Introduction

In gas tungsten arc welding (GTAW)the arc and the weld pool are protectedagainst the influence of atmospheric gasesby a shielding gas. Contamination of theshielding gas leads, among other things, toarc instability, oxidation, porosity, andspatter. Furthermore, atmospheric gasessuch as oxygen, carbon dioxide, or nitro-gen affect the characteristics of the plasmaand influence the arc spots at the cathodeand anode. Therefore, one important goalof welding torch development is to gener-ate an optimal gas flow through the weld-ing torch in order to guarantee a stableand protective shielding gas coverage. Toachieve this, it is most important to avoidflow separation and turbulence in theshielding gas nozzle.

In order to minimize the experimentaleffort by performing numerous weldingexperiments, computational fluid dynam-ics and gas flow diagnostics can be used.

In prior work, attempts were made tofollow this route. As described in Refs. 5and 13, the computational fluid dynamicswere used to optimize the welding fumeexhaustion. However, in these simulations,the arc was either neglected or signifi-cantly simplified by being modeled as asource of thermal energy with a preset mo-mentum. In Refs. 1 and 8, the commercialsoftware ANSYS CFX was used with acontained arc module to calculate theshielding gas flow and the diffusion. How-ever, the models used were based on as-sumptions and many simplifications.Moreover, the torch geometry was oftensimplified in order to reduce the numeri-cal mesh size. Thus, verified experimentalfindings are needed for proofing and cali-brating of these models.

To analyze gas flow fields, particle-based methods such as the laser doppleranemometry (LDA) and the particle

image velocimetry (PIV) can be used. ByZschetzsche (Ref. 2) the applicability ofboth methods for the measurement of gasflow in arc welding was tested and the PIVmethod was adapted to measure differentwelding processes. The method enabled anonintrusive and temporally resolved de-tection of a two-dimensional gas flow fieldin GTAW and gas metal arc welding(GMAW). However, LDA and PIV meas-urements are extremely cost-intensive andrequire a high measuring technique effort.An easier way to visualize gas flows is theSchlieren technique, which has beenknown since the 17th century (Refs. 3, 7,14–16). Typical applications where theSchlieren measuring method was previ-ously used are airplane aerodynamics, bal-listics, and ventilation technology (Ref. 6).

Schlieren studies of electrical dis-charges (arcs) were first carried out byToepler (Ref. 3). In the field of cuttingtechnology, oxygen cutting analyses werecarried out by the Schlieren technique inthe 1930s (Ref. 9). Gas flow studies of arcsby the Schlieren technique are especiallyused in plasma cutting processes and ther-mal spraying (Ref. 6). Gas flow visualiza-tion of plasma cutting arcs and theinteraction of the arc with the workpieceare known from investigations by Settles(Ref. 10). These investigations can be ex-tended to image the gas flow and turbu-lences below the workpiece as well. Inorder to detect instabilities in the plasma-cutting process, Heberlein (Ref. 11) usedthe Schlieren technique in combinationwith current and potential measurementsas well as acoustic recordings. An expla-nation of the relationship between nozzledesign and cutting quality was derivedbased on Schlieren images.

In contrast, Schlieren measurements ofwelding processes are not so common. Forplasma arc welding with alternating cur-rent, McClure and Garcia (Ref. 4) de-

SUPPLEMENT TO THE WELDING JOURNAL, JANUARY 2014Sponsored by the American Welding Society and the Welding Research Council

Visualization of Gas Flows in Welding Arcsby the Schlieren Measuring Technique

The influence of typical welding parameters on the gas flow for the GTAW, GMAW,and PAW processes is demonstrated using the high-speed Schlieren technique

BY E. SIEWERT, G. WILHELM, M. HÄSSLER, J. SCHEIN, T. HANSON, M. SCHNICK, AND U. FÜSSEL

KEYWORDS

Shielding Gas Gas ContaminationGas Flow DynamicsGas Tungsten ArcGas Metal ArcPlasma Arc

Dipl.-Ing. E. SIEWERT, Dr.-Ing. G. WILHELM,M. HÄSSLER, Prof. Dr.-Ing. J. SCHEIN, and Dr.T. HANSON are with Center of Excellence AAP(advanced arc processes), a coop of Linde AG Co.and the Lab of Plasma Technology, University of theGerman Federal Armed Forces, Munich, Germany.Dipl.-Ing. M. SCHNICK and Prof. Dr.-Ing. U.FÜSSEL are with the Department of Joining Engi-neering and Assembly Technology, University ofTechnology, Dresden, Germany.

ABSTRACT

Gas flows in and around welding arcs have a strong influence on the welding process.Atmospheric gases reach the arc due to turbulences and diffusion mechanisms and this af-fects the arc and the weld pool. Using optical analysis of the gas flow during welding withand without the arc present reveals possible mixing and thus the causes of contaminationcan be determined. The Schlieren method offers a simple way to do this. In this paper, thesetup of a Schlieren measuring system and the influence of the most relevant setting pa-rameters are described as well as their influence on the Schlieren images.

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scribed the necessity for a gas flow analy-sis. However, their work contained no cor-responding results or Schlieren images.Allemand and Schroeder (Ref. 12) usedthe Shadowgraph method (Ref. 6) in orderto visualize the drop transfer during gasmetal arc welding. For illumination, a He-Ne laser was used. The photographs are,however, overexposed due to the presenceof the arc and the drop transfer was diffi-cult to observe.

This paper describes an attempt to usethe Schlieren technique to visualize theshielding gas flