Optical and Spectroscopic Study of a Submerged Arc Welding ... · Introduction Submerged arc welding (SAW) is a widely used joining process in a great ... While the fundamentals of

WELDING RESEARCH

DECEMBER 2016 / WELDING JOURNAL 491-s

Introduction Submerged arc welding (SAW) is awidely used joining process in a greatvariety of industries. This includesshipbuilding, construction, and theenergy sector with the production ofpipelines, wind towers, and offshorefoundations. While the fundamentalsof the process have not changed, therehave been improvements in filler ma-terials and power source technologies.

However, one basic characteristic ofthe process is the restricted observ-ability of the wire, arc, and droplet be-havior due to the flux covering thecavern and molten bath. This restrict-ed observability affects the processfrom being well understood, comparedto other arc welding processes [gasmetal arc welding (GMAW) and gastungsten arc welding (GTAW)]. Thecomplex chemical reactions leading tospecified mechanical properties of the

joint, which depend on the droplettransfer and parameters in SAW, havenot yet been completely described.The understanding of these mecha-nisms would support the developmentof more sophisticated process vari-eties, as can be seen in other arc weld-ing processes, because modern powersources are capable of a variety of different waveforms and current patterns. In this work, the processes insidethe SAW cavern have been recorded at5000 frames/s (fps). There are only afew preceding papers on high-speedimaging in SAW. Two kinds of ap-proaches to achieve these images canbe found in literature. Tybus (Ref. 1)used two-quartz-borosilicat windowson both sides of the process. By weld-ing between those windows, high-speed images could be recorded to ana-lyze the process from the side. Thishad a strong impact on the process,since it changed the shape of the cav-ern drastically. In addition, the imageswere of low quality due to the spatterand smoke residue adhering to thewindows. The second approach can be foundin the dissertation from Franz (Ref. 2).He used a ceramic tube that he posi-tioned in front of the weld path. Bywelding over it, he could observe theprocess with less disturbance. The ma-terial of the tube was chosen to matchthe flux. In addition, he compensatedfor the loss of pressure inside the cav-ern by adding a shielding gas. This alsokept the tube and the attached windowclear of debris. Within these investiga-

Optical and Spectroscopic Study of a Submerged Arc Welding Cavern

A combination of highspeed imaging and spatically resolved spectroscopy at 5000 fps wasperformed on a submerged arc welding process using a thingauge steel tunnel

BY G. GÖTT, A. GERICKE, K.-M. HENKEL, AND D. UHRLANDT

ABSTRACT For the first time, a combination of highspeed imaging and spatially resolvedspectroscopy at 5000 fps was performed on a submerged arc welding process. This wasachieved by inserting a thingauge steel tunnel into the flux and aligning the diagnosticsaccordingly. Four processes were observed; both direct current electrode positive (DCEP)and direct current electrode negative (DCEN), as well as alternating current (AC) at 600 Aand DCEP with a higher current at 1000 A. The videos show an erratic droplet transferwith a lot of spatter that was caught by the cavern walls and directed into the weld pool.Additionally, flux was molten at the top of the cavern close to the electrode and mergedinto the droplet that was still attached to the wire. The cavern walls were a mixture ofsolid flux that was partially falling into the weld pool and molten flux, which created asmooth wall. The surface properties of the cavern wall behind the process was mostlysmooth and merged with the weld pool, which created a solidifying layer of slag on topof the slowly cooling weld joint. The observed processes showed only a slight change inchemical composition of main alloying elements in the solidified weld joint, while theoxygen content varied significantly in the droplet stage and weld joint between theprocesses. The highspeed images indicated a correlation between dropletflux interaction and oxygen content. The spatially resolved spectra showed intense selfreversedlines of Na, Ca, and Mn. Fe lines suggested that the arc was also dominated by metalvapor. Especially during the AC process, a fluctuating emission of Mn lines was observed,which correlated with the frequency of the shifting polarity.

KEYWORDS • Submerged Arc Welding (SAW) • HighSpeed Video • Metal Transfer • Cavern • Spectroscopy • Droplet • Flux • Oxygen Content

G. GÖTT (g.goett@inpgreifswald.de) and D. UHRLANDT are with the Leibniz Institute for Plasma and Technology, Greifswald, Germany. A. GERICKE and K.M. HENKEL are with Fraunhofer Application Center Large Structures in Production Engineering, Rostock, Germany.

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WELDING JOURNAL / DECEMBER 2016, VOL. 95492-s

tions, the droplet behavior and materi-al transfer were described as to theirdependence on welding parametersand filler materials. Basic statementsabout arc behavior and electromagnet-ic blowing effects in SAW could be de-scribed. A more recent publication,which adapted this method, is fromMendez (Ref. 3). He used a tube madeout of rolled steel sheets open at bothends. He found a droplet detachmentfrequency of approximately 9 Hz at500-A DCEP, and 13 Hz at 500-A AC.At 1000-A DCEP, he found that a ta-pering electrode tip with a buried arcand a molten tail was ejected through amechanism resembling an electromag-netic kink instability. Also, no obvioussigns of external gas entrainment weredetected. Nevertheless, the flow of gashas to be chosen within narrow limits.Otherwise, it can affect the natural at-mosphere inside the cavern. In other papers, the process was ob-served with x-rays. The first investiga-tors were Ostapenko and Grebelnik(Refs. 4, 5), who recorded singleframes. The images had a poor resolu-tion and could not reflect the dynam-ics of the process. Eichhorn (Ref. 6)managed to create a 500 fps x-ray filmthat could resolve spatter with a diam-eter of 0.16 mm. The advantages ofthis method were the lack of influenceon the process, the long possible ob-

servation time, andthe great level offreedom in observa-tion perspective. Onthe other hand, thetwo disadvantageswere that the shad-ow image did notshow details of thesurfaces (being per-pendicular to the in-cident ray) and thenecessary safety pre-cautions due to the

harmful radiation. A detailed observa-tion of the cavern structure, themolten droplet, or even the arc behav-ior was greatly restricted. X-ray obser-vation in SAW was also performed inRefs. 8–11. The main outcome ofthese investigations was the additionalknowledge in the basic droplet behav-ior as it depends on welding parame-ters, polarity, and used welding fluxes.

Experimental Apparatusand Procedure Two major improvements havebeen achieved, supplementing previ-ous experiments. First, a thin-gaugemetal foil was used as tunnel materialto keep the disturbance of the processas small as possible. It can be seen inFig. 1 ending under the flux. The ma-terial was steel foil with a very lowamount of alloying elements (Table 1),and it had a thickness of 25 μm, whichreduced the effect of additional mate-rial to a negligible amount. This tun-nel was placed in two different ways. One way is shown in Fig. 1, whichgives a side view of the process. Thesecond way was to put the tunnelalong the welding direction. In thisway, a front view of the process couldbe achieved. Similar to the setup inFranz (Ref. 2), a shielding gas was in-

troduced into the tunnel. This helpedto keep the cavern from collapsing dueto the open channel, and to keep at-mospheric gases outside. The addi-tional tube brazed to the tunnelserved as a gas inlet. Second, a spatially resolved high-

speed spectrometer system was addedto the setup. It had to monitor theprocess from the same direction as thehigh-speed camera since the tunnelhad a narrow angle of aperture. Thiswas achieved by a 90-deg mirror thatwas placed in front of the lens at itsblind spot — Fig. 1. This blind spotdesign was related to the internal mirror positions of the lens. The mirror lens was a long-distancemicroscope from Questar Corp. calledQM 1. In combination with a high-speed camera (HSC; MotionPro Y4-monochrome from Integrated DesignTools, Inc.) and an infrared filter, theimages could be recorded at 5000 fpswith only a slight disturbance causedby the arc. This was sufficient to playback the fast processes inside the cav-ern and give a visual overview of theprocesses concerning the metal trans-fer and flux behavior. The acquisitionwas synchronized with the secondcamera (MotionPro Y4-monochrome),recording the high-speed spectra fromthe 0.5-m monochromator (PrincetonInstruments Acton SP2500). By doingso, it was possible to find the connec-tion between high-speed camera images and the spectra. The spectrometer was chosen dueto several advantages. It has a highspectral resolution to determine whichspecies are present inside the arc. Pre-liminary trials showed acquisitionswith a mini spectrometer do not pro-vide sufficient resolution to deter-mine, and distinguish between,species present in the cavern. Thespectrometer was equipped with threegratings with different groove densi-

Fig. 1 — Setup with highspeed camera and spectrometer.

Fig. 3 — Snapshot of the SAWDCEP process.

Fig. 2 — Chemical compositions of SAWslags.

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ties to be able to change the resolu-tion. The spatial distribution could berecorded with an optical system thatcontained spherical and several planarmirrors, an edge filter, and an ad-justable aperture. Therefore, it waspossible to distinguish between thedifferent areas inside the cavern anddetermine where the different specieswere located. Optical emission spectroscopy(OES) had been performed with aSpectromaxx by SPECTRO. The deter-mination of oxygen had been imple-mented through carrier-gas melt ex-traction with a Bruker Elemental G8GALILEO ON/H analyzer. After finish-ing the welding process, the remainingdroplets were collected and carefullycleaned from the remaining scale forthe carrier-gas melt extraction. In this setup, the welding head wasfixed and the base material was movedby using a linear table with a constantvelocity of 1000 mm/min. This was nec-

essary to keep the arc in constant focusfor the optical diagnostics. An overviewof the whole setup is shown in Fig. 1. Itconsists of two HSCs and a spectrome-ter. The welding was performed with aninverter power source (Lincoln AC/DC1000) with a maximum current of±1000 A. A constant current weldingcharacteristic was chosen. In this paper, the single-wire SAWprocess was analyzed with four vary-ing parameter settings. The four pa-rameter changes that were observedwith the diagnostics are given in Table2. The materials were not altered. Thewire was a Lincoln Electric L50M (ENISO 14171 S3Si) with a diameter of 4 mm, and the base material was anEN 10025 S355 J2+N. The mainchemical composition of the materialsis listed in Table 1. The flux used was aLincolnweld 8500 (EN 760 – S A FB 1)with a basicity index of 2.9 and a neu-tral chemical behavior. The flux com-position is listed in Table 3. The weld-ing and wire-feed speed was constant.The height of the pile of flux was keptconstant as well. This was necessary tokeep the basic conditions steady. The pressure that the flux appliedto the cavern was about 0.05 g/mm².The gas pressure that impinged on thecavern through the tunnel had to befinely tuned to the pressure inside the

cavern. If the shielding gas pressure istoo high, it will be injected into thecavern’s atmosphere and influence theprocess. In the case of argon (Ar), itwould change the process to a spraytransfer similar to the GMAW process.If the pressure is too low, the cavernwill shrink, which is visible in the weldjoint profile. In addition, the tunnel tends to beclogged with debris. With a balancedsetting of the gas pressure, the influ-ence on the process is minimized andthe view into the cavern is unobstruct-ed. The best results were achieved byusing Ar at an overpressure of 25mbar. Carbon dioxide (CO2) and Arwere investigated as applicable shield-ing gases. None of the gases in the pre-liminary trials changed the chemicalcomposition of the weld deposit.Nonetheless, changes in the chemicalcompounds of the molten slag, investi-gated by x-ray fluorescence (XRF),were observed using CO2 — Fig. 2.Furthermore, the measured short-circuit frequency changed from 3.6 Hzin the unaffected welding process to4.2 Hz by injecting CO2 as a shieldinggas. Short-circuit frequency was con-stant while using the inert gas Ar. Thisindicated that the use of CO2 as ashielding gas is more invasive to theprocess.

Fig. 4 — Front view illustrating material transfer of the DCEP process with 600 A (Ref. 16, SOM2).

