001800 0,0 1,0 2,0 3,0 4,0 5,0 18001800 0,0 1,0 2,0 3,0 4,0 5,0 10001000 10001000 10001000 14001400 [s] 14001400 [s] 14001400 [s] 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 10001000 10001000 10001000 [s][s] [s][s] [s][s] 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 [K] 800 W [K] 800 W [K] 800 W [s][s] 2600 [s][s] 2600 [s][s] 2200 2200 2200 [K][K] 880000 WW 26002600 [K][K] 880000 WW 26002600 [K][K] 880000 WW 1800 1800 1800 22002200 22002200 22002200 1400[K][K] 880000 W W 26002600[K][K] 880000 W W 26002600[K][K] 880000 W W 18001800 1400 1400 22002200 18001800 18001800 1000 220010002200 220010002200 14001400 14001400 14001400 18001800 0,0 1,0 2,0 3,0 4,0 5,0 18001800 0,0 1,0 2,0 3,0 4,0 5,0 18001800 0,0 1,0 2,0 3,0 4,0 5,0 10001000 10001000 10001000 14001400 [s] 14001400 [s] 14001400 [s] 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 10001000 10001000 10001000 [s][s] [s][s] [s][s] 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 0,00,0 1,01,0 2,02,0 3,03,0 4,04,0 5,05,0 [s][s] [s][s] [s][s]

Materials 2018, 11, 1388 9 of 10

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−1) near 40 times higher than argon’s thermal conductivity (0.018 W·m−1·K−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:

1. Helium and argon process gases have different effects on LMD. Its inﬂuence is not negligible and must be taken into account. 2. The increment of helium concentration in the gases of the LMD processes based on argon will have three effects. The first one is a slight reduction of the height of the clads. Second, an increase of the temperature of the melt pool. Last, smaller wet angles are obtained for higher helium concentrations. 3. However, some variations can be neglected due to its small values, like width and dilution depth, since helium concentration seems to have no special inﬂuence on these parameters.

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. Materials 2018, 11, 1388 10 of 10

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.

References

1. Gibson, I.; Rosen, D.; Stucker, B. Directed Energy Deposition Processes. In Additive Manufacturing Technologies, 2nd ed.; Springer: New York, NY, USA, 2015; pp. 245–268. ISBN 978-1-4939-2112-6. 2. Cortina, M.; Arrizubieta, J.I.; Ruiz, J.E.; Lamikiz, A.; Ukar, E. Design and manufacturing of a protective nozzle for highly reactive materials processing via Laser Material Deposition. Procedia CIRP 2018, 68, 387–392. [CrossRef] 3. Li, P.; Yang, T.; Li, S.; Liu, D.; Hu, Q.; Xiong, W.; Zeng, X. Direct laser fabrication of nickel alloy samples. Int. J. Mach. Tools Manuf. 2015, 45, 1288–1294. [CrossRef] 4. Kumar, L.J.; Nair, C.K. Laser metal deposition repair applications for Inconel 718 alloy. Mater. Today Proc. 2017, 4, 11068–11077. [CrossRef] 5. Gasser, A. Laser Metal Deposition. In Tailored Light 2, 1st ed.; Poprawe, P., Ed.; Springer: Berlin/Heidelberg, Germany, 2011; pp. 15–40. ISBN 978-3-642-01236-5. 6. Arrizubieta, J.I.; Tabernero, I.; Ruiz, J.E.; Lamikiz, A.; Martínez, S.; Ukar, E. Continuous coaxial nozzle design for LMD based on numerical simulation. Phys. Procedia 2014, 56, 429–438. [CrossRef] 7. Elmer, J.W.; Vaja, J.; Carlton, H.D.; Pong, R. The Effect of Ar and N2 Shielding Gas on Laser Weld Porosity in Steel, Stainless Steels, and Nickel. Weld. J. 2015, 94, 313s–325s. 8. Petrat, T.; Graf, B.; Gumenyuk, A.; Rethmeier, M. Laser Metal Deposition as repair technology for a gas turbine burner made of Inconel 718. Phys. Procedia 2016, 83, 761–768. [CrossRef] 9. Raymond, C.B.; Randy, P.S. Additively Manufactured Inconel Alloy 718. In Proceedings of the 7th Symposium on Superalloy 718 and Derivatives, Pittsburgh, PA, USA, 10–13 October 2010. [CrossRef] 10. Patschgera, A.; Sahibb, C.; Bergmannc, J.P.; Basticka, A. Process Optimization through Adaptation of Shielding Gas Selection and Feeding during Laser Beam Welding. Phys. Procedia 2011, 12, 46–55. [CrossRef] 11. Wester, R. Absorption of Laser Radiation. In Tailored Light 2, 1st ed.; Poprawe, P., Ed.; Springer: Berlin/Heidelberg, Germany, 2011; pp. 216–224. ISBN 978-3-642-01236-5. 12. Haynes International. Haynes(R) 718 Alloy Plate Test Certificate; Haynes International: Kokomo, IN, USA, 2018. 13. Oerlikon Metco. MetcoClad 718 Material Product Data Sheet; Oerlikon Metco: Pfäfﬁkon, Switzerland, 2018. 14. Praxair, Inc. Praxair Argon Product Data Sheet; Praxair, Inc.: Danbury, CT, USA, 2018. 15. Praxair, Inc. Praxair Helistar 25 Product Data Sheet; Praxair, Inc.: Danbury, CT, USA, 2018. 16. Praxair, Inc. Praxair Helistar 50 Product Data Sheet; Praxair, Inc.: Danbury, CT, USA, 2018. 17. LumaSense Technologies, Inc. IMPAC Pyrometer IGAR 12-LO MB 25 Product Data Sheet; LumaSense Technologies, Inc.: Santa Clara, CA, USA, 2018. 18. Toyserkani, E.; Khajepour, A.; Corbin, S. Laser Cladding, 1st ed.; CRC Press LLC: Boca Raton, FL, USA, 2005; pp. 11–50. ISBN 0-8493-2172-7. 19. Bennett, J.; Wolff, S.; Hyatt, G.; Ehmann, K.; Cao, J. Thermal effect on clad dimension for laser deposited Inconel 718. J. Manuf. Process. 2017, 28, 550–557. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).