Influence of the CO2 Content in Shielding Gas on the Temperature

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Journal of Manufacturing and Materials Processing

Article Inﬂuence of the CO2 Content in Shielding Gas on the Temperature of the Shielding Gas Nozzle during GMAW Welding

Martin Lohse 1,* , Marcus Trautmann 1, Uwe Füssel 1 and Sascha Rose 2

1 Institute of Production Engineering, Technische Universität Dresden, 01062 Dresden, Germany; [email protected] (M.T.); [email protected] (U.F.) 2 Alexander Binzel Schweisstechnik GmbH & Co. KG, 35418 Buseck, Germany; [email protected] * Correspondence: [email protected]

Received: 11 November 2020; Accepted: 2 December 2020; Published: 4 December 2020

Abstract: Gas metal arc welding torches are commonly chosen based on their current-carrying capacity. It is known that the current-carrying capacity of welding torches under CO2 is usually higher than under argon dominated shielding gases. In this publication, the extent to which this can be attributed to the shielding gas dependent arc radiation is investigated. For this purpose, the influence of the shielding gas on the thermal load of the shielding gas nozzle of a GMAW torch was calorimetrically measured. These experiments were carried out for four different shielding gases (argon, CO2, and two argon/CO2 mixtures). The measurements were all performed at an average current of 300 A. The welding current was set by adjusting the wire feed rate or the voltage correction. For each case, a separate set of experiments was done. It is shown that the changed arc radiation resulting from the different shielding gases has an influence on the heat input into the gas nozzle, and thus into the torch. For the same shielding gas, this influence largely correlates with the welding voltage.

Keywords: GMAW; radiation; heat input; argon; CO2

1. State-of-the-Art In gas metal arc welding (GMAW), the filler material is fed via a melting wire electrode. The molten material is protected from the surrounding atmosphere by a shielding gas. In Europe, argon is mainly used as a shielding gas, with other gases such as carbon dioxide, oxygen, hydrogen, nitrogen, or helium being added, depending on the application [1,2]. The choice of the shielding gas is primarily determined by the materials to be welded [3]. Shielding gases with oxidative components, such as CO2 and O2, are mainly used for unalloyed and low-alloyed steels. The loss of alloy elements due to oxidation must be compensated by over-alloying the filler material [4]. The use of inert shielding gases such as argon and/or helium can prevent alloying elements from oxidizing. Pure CO2 as a shielding gas is highly resistant to pore formation, but is rarely used in Europe because of the risk of carburization of the parts to be joined and the high level of spatter formation [1,3–5]. A summary of different shielding gases and shielding gas mixtures with application areas and recommendations can be found in [2]. Besides an influence on the formation of the molten bath, the shielding gas also has an influence on the arc. The drop transfer and the energy input into the electrode as well as the work piece can vary. Detailed investigations were carried out in the AiF project 17431 N [6]. For shielding gases containing argon with admixtures of max. 10% CO2, the droplet transfer is uniform and mainly symmetrical to the wire axis. For CO2 contents above 10%, the drop separation is increasingly asymmetrical and leads to

J. Manuf. Mater. Process. 2020, 4, 113; doi:10.3390/jmmp4040113 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2020, 4, 113 2 of 13

the formation of larger drops. Thus, the drops hit the melt pool in a larger area. For low CO2 admixtures (<18%), the fusion penetration profile is finger-shaped and becomes lenticular with increasing CO2 content, whereby the fusion penetration depth and seam height decrease. With increasing CO2 content, the arc length continues to decrease. The influence of the shielding gas on the torch components has not been scientifically investigated so far. The design of welding torches is based on the current carrying capacity as a function of the duty cycle, which is determined, e.g., according to DIN EN 60974-7. This specifies argon for MIG welding of aluminium and argon-CO2 mixed gases with 15 to 25% CO2 content for MAG welding of mild steel. The resulting permissible currents for CO2 at a constant duty cycle are specified by the manufacturers as approximately 30–50 A (10–20%) higher for CO2 than for argon [7,8]. These specifications apply to both gas-cooled and liquid-cooled torches. This circumstance is usually explained with the increased cooling effect of CO2, whereas no proof is provided. There are also no studies on this topic in the literature. Only in the work of Haelsig [9], it is determined by calorimetric measurements on a GMAW process that approximately 7–9% of the electrically supplied power of the welding process is dissipated by cooling the torch. Depending on the process, this can be up to a Kilowatt and explains the need for water-cooling in welding torches under high loads. However, no publication could be found on how large the contribution of heat radiation is, especially for different shielding gases. Nevertheless, there are publications on the temperature-dependent material data of different shielding gases, which were calculated assuming the local thermodynamic equilibrium. Data can be found, for example, in [10] for argon and in [11] for CO2. Selected values are shown in Figure1. In the area of the shielding gas nozzle, temperatures of approximately 300–900 K occur. In this temperature range, both the specific heat capacity and the thermal conductivity of CO2 are in part significantly higher than those of argon. This leads to a higher cooling effect of CO2 compared with argon in this temperature range. However, no quantification of this effect can be derived from the material data alone, as the formation of the gas flow also has an influence on the heat transfer. The density and viscosity of the gases also play a role. In the temperature range of 300–900 K, the viscosity of argon is about double as high as for CO2, while the density of CO2 is about 10% higher than that of argon.