Table 1 — Main Chemical Composition of Used Materials (Values in wt%)

Element C Si Mn P S Al Ti B Fe

Foil 0.0038 0.0290 0.2470 0.0260 0.0110 – – – 99.2440 Base Material 0.0720 0.4060 1.4100 0.0190 0.0250 0.0230 0.0029 0.0042 97.8500 Wire (factory certificate) 0.0800 0.3500 1.6000 <0.01 <0.01 <0.01 – – >97.83 DCEP 600 A 0.1300 0.3100 1.5500 0.0130 0.0069 0.0086 0.0042 0.0055 97.8200 DCEN 0.1080 0.2970 1.4700 0.0170 0.0053 0.0028 0.0027 0.0061 97.9400 AC 0.0890 0.3070 1.4400 0.0280 0.0066 0.0030 0.0036 0.0074 97.9400 DCEP 1000 A 0.1110 0.2980 1.5600 0.0170 0.0050 0.0090 0.0040 0.0058 97.7600

Table 2 — Parameter Variation

Process Identifier Current Voltage

DC+ 600 A 30 V DC −600 A −30 V AC 600 A 30 V DC++ 1000 A 34 V

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Results and Discussion In the following subsection, someof the phenomenon observed with thehigh-speed camera is presented. In or-der to understand the still frames, thesupporting online material is recom-mended (Ref. 16). This will help to en-hance understanding on what eachpart in the frames represents, and itwill facilitate identification of thoseparts. For a better understanding ofthe findings and explanations, Fig. 3shows a snapshot of the clearly visiblemoment in the DCEP process. It is helpful to keep in mind fromwhat perspective the process was ob-served. In this case, it was from a lowangle just above the surface of thebase material. The tunnel was the outer-limiting part of each image, andmoving parts like the droplets, fluxgrains, weld pool surface, and the wirewere visible. The only light source wasthe arc itself, except for the hot sur-faces that represented a very smallpart of the total emission. Therefore,particles in front of the arc are seen asshadows. Particles next to or behindthe arc are illuminated and can be seenas bright spots. The surface of the liq-uid metal has a low emissivity. There-fore, it has a high reflectivity. It wasperceived as a reflecting surface simi-lar to mercury at room temperature. High-speed images of the DCEPprocess with 600 A. The videosshowed different effects. In Fig. 5, theDCEP process, with its SAW basic pa-rameters, is shown in a front view, andin the following section the generalfindings are presented and discussed.Later on, the characteristics of theother parameter sets are discussed incomparison to the DCEP process. Thefront view of the DCEP process

showed a stable arc behavior the entirerecorded time. The droplet transferwas turbulent and changed randomlybetween short circuiting dropping, ex-ploding, and repelling — Fig. 4 andRef. 16, SOM1. Most of the time, flux grains, andsmall metal and slag droplets, weresplattering through the cavern area.The analyzed videos show more or lessturbulent processes inside the cavern,although the SAW process is knownfor a high grade of stability andsmooth weld joints. These are proba-bly a consequence of the slow solidifi-cation of the molten weld pool and thesmoothing effect of the freezing slag.The reaction between flux and metalhappens preferably at the contactpoint between molten droplet and cav-ern wall in the welding direction,where flux is continuously molten andabsorbed by the droplet. This reaction is clearly visible inRef. 16 (SOM2) as a front view, wherethe absorbed molten flux also leads toa change of emissivity in the metaldroplet. This is probably the place andstate in the welding process where themost intensive chemical slag metal re-actions take place due to the sphericaldroplet shape (positive ratio of absorb-ing-surface area to volume), the highreaction temperatures, and the con-stant flux supply. In the lower part ofthe frame, the base material withsome flux and metal droplets can beseen. Obscured by the base materialsurface is a settling, which builds theweld pool, respectively the emergingweld joint — Figs. 5 and 6. It is createdby the arc pressure onto the liquidweld pool. In the upper left corner ofFig. 5, parts of the melting tunnel arevisible. Other effects take place behind the

wire. One is the kinking of the undu-loid, a mathematical term used to de-scribe the geometry of the long moltenmetal droplet that is still attached tothe upper electrode. Magnetic forcesdrive the kinking and throw moltenmetal to the back. This effect can alsobe seen in GMAW processes with highcurrents (Ref. 16, SOM3). The cavern was stable the entire ob-served time, with just a few flux grainsfalling from the walls. This means theminimum internal cavern pressurewas equal to the pressure applied bythe flux on top of the cavern. The cav-ern had a half-ovoloid shape with aminimum width of 12 mm based onthe given scale and visible wire diame-ter. Figure 6 (SOM4) shows the rearpart of the cavern where different ef-fects appear compared to the frontpart. The view is mostly obscured bythe debris coming from the fallingflux. On the left side of this image, thesloping surface of the weld pool can beseen. In the center of the frame a partof the wire is visible, and left of thewire the molten cavern ceiling ap-pears. It merges into the weld pool,visible on the left end of the frame.This part is where the cavern surfacein this area is mostly molten and migrates toward the metal surface. As soon as the cavern surface getsin contact with the still molten weldjoint, the cooling process starts be-cause there is no heat input any more.Once the molten flux is cooled, it willpeel off the weld joint as slag. Thiscontributes to the high weld quality,since the cooling is slowed down andthe atmospheric gases are held backduring this process. In contrast to thesmooth surface of the slag, oncecooled, it can be said that most partsof the cavern wall are not as smooth.

Fig. 5 — Front view of DCEP process with 600 A and flux melting into the droplet — see red marking (Ref. 16, SOM2).

Fig. 6 — Side view behind the process showing the weld pooland the cavern ceiling (Ref. 16, SOM4).


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The inner cavern surface consists ofsolid flux grains, molten slag, and sol-id particles merging into a moltenstage (Ref. 16, SOM5). Within a shorttime of exposure to the heat source,the surface of the flux grains startsmelting with visible outgassing oreven boiling on the surface. The over-exposed region at the center is the arcwith the hot wire tip. The emittedlight illuminates the whole cavern. Thearc contains mostly metal vapor andnonmetallic elements, like calcium(Ca), according to the recorded spec-tra. On the right-hand side in eachframe, the front wall can be seen be-

cause this is a side view of the process.Flux grains in different sizes are fallingthrough the cavern or along the cavernwall. These grains with a melting sur-face appear in the rear part of the cav-ern relative to welding direction aswell, which is supposed to be the cold-est part because of its maximum distance to the burning arc. The flux used was agglomeratedand fluoride-basic. These fluxes typi-cally start melting at around 1200°–1400°C depending on chemical compo-sition and the relation of mineral con-stituents (Refs. 17–19). There is thepossibility of reactions and forming of

different compounds or crystal phasesbefore melting of the flux grains. Onehas to be aware that these compoundshave different, usually lower, meltingand solidification points compared tothe separate components listed inTable 3 (Ref. 17). For a first approximation of the in-ternal cavern temperature close to thesurface, these effects were disregard-ed. Since the fusing process of the fluxgrains is visible in the high-speed im-ages, the cavern surface must rapidlyexceed the melting temperature of theflux. This is possible due to the tem-peratures of the arc and the liquid

Table 3 — Main Chemical Composition of Used Welding Flux and Melting Temperature Tmelt (Values in wt% and C, respectively)

Chem. Comp. SiO2 MnO MgO CaF2 Na2O Al2O3 CaO K2O TiO2 Metal Alloys

CONCN in % 13 1 30 24 2 19 8 1 1 1 Tmelt in C 1713 1650 2852 1423 1275 2050 2575 ± 5 2575 ± 5 1855 –

Fig. 7 — Side view shows the flux falling on the righthand side (Ref. 16, SOM5).