Figure 1. Speciﬁc heat and thermal conductivity of argon and carbon dioxide in comparison [10,11].

The arc temperatures and thus the radiated heat also diﬀer between the shielding gases. Corresponding temperatures were published in [12,13] and are shown in Figure2. In [ 12], temperatures of approximately 9000–10,000 K in the metal-vapor dominated arc core were determined for an arc in an argon shielding gas; see Figure2. Outside this range, the temperatures in the plasma were approximately 12,000 K. In [13], shielding gas mixtures of CO2 with high argon content and pure CO2 were investigated. It is shown that the minimum temperature in the arc core is more pronounced for argon-rich shielding gas mixtures than under pure CO2; see Figure2. The determined arc temperatures for an argon mixture with 18% CO2 were about 10,000 K in the arc core and between 11,000 K and almost 14,000 K in the plasma. The temperatures for an arc under pure CO2 shielding gas, however, are comparatively lower at approximately 8000—9000 K.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 3 of 13 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 3 of 13 The arc temperatures and thus the radiated heat also differ between the shielding gases. CorrespondingThe arc temperatures temperatures and were thus published the radiated in [12,13]heat also and differ are shownbetween in the Figure shielding 2. In gases. [12], temperaturesCorresponding of temperaturesapproximately were 9000–10,000 published K in in [12,13] the metal-vapor and are shown dominated in Figure arc core2. In were[12], determinedtemperatures for of an approximatelyarc in an argon shielding9000–10,000 gas; Ksee in Figure the 2.metal-vapor Outside this dominated range, the temperaturesarc core were in determined for an arc in an argon shielding gas; see Figure 2. Outside this range, the temperatures in the plasma were approximately 12,000 K. In [13], shielding gas mixtures of CO2 with high argon the plasma were approximately 12,000 K. In [13], shielding gas mixtures of CO2 with high argon content and pure CO2 were investigated. It is shown that the minimum temperature in the arc core is content and pure CO2 were investigated. It is shown that the minimum temperature in the arc core is more pronounced for argon-rich shielding gas mixtures than under pure CO2; see Figure 2. The more pronounced for argon-rich shielding gas mixtures than under pure CO2; see Figure 2. The determined arc temperatures for an argon mixture with 18% CO2 were about 10,000 K in the arc core J.anddetermined Manuf. between Mater. arc Process.11,000 temperatures2020 K ,and4, 113 almost for an 14,000 argon K mixture in the plasma.with 18% The CO temperatures2 were about 10,000for an K arc in underthe arc 3pure ofcore 13 and between 11,000 K and almost 14,000 K in the plasma. The temperatures for an arc under pure CO2 shielding gas, however, are comparatively lower at approximately 8000—9000 K. CO2 shielding gas, however, are comparatively lower at approximately 8000—9000 K.

Figure 2. Measured arc temperatures for a gas metal arc welding (GMAW) pulse process. Values Figure 2. Measured arc temperatures for a gas metal arc welding (GMAW) pulse process. Values under underFigure argon 2. Measured determined arc temperaturesin [12] for an forarc a1040 gas msmetal after arc the welding start of (GMAW) the high currentpulse process. phase andValues 1.7 argonunder determined argon determined in [12] for in an[12] arc for 1040 an msarc after1040 thems startafter of the the start high of current the high phase current and 1.7phase mm and above 1.7 mm above the workpiece. Values for 82% argon/18% CO2 and CO2 determined in [13] for an arc 980 the workpiece. Values for 82% argon/18% CO2 and CO2 determined in [13] for an arc 980 ms after the msmm after above the the start workpiece. of the high Values current for phase 82% argon/18%approximately CO2 1.5and mm CO above2 determined the workpiece. in [13] for The an welding arc 980 startms after of the the high start current of the phasehigh current approximately phase appr 1.5oximately mm above 1.5 the mm workpiece. above the The workpiece. welding current The welding at the current at the time of measurement was about 420 A for argon, about 250 A for 82% argon/18% CO2, time of measurement was about 420 A for argon, about 250 A for 82% argon/18% CO , and about 330 A current at the time of measurement was about 420 A for argon, about 250 A for 82%2 argon/18% CO2, and about 330 A for CO2. for CO . and about2 330 A for CO2. The radiation behaviour can be described by the net emission coefficien coeﬃcientt (NEC). In [14,15], [14,15], The radiation behaviour can be described by the net emission coefficient (NEC). In [14,15], corresponding values for argon and CO2 in the temperature range from 300 K to 30,000 K are corresponding values for argon and CO2 in the temperature range from 300 K to 30,000 K are published corresponding values for argon and CO2 in the temperature range from 300 K to 30,000 K are andpublished they are and shown they are in Figure shown3. in Figure 3. published and they are shown in Figure 3.

Figure 3. Net emission coefficient of argon and carbon dioxide compared. Data is taken from [14,15]. FigureFigure 3.3. NetNet emissionemission coecoefficientﬃcient ofof argonargon andand carboncarbon dioxidedioxide compared.compared. DataData isis takentaken fromfrom [[14,15].14,15].

From the published temperatures, it follows that the NEC for an arc under argon is between 7 3 9 3 6 3 1.0 10 W/m and 3.5 10 W/m . For a CO2 arc, the NEC decreases to values of 9.0 10 W/m and × 7 3 × × 6.7 10 W/m . When CO2 is used as a shielding gas, this results in lower heat radiation, and thus a × lower thermal load on the torch components due to the arc radiation. However, higher arc temperatures were measured for a shielding gas mixture of 82% argon and 18% CO2. For these values, the NEC is between 1.8 108 W/m3 and 5.3 109 W/m3, so that a higher radiation load of the torch components × × could be concluded. Based on the material data alone, the heat input into the torch components cannot be quantiﬁed, as other parameters, such as the radiant volume, i.e., the arc size, must be known. J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 4 of 13

From the published temperatures, it follows that the NEC for an arc under argon is between 1.0 × 107 W/m3 and 3.5 × 109 W/m3. For a CO2 arc, the NEC decreases to values of 9.0 × 106 W/m3 and 6.7 × 107 W/m3. When CO2 is used as a shielding gas, this results in lower heat radiation, and thus a lower thermal load on the torch components due to the arc radiation. However, higher arc temperatures were measured for a shielding gas mixture of 82% argon and 18% CO2. For these values, the NEC is between 1.8 × 108 W/m3 and 5.3 × 109 W/m3, so that a higher radiation load of the torch components could be concluded. Based on the material data alone, the heat input into the torch components cannotJ. Manuf. be Mater. quantified, Process. 2020 as, 4 ,other 113 parameters, such as the radiant volume, i.e., the arc size, must4 of be 13 known.

2.2. Aim Aim of of the the Study Study AccordingAccording to to the the state-of-the-art, state-of-the-art, GMAW GMAW torches, torches, which which are are designed designed according according to to DIN DIN EN EN 60974-7, can generally be loaded with a higher current under a CO shielding gas than under argon. 60974-7, can generally be loaded with a higher current under a CO22 shielding gas than under argon. TheThe published published material material data data of of the the shielding shielding gases gases and and the the temperatures temperatures in in the the arc arc already already give give a a possiblepossible explanation explanation for for this this “cooling “cooling effect”. effect”. However, However, no no quantification quantification of of individual individual effects effects of of the the heatheat transfer transfer from from the the arc arc and shielding gas to the torch components has yet been made. TheThe heat heat input input into into a a GMAW GMAW welding torch torch is is determined by by convection, convection, solid solid state state radiation, radiation, andand arc arc radiation. radiation. In In this this work, work, the the heat, heat, which which is introduced is introduced into into the theshielding shielding gas gasnozzle nozzle by the by arc the radiation,arc radiation, is determined is determined calorimetrically. calorimetrically. It is It assumed is assumed that that the the radiation radiation influence influence on on other other torch torch componentscomponents exhibitsexhibits analogousanalogous behaviour. behaviour. For For the th calorimetrice calorimetric measurement, measurement, the shielding the shielding gas nozzle gas nozzleis cooled is cooled down todown almost to almost room temperature. room temperature. Thus, aThus, convective a convective heat transfer heat transfer can be omitted.can be omitted. 3. Experimental Setup 3. Experimental Setup To determine the radiation input by the arc in an isolated manner, a heavily cooled shielding gas To determine the radiation input by the arc in an isolated manner, a heavily cooled shielding nozzle is used, which absorbs the majority of the arc radiation in the direction of the torch. Figure4 gas nozzle is used, which absorbs the majority of the arc radiation in the direction of the torch. Figure shows on the left side the principle of the heat sink, which serves as a shielding gas nozzle. The heat 4 shows on the left side the principle of the heat sink, which serves as a shielding gas nozzle. The heat sink consists of a round copper piece with an internal cooling channel for water-cooling. On the bottom sink consists of a round copper piece with an internal cooling channel for water-cooling. On the side, a shortened gas nozzle is screwed in. The heat dissipated by the cooling water is determined bottom side, a shortened gas nozzle is screwed in. The heat dissipated by the cooling water is calorimetrically. Because the heat sink is cooled down to almost room temperature and is insulated determined calorimetrically. Because the heat sink is cooled down to almost room temperature and against the torch, the cooling capacity corresponds to the heat input by radiation. is insulated against the torch, the cooling capacity corresponds to the heat input by radiation.