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weld pool. Since the arc is metal-vapordominated (see section on high-speedspectroscopy), it might have a coretemperature of approximately7000–10,000 K as other research sug-gests (Ref. 20). In addition, between 10 and 20% ofthe electrical energy put into theprocess is converted into radiative en-ergy in arc welding processes (Ref. 21).This radiative energy from the arc plusthe heat from the weld pool, which isall trapped inside the cavern, leads to aheat accumulation. Therefore, a quickheating of the flux is to be expected.To verify these statements further,more precise investigations have to beconducted. High-speed images of the DCENprocess with 600 A. In the directcurrent process with electrode nega-tive polarity (DCEN), as shown in Fig.8 and Ref. 16 (SOM6), the arc is muchshorter than in the DCEP process andalmost not visible. The droplet trans-fer happens beneath the surface of thebase metal in a weld pool depression.Therefore, it is not visible. The flicker-ing and fast drifting of regions, withhigh emissivity on the droplet, indi-cates a less stable arc behavior. Thiscould be explained by the high amountof CaF2 in the flux, which is wellknown for destabilizing the arc. Cath-ode spots appeared all over thedroplet, mainly in the upper regionnear the solid wire. This can be ex-plained with lower temperatures andlower electrical resistance. The cavernshowed less volume compared to theDCEP process, which is probably a re-sult of lower internal pressure due tolower temperatures and a smalleramount of metal vapor. This is a resultof a less concentrated arc on the wire,which is a typical feature of a DCENprocess (Ref. 11). High-speed images of the DCEPprocess with 1000 A. While the cav-ern walls were largely covered withflux grains in the DCEP process with600 A, more flux was molten in theDCEP process with 1000 A. Thismakes the observation more difficultdue to high-viscosity slag moving inthe visual field. This can be seen inRef. 16 (SOM7) as a front view. Herethe breakup of the cavern wall, con-sisting of high-viscosity molten slag, isclearly visible as well. The concentrat-ed arc attachment on the droplet

shows a stable behavior with a higheremissivity compared to the lower cur-rent in DCEP. The material transfermainly takes place in a streaming way.Due to the high amount of completelymolten slag, the interior temperatureof the cavern is supposed to be signifi-cantly higher than a DCEP processwith 600 A. From this we can deducethat there was a higher amount of va-porized metal and elements from theflux in the cavern atmosphere. High-speed images of the ACprocess with 600 A. The alternate cur-rent process was conducted with asquare wave and a frequency at 100 Hz.As can be seen in Ref. 16 (SOM8) (as afront view), the process is very stabledespite the continuous changing of po-larity. This is attributed to the fast ratesof current changes due to the inverterpower source technology. The positiveand negative half-waves can be identi-fied because the cathode spots appearon the wire during negative polarity.Compared to the DCEN process, thenegative half-wave appears more stable

with cathode spots running over thedroplet. It is noticeable that the arc of-ten moves to the upper part of thedroplet and enlarges it by melting thesolid wire in the negative half-wave.This can be explained with the preferredmovement of the cathode spots intocooler regions on the wire due to lesselectrical resistance. It leads to an accel-erated fusing of the wire and an increased deposition rate (Ref. 11). High-speed spectroscopy. In thispart, the results of the spectroscopicmeasurements are presented. The aimof these measurements was to find asuitable spectral range to analyze thearc atmosphere and to identify signifi-cant changes within the process. Thiscould help to enhance the understand-ing of the chemistry and mechanics inthe cavern. Similar to the high-speed videos,the spectra were recorded from a lowangle. The setup allowed a correlationbetween the spectra (Fig. 9) and theimages (Fig. 10). The spectra wererecorded along a vertical line across

Fig. 8 — Front view of a DCEN process (Ref. 16, SOM6).

Fig. 9 — Spectra from recording the positive phase (red line) and the negative phase(black line) corresponding to the left and right images in Fig. 10.


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the tunnel (Fig. 1). The upper part inthe spectra represents the upper re-gion in the middle of the images andthe lower part in the spectrum, whichis the lower part in the middle of theimage. Figure 10 shows frames fromRef. 16 (SOM9). The images showboth high-speed images of the processand synchronized spectra in combina-tion. The recorded spectra were domi-nated by iron (Fe), calcium (Ca), self-reversed sodium (Na) lines, and man-ganese (Mn) (compare to Fig. 9). Noticeable is the pair of Na lines atthe center with around 589 nm, whichappear as one dark stripe because theyare strongly self-reversed. Most linesbelow 580 nm are from Fe vapor. Boththe line groups between 610 and 620nm, and 643 and 652 nm, and withthree lines each are from Ca. In thecenter image of Fig. 10 there is anoth-er set of spectral lines visible ataround 601–602 nm, which are miss-ing in the frames before and after (see left and right image in Fig. 10).These lines originate from Mn (601.4,601.7, and 602.2 nm), and are only detectable during the phase aroundcurrent zero. This phase obviously has a lowerarc temperature, and the compositionof the plasma allows these lines to beemitted. Just before the positive phaseat higher currents, there are manymore Fe lines visible and the Na line isalso more intense. A similar situationis visible in the negative phase. Itmight be because of the low boilingtemperature of Mn that its lines arevisible at all. Otherwise, the spectra show an Fe-dominated arc. This is consistent withthe earlier presented observation madeby the high-speed imaging. Both sug-