FigureFigure 4. 4. DesignDesign of of the the gas gas nozzle nozzle cooling cooling an andd current-carrying current-carrying torch torch parts. parts. ( (LeftLeft)) principle principle sketch; sketch; ((RightRight)) design design of of the the torch. torch.

In Figure4, on the right side, the heat sink is mounted on a liquid cooled 500 A GMAW torch from ABICOR Binzel, which in turn is mounted on a three-axis gantry. For the experiments, a power source from EWM “Phoenix 521 Expert Puls forceArc” was used.

The process was recorded with the help of a single-lens reflex (SLR) camera and a high-speed camera in order to assess the visible part of the arc in a qualitative manner. Furthermore, current and voltage were recorded with a sampling rate of 10 kHz. A filter with 810 nm was used for the recordings with the SLR camera. For the recordings with the high-speed camera, a combination of an 810 nm and an 808 nm filter was used. For all experiments, a constant welding current of 300 A and a constant contact tip distance of 17 mm were set. These parameters were chosen following the AiF project [6], in order to be able to run J. Manuf. Mater. Process. 2020, 4, 113 5 of 13 the same welding current for all shielding gases investigated. The welding current was adjusted via the wire feed rate or voltage correction. This results in two measurement series:

Measurement series 1: adjusted wire feed (constant voltage correction); • Measurement series 2: adjusted voltage correction (constant wire feed). • 4. Heat Input with Adjusted Wire Feed Rate Table1 below lists the process parameters for the experiments. In order to set a current of 300 A for the various shielding gases, a characteristic curve for the shielding gas was set at the power source by selecting the appropriate job. The welding current was determined by direct measurement using a Dewetron measuring device and averaged over a time of approximately 5 s. The wire feed rate was then adjusted until an average current of approximately 300 A could be measured.

Table 1. Process parameters of the experiments.

Welding Position According to DIN EN ISO 6947 Flat Position type of welding Overlap joint mainly free of short circuits droplet transfer (determined short circuit time in the results) wire diameter [mm] 1.2 welding current [A] 300 voltage correction [V] 1 wire feed [m/min] variable wire material G3Si1/DIN 8559: SG2/AWS: A5.18 power supply direct current, not pulsed, wire positive contact tip distance [mm] 7 welding speed [mm/s] 12 welding time [min] ~4 weld seam length [m] ~3 100% Argon Argon 97.5% CO 2.5% shielding gas composition 2 Argon 82% CO2 18% 100% CO2 ﬂow rate of the shielding gas [l/min] 20 ﬂow rate of cooling water for calorimetry [l/min] 1.7

In order to minimize the inﬂuence of process ﬂuctuations and of the heating of the torch on the results, a seam length of 3 m is welded. For this purpose, the torch was moved over the work piece in a meandering manner. The welding speed was 12 mm/s and the length of one meander path 600 mm. The distance between the individual paths was 15 mm. Five connecting paths were welded, resulting in a welding time of approximately 4 min.

4.1. Parameter Table2 summarizes the determined wire feed rates. Furthermore, the measured current, voltage, and calculated short circuit time, as well as electrical power, are shown. J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 6 of 13

J.4.1. Manuf. Parameter Mater. Process. 2020, 4, 113 6 of 13 Table 2 summarizes the determined wire feed rates. Furthermore, the measured current, voltage, and calculated short circuit time, as well as electrical power, are shown. Table 2. Process parameters for the individual shielding gases.

Table 2. ProcessArgon parameters 100% forArgon the individual 97.5% shieldingArgon 82% gases. Argon 0% Shielding Gas CO2 0% CO2 2.5% CO2 18% CO2 100% Argon 100% Argon 97.5% Argon 82% Argon 0% ShieldingWire Gas feed [m/min] 7.6 9.0 9.0 10.5 CO2 0% CO2 2.5% CO2 18% CO2 100% Wire feedShort [m/min] circuit time [%] 7.6 0 9.0 0 0 9.0 10 10.5 Short circuitaverage time current[%] [A] 0 300 0301 302 0 304 10 average current [A] 300 301 302 304 average voltage [V] 30.18 27.78 31.25 26.96 average voltage [V] 30.18 27.78 31.25 26.96 average averagepower [W] power [W] 9050 9050 8360 8360 9440 9440 8200 8200

4.2. Current and Voltage Measurement Figure5 5 shows shows the the current current and and voltage voltage curves curves of of the the welding welding experiments. experiments. FromFrom thethe curves curves of of thethe voltage,voltage, itit cancan be seen that the drop transition is short-circuit free for all measurements except for 100%100% COCO22.

FigureFigure 5.5. Current and voltage for anan argonargon shieldingshielding gasgas withwith 0%,0%, 2.5%,2.5%, 18%,18%, andand 100%100% COCO22..

Figure5 shows that the scatter of the measured values increases with increasing CO 2 content in the shielding gas. At the same time, the visible part of the arc shortens, as shown in Figure6. From the electrical signals, a short-circuit proportion of approximately 10% results for 100% CO2, while the process runs without a short circuit for the other gases. The photos shown in Figure6 illustrate the visible arc length and were taken with the SLR camera and an 810 nm ﬁlter. J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 7 of 13 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 7 of 13

Figure 5 shows that the scatter of the measured values increases with increasing CO2 content in Figure 5 shows that the scatter of the measured values increases with increasing CO2 content in the shielding gas. At the same time, the visible part of the arc shortens, as shown in Figure 6. From the shielding gas. At the same time, the visible part of the arc shortens, as shown in Figure 6. From the electrical signals, a short-circuit proportion of approximately 10% results for 100% CO2, while the the electrical signals, a short-circuit proportion of approximately 10% results for 100% CO2, while the J.process Manuf. Mater.runs Process.without2020 a, 4short, 113 circuit for the other gases. The photos shown in Figure 6 illustrate7 of the 13 process runs without a short circuit for the other gases. The photos shown in Figure 6 illustrate the visible arc length and were taken with the SLR camera and an 810 nm filter. visible arc length and were taken with the SLR camera and an 810 nm filter.

Figure 6. Visible arc length of the examined shielding gases in qualitative comparison. FigureFigure 6.6. Visible arc lengthlength ofof thethe examinedexamined shieldingshielding gasesgases inin qualitativequalitative comparison.comparison. 4.3. Heat Input Measurement 4.3. Heat Input Measurement Figure7 7 shows shows thethe heatheat inputinput intointo thethe gas gas nozzle nozzle over over the the measuring measuring timetime calculatedcalculated fromfrom thethe Figure 7 shows the heat input into the gas nozzle over the measuring time calculated from the measuring data.data. measuring data.

Figure 7. Heat input into the gas nozzle for the examined shielding gases. FigureFigure 7.7. Heat input into the gasgas nozzlenozzle forfor thethe examinedexamined shieldingshielding gases.gases. For the calculation of the average heat input, measured values between 200 and 250 s are ForFor thethe calculation calculation of of the the average average heat heat input, input, measured measured values values between between 200 and 200 250 and s are 250 selected, s are selected, as steady state measurement can be assumed. asselected, steady as state steady measurement state measurement can be assumed. can be assumed. In Figure 8, the corresponding values including their variance over the considered period are InIn FigureFigure8 8,, the the corresponding corresponding values values including including their their variance variance over over the the considered considered period period are are shown in a diagram. shownshown inin aa diagram.diagram. From the results, it can be seen that the heat input is clearly dependent on the choice of shielding gas. By adding CO2 to the shielding gas argon, the heat input introduced into the gas nozzle by radiation is reduced. This reduction is the least pronounced at a CO2 content of 18%. For a CO2 content of 2.5%, the reduction is already 40% and, for 100% CO2, 70%. Taking into account the time in which the welding process is in short circuit, and thus does not emit any radiation, it can be estimated that the heat input at 100% CO2 is approximately 10% higher, resulting in a value of approximately 81 W. This diﬀerence is small enough for the statements derived so far to stay valid.

J. Manuf. Mater. Mater. Process. Process. 20202020,, 44,, x 113 FOR PEER REVIEW 8 of 13

Figure 8. AverageAverage heat input into the gas nozzle for the examined shielding gases with variance over the evaluated period at 300 A.