gest that the maincurrent path is situ-ated below thedroplet that is stillattached to the wire.Therefore, thedroplet transfer inSAW has similaritiesto a CO2 GMAWprocess, although itsatmosphere is differ-ent. In CO2, themain current pathexits at the wire tipin contrast to the Ar-dominated GMAWprocesses where itexits the wire abovethe liquid part of thewire. This mecha-nism is necessary incertain GMAW processes to achieveglobular and spray transfer (Ref. 21).The same effect was stimulated in SAWwhen the shielding gas introduced intothe tunnel was set to an excessivelyhigh pressure and Ar entered the cav-ern. Under these circumstances, thedroplet transfer changed to a constrict-ed spray transfer without any short cir-cuits. This had to be avoided to main-tain a diagnostic method with as littleinfluence as possible. Chemical Analysis. The weldjoints were analyzed by OES. The sam-ples for the OES of the weld metalwere collected from bead-on-platewelds with eight layers to avoid dilu-tion with the base material. Only a fewchanges were found in the chemicalcomposition of the main alloying ele-ments within the varying processes(Table 1). This can be attributed to thechemically neutral character of theflux and the low amount of alloying el-

ements in the wire. As can be ob-served, slight changes occur by varyingpolarity with most melting loss of al-loying elements in DCEN and ACprocesses. This is especially the casefor alloying elements with a high affin-ity to oxygen like carbon, aluminum,and titanium. Oxygen is an important element inwelding metallurgy and can act bothpositively and negatively on mi-crostructure formation. In a balanced,low amount, oxygen plays an impor-tant role in nucleation and can sup-port a fine-grained microstructure for-mation with improved toughness andtensile strength. In interaction with ti-tanium, boron, or other microalloyingelements, this effect is enhanced (Ref.12). In contrast, a high amount of oxy-gen in the weld joint leads to embrit-tlement and porosity. Therefore, opti-mized oxygen content is ideal for ade-quate mechanical properties. In sub-

Fig. 10 — Three successive acquisitions of an AC process a fewhundred µs apart. They go from positive phase via current zeroto negative. Marked is the change in the spectrum from Mn(Ref. 16, SOM9).

A B

C


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merged arc welding, the main sourcesof oxygen are the decomposed fluxconstituents, which contaminate thedroplet (Ref. 23). In the determination of oxygencontents, significant changes were de-tected between the observed processesin droplet and weld joint. The highestvariations were measured between dif-ferent settings of polarity (DCEP,DCEN, and AC). Figure 11 shows thetransient amount of oxygen in thewire, droplet, and solid-weld jointstage within varying processes. As can be seen, the amount of oxy-gen rises in the droplet in all speci-mens. The value of oxygen content de-pends mainly on the polarity but alsoon the current. It is highest in DCEPcompared to DCEN, and slightly in-creases with higher currents. Afterpassing over into the weld pool and so-lidifying, the oxygen content drops toa lower level in all investigatedprocesses. This can be explained bytwo separate effects. First, the droplet and the oxygen itcontains are “dissolved” in the weldpool. Second, the thermochemical de-oxidation effects in the weld pool re-duce the oxygen content of the weldmetal further. This happens throughthe oxidation of alloying elementswith a high affinity to oxygen, like sili-con or aluminum. These oxidized com-pounds are transferred from the weldmetal through the slag-metal interfaceinto the slag. However, the level of thefinal oxygen content in the solid weldjoint seems to be determined by theoxygen content in the droplet stage. It is known that a lower oxygen con-tent in weldments can be achieved byDCEN polarity due to electrochemicalreactions (Refs. 13–15). Supposedly,this is one of the main reasons for thefinal amount of oxygen in SAW. Otherinvestigations show that the final oxy-gen content depends on the weld solidi-fication time as well (Ref. 23). Referring to the high-speed images,the oxygen content also seems to beinfluenced by the droplet-flux interac-tion and arc length, which are deter-mined by the chosen polarity. As canbe seen in Ref. 16 (SOM1) (DCEP) anddescribed in an earlier subsection, theflux merges directly into the droplet.The molten droplet is situated in theupper part of the cavern and in directcontact with the cavern wall, respec-