4.4. InterimFrom the Summary results, it can be seen that the heat input is clearly dependent on the choice of shielding gas. ByIn thisadding series CO of2 measurements, to the shielding the gas influence argon, of the the heat shielding input gas introduced on the heat into input the by gas arc nozzle radiation by radiation is reduced. This reduction is the least pronounced at a CO2 content of 18%. For a CO2 content into the gas nozzle was investigated. The admixture of CO2 in argon leads to a reduction of the heat of 2.5%, the reduction is already 40% and, for 100% CO2, 70%. Taking into account the time in which radiation into the torch components by up to 40%. The use of 100% CO2 leads to a reduction of 70%. the welding process is in short circuit, and thus does not emit any radiation, it can be estimated that Figure8 shows a local maximum in heat input at 18% CO 2. This result is interesting, because Figure6 the heat input at 100% CO2 is approximately 10% higher, resulting in a value of approximately 81 W. shows that the arc length decreases continuously with the increasing CO2 content. Consequently, Thisthe radiant differencevolume is small also enough decreases for steadily.the statements From thederived publications so far toon stay arc valid. temperature, however, it is known that the temperature in the arc is higher for 18% CO2 than for argon or CO2; see Figure2 . 4.4.Consequently, Interim Summary this e ffect can reduce the influence of the reduced arc length, and thus lead to the localIn maximum. this series of measurements, the influence of the shielding gas on the heat input by arc radiation into the gas nozzle was investigated. The admixture of CO2 in argon leads to a reduction of 5. Heat Input with Constant Wire Feed Rate the heat radiation into the torch components by up to 40%. The use of 100% CO2 leads to a reduction of 70%.From the experiments carried out so far, it can be seen that the GMAW process has very different arc lengthsFigure for di 8ff showserent shielding a local maximum gases at constant in heat current input andat 18% adjusted CO2. wireThis feed.result Therefore, is interesting,the measured because Figureheat input 6 shows into the that shielding the arc gas length nozzle cannotdecreases be clearlycontinuously assigned with to the the influence increasing of the CO shielding2 content. gas. Consequently,As a result, the the tests radiant are performed volume also with decreases constant steadily. arc length. From The the procedure publications from on the arc AiF temperature, project [6] however,was applied. it is known that the temperature in the arc is higher for 18% CO2 than for argon or CO2; see FigureFor 2. theConsequently, test, the same this characteristic effect can reduce curve isthe used influence for all shieldingof the reduced gases, arc which length, corresponds and thus to lead the tosame the setlocal job maximum. at the power source, and the same wire feed is used. Then, the voltage correction is set so that the same welding current results for each shielding gas. This means that the same deposition rate 5.is Heat set for Input all tests with and, Constant consequently, Wire Feed the arcRate length should be the same. From the experiments carried out so far, it can be seen that the GMAW process has very different 5.1. Parameter arc lengths for different shielding gases at constant current and adjusted wire feed. Therefore, the measuredTable 3heat summarizes input into the the determined shielding gas voltage nozzle corrections cannot be for clearly the examined assigned shielding to the influence gases and of thethe shieldingadjusted wiregas. feed.As a result, Furthermore, the tests the are measured performed current with constant and voltage arc length. and the The calculated procedure short from circuit the AiFtime project and power [6] was are applied. shown. With these values, the deviation of the average welding current from the targetedFor the 300 test, A was the the same smallest. characteristic curve is used for all shielding gases, which corresponds to the same set job at the power source, and the same wire feed is used. Then, the voltage correction is set so that the same welding current results for each shielding gas. This means that the same deposition rate is set for all tests and, consequently, the arc length should be the same.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 9 of 13

5.1. Parameter Table 3 summarizes the determined voltage corrections for the examined shielding gases and the adjusted wire feed. Furthermore, the measured current and voltage and the calculated short J. Manuf.circuit Mater. time Process. and power2020, 4, are 113 shown. With these values, the deviation of the average welding current9 of 13 from the targeted 300 A was the smallest.

TableTable 3. 3.Process Process parameters parameters forfor thethe individualindividual shielding gases. gases.

ArgonArgon 100% 100% Argon 97.5% 97.5% ArgonArgon 82% 82% ArgonArgon 0% 0% ShieldingShielding Gas Gas COCO2 0%2 0% CO2 2.5%2.5% COCO2 18%2 18% CO2 CO100%2 100% VoltageVoltage correction correction [V] [V] −4 4 −2.52.5 4.94.9 14.9 14.9 − − WireWire feed feed [m/min] [m/min] 8.3 8.3 8.3 8.3 8.3 8.3 8.3 ShortShort circuit circuit time time [%] [%] 0 0 0 0 0 0 0 0 averageaverage current current [A] [A] 302 302 303 303 303 303 286 286 averageaverage voltage voltage [V] [V] 28.15 28.15 29.53 36.92 36.92 46.24 46.24 averageaverage power power [W] [W] 8500 8500 8950 11,190 11,190 13,220 13,220

5.2. Current and Voltage Measurement 5.2. Current and Voltage Measurement Figures 9 and 10 show the current and voltage curve of the welding experiments. The welding Figures9 and 10 show the current and voltage curve of the welding experiments. The welding process is short-circuit-free for all measurements. process is short-circuit-free for all measurements.