tively, with the flux. At this point, theflux, consisting of a wide range of ox-ides, raises the oxygen content of thedroplet due to its absorption. Within the DCEN process (Ref. 16,SOM6), the arc is much shorter andthe droplet is closer to the weld pool.Therefore, the droplet transfer is notclearly visible most of the observedtime. There is only a slight droplet-flux interaction visible, as opposed tothe DCEP process. This could lead to alower amount of oxygen in the dropletdue to less absorption of flux with itsoxidic compounds. The AC process shows characteristicsof both DCEP and DCEN in the high-speed images (Ref. 16, SOM8). It has amedium oxygen content in droplet andsolid weld joint, which is between theDCEP and DCEN processes. The highest amount of oxygen wasdetected in the DCEP process withhigh current (I = 1000 A). Referring tothe high-speed images (Ref. 16,SOM7) and the explanations in an ear-lier subsection, the flux is completelymolten because of the high energy in-put and heat in the cavern. This leadsto an increased reactivity of flux com-

pounds, and most likely an increasedoxygen range in the cavern. These re-actions could result in a higher oxygencontent in the droplet.

Conclusion In the presented investigations, acombined and synchronized methodfor high-speed imaging and spatial-resolved spectroscopy in submergedarc welding (SAW) was introduced. Itwas shown that there is a minimuminvasive influence on the processachieved by using this method. Basedon the high-speed images, detailed de-scriptions of the process were madeconcerning the nature, behavior, andsize of cavern, droplet, and arc in dif-ferent polarities (DCEN, DCEP, andAC) and welding currents (600 and1000 A) in submerged arc single-wirewelding. Through observation of the physi-cal state of the flux and slag inside thecavern, estimations of cavern temper-atures could be made. Based on thisand in combination with the results ofthe spectroscopy, the main compo-nents of the cavern atmosphere are

Fig. 11 — Oxygen content in the wire, droplets, and weld joint.


WELDING RESEARCH


likely iron vapor and dissociated fluxcomponents. With varying polarities,DCEN welding shows the lowest cav-ern temperatures, and the DCEPprocess shows the highest, founded onthe condition of the flux and slag in-side the cavern. As expected, at thesame time, cavern temperature riseswith increasing welding current, whichis visible by the completely moltenflux around the cavern. The oxygen contents were deter-mined in the wire, droplets, and weldjoints with varying polarities andwelding currents. It was found that inall investigated process varieties, theoxygen content rises in droplet stageand drops in molten bath, respectively,in the weld joint. The final oxygencontent seems to be determined bywelding polarity in a wide range. Low-est oxygen contents were achievedwith a maximum percentage of cath-ode burning time on the electrode(DCEN and AC). This can be explainedby electrochemical reactions and inconsideration of the high-speed im-ages by droplet-flux interaction andarc length, which are determined bythe chosen polarity. It was shown thathigh-speed imaging can help explainfundamental activities and reactionsin the SAW cavern. Additional spectramay allow us to draw conclusionsabout the mechanisms behind theseeffects. In future investigations, high-speedimages will be correlated with electri-cal characteristics (current and volt-age) of the process, which describes re-curring patterns in process behaviorby means of current-voltage courses.Furthermore, different welding pa-rameters, positions, SAW multiwireprocesses, and materials (e.g., austen-ites) are to be examined.

This joint research project (IGF-Pro-ject Nr.:18579 BR / 1) of the researchassociation of the German Welding So-ciety (DVS) has been funded by the Aifwithin the program for sponsorship byIndustrial Joint Research (IGF) of theGerman Federal Ministry of EconomicAffairs and Energy based on an enact-ment of the German Parliament.Thanks also to Cindy Gött for improv-ing the language.

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References

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Optical and Spectroscopic Study of a Submerged Arc Welding ... · Introduction Submerged arc welding (SAW) is a widely used joining process in a great ... While the fundamentals of

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