Figure 9. Current and voltage for an argon shield gas with 0%, 2.5%, 18%, and 100% CO2 content. Figure 9. Current and voltage for an argon shield gas with 0%, 2.5%, 18%, and 100% CO2 content.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 10 of 13

It can be seen from Figure 9 that the scatter of the measured values increases with the increasing CO2 content in the shielding gas. At the same time, the visible part of the arc lengthens, as shown in Figure 11. Starting from an admixture of 18% CO2, the arc cone is visibly enlarged. The photos shown in Figure 10 were taken with a high-speed camera and a combination of an 808 nm and an 810 nm filter.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 10 of 13

Figure 10. Visible arc length of the examined shielding gases in qualitative comparison.

Similar to the results of the AiF project [6], the same arc length was expected for the different shielding gases owing to the constant current and constant wire feed. In the respective project, the process is illuminated with a laser and recorded with the help of a filter that only allows the laser radiation to pass. Thus, it could be proven that the distance of the solidus line on the welding wire to the surfaceFigure of the 10. workVisible piece arc is length approximately of the examined the shieldingsame for gasesconstant in qualitative current comparison.and wire feed values. Figure 10 showsFigure that 10. at Visible least arcthe length visible of partthe examined of the arc shie islding nevertheless gases in qualitative of a different comparison. length. It can be seen from Figure9 that the scatter of the measured values increases with the increasing CO5.3.2 HeatcontentSimilar Input in Measurementto the the shielding results of gas. the AtAiF the project same [6], time, the the same visible arc length part of was the expected arc lengthens, for the as different shown in shielding gases owing to the constant current and constant wire feed. In the respective project, the Figure 11. Starting from an admixture of 18% CO2, the arc cone is visibly enlarged. processFigure is 11 illuminated shows the with heat ainput laser intoand therecorded gas nozzle with theover help the ofmeasuring a filter that time. only allows the laser radiation to pass. Thus, it could be proven that the distance of the solidus line on the welding wire to the surface of the work piece is approximately the same for constant current and wire feed values. Figure 10 shows that at least the visible part of the arc is nevertheless of a different length.

5.3. Heat Input Measurement Figure 11 shows the heat input into the gas nozzle over the measuring time.

Figure 11. Heat input into the gas nozzle for the examined shielding gases. Figure 11. Heat input into the gas nozzle for the examined shielding gases.

The photos shown in Figure 10 were taken with a high-speed camera and a combination of an 808 nm and an 810 nm filter. Similar to the results of the AiF project [6], the same arc length was expected for the different shielding gases owingFigure to 11. the Heat constant input into current the gas nozzle and constantfor the examined wire feed. shielding In gases. the respective project, the process is illuminated with a laser and recorded with the help of a filter that only allows the laser radiation to pass. Thus, it could be proven that the distance of the solidus line on the welding wire to the surface of the work piece is approximately the same for constant current and wire feed values. Figure 10 shows that at least the visible part of the arc is nevertheless of a different length.

5.3. Heat Input Measurement

Figure 11 shows the heat input into the gas nozzle over the measuring time. For all measurements, it can be seen that the heat input has settled after 100 s. For the measurements with 18% or 100% CO2, a medium to strong decrease of the energy input can be seen in the middle of the measurement. This was attributed to a thermal deformation of the sheets. As soon as the deformed area is left, the energy input rises again to a constant average value. For the calculation of the J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 11 of 13 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 11 of 13 For all measurements, it can be seen that the heat input has settled after 100 s. For the For all measurements, it can be seen that the heat input has settled after 100 s. For the measurements with 18% or 100% CO2, a medium to strong decrease of the energy input can be seen measurements with 18% or 100% CO2, a medium to strong decrease of the energy input can be seen in the middle of the measurement. This was attributed to a thermal deformation of the sheets. As J.in Manuf. the Mater.middle Process. of the2020 measurement., 4, 113 This was attributed to a thermal deformation of the sheets.11 of 13As soon as the deformed area is left, the energy input rises again to a constant average value. For the soon as the deformed area is left, the energy input rises again to a constant average value. For the calculation of the average heat input, measuring values between 200 and 250 s are chosen, because calculation of the average heat input, measuring values between 200 and 250 s are chosen, because averagehere, steady heat input,values measuring are available values for all between measurements. 200 and 250 s are chosen, because here, steady values here, steady values are available for all measurements. are availableIn Figure for 12, all the measurements. corresponding mean values including their variance over the observed period In Figure 12, the corresponding mean values including their variance over the observed period are Inshown Figure in 12a diagram., the corresponding mean values including their variance over the observed period are shown in a diagram. are shown in a diagram.

Figure 12. Average heat input into the gas nozzle for the examined shielding gases with variance over FigureFigure 12. 12.Average Average heat heat input input into into the the gas gas nozzle nozzle for for the the examined examined shielding shielding gases gases with with variance variance over over the evaluated period. thethe evaluated evaluated period. period. Figure 12 shows that, in contrast to the previous investigations, the heat input increases with the FigureFigure 12 12 shows shows that, that, in in contrast contrast to to the the previous previous investigations, investigations, the the heat heat input input increases increases with with the the increasing CO2 content. There is also no local maximum at a content of 18% CO2 in the shielding gas. increasingincreasing CO CO2 2content. content. There There is is also also no no local local maximum maximum at at a a content content of of 18% 18% CO CO2 2in in the the shielding shielding gas. gas. Figure 13 shows the results of both test series in a diagram. In addition, the measured mean FigureFigure 13 13 shows shows the the results results of of both both test test series series in in a a diagram. diagram. In In addition, addition, the the measured measured mean mean current and the measured mean voltage are shown at each entry. From this comparison, it can be currentcurrent and and the the measured measured mean mean voltage voltage are are shown shown at at each each entry. entry. From From this this comparison, comparison, it it can can be be seen that the radiation-induced energy input can vary considerably even with the same shielding gas. seenseen that that the the radiation-inducedradiation-induced energy input input can can vary vary considerably considerably even even with with the the same same shielding shielding gas. Thus, the largest and smallest measured heat input was determined for 100% CO2. Therefore, the gas.Thus, Thus, the thelargest largest and and smallest smallest measured measured heat heat input input was was determined determined for for 100% 100% CO CO2. Therefore,2. Therefore, the radiation-related heat input into the shielding gas nozzle can be increased or reduced for the same theradiation-related radiation-related heatheat input input into into the the shielding shielding gas gas nozzle nozzle can can be be increased oror reducedreduced forfor the the same same shielding gas by adjusting the process voltage. shieldingshielding gas gas by by adjusting adjusting the the process process voltage. voltage.

Figure 13. Average heat input into the gas nozzle for the examined shielding gases from the two

test series. J. Manuf. Mater. Process. 2020, 4, 113 12 of 13

6. Summary With the measurements carried out, the influence of the shielding gas in GMAW welding on the energy input into the shielding gas nozzle due to radiation was quantified. For the evaluation of the results, it has to be considered that the different process gases partly differ strongly in their process voltage and arc characteristics. In the first part of the experiments, the process parameters were selected by adjusting the characteristic curve and the wire feed to the shielding gas. These tests showed a reduction of the radiation-induced energy input into the shielding gas nozzle for increasing CO2 contents, whereby a local maximum exists for a mixture of 82% argon and 18% CO2. At the same time, a decrease of the visible arc length and, consequently, of the radiant volume was observed with the increasing CO2 content. This decrease is presumably counteracted by higher temperatures in the arc at 18% CO2 compared with 100% argon or 100% CO2. In the second part, the process parameters were selected with the aim of achieving a constant radiating volume. Therefore, the selection of parameters was analogous to the results from [6]. However, the investigations in this publication show that the visible arc length for increasing CO2 contents in the shielding gas nevertheless increases steadily. The experiments also show that an increase in the CO2 content in the shielding gas is accompanied by an increase in the radiation-related energy input into the shielding gas nozzle. In the tests carried out, the smallest as well as the largest radiation-related energy input into the shielding gas nozzle was determined for an arc below 100% CO2. This effect was achieved with a massive increase of the welding voltage. This also increased the arc length. However, this correlation does not allow for a causation. On the contrary, under any shielding gas, the radiation-related energy input into the shielding gas nozzle can be increased by increasing the voltage. Using standard parameters, it can be assumed that an arc under CO2 emits less radiation into the shielding gas nozzle than an arc under argon–CO2 mixtures. However, with the tests performed, this finding cannot be addressed to a cooling effect of the CO2 shielding gas compared with argon. In future research work at the TU Dresden, the cooling effect of the shield gases will be directly investigated. This knowledge can further be used to develop welding processes with less radiation emission, keeping in mind that health and safety regulations restrict the allowable ozone levels, which are directly linked to arc radiation emission [16].

Author Contributions: Data curation, M.L.; Formal analysis, M.L.; Methodology, M.T.; Project administration, M.T.; Resources, U.F.; Supervision, M.T. and U.F.; Validation, M.L., M.T. and S.R.; Visualization, M.L.; Writing—original draft, M.L.; Writing—review & editing, M.L., M.T. and S.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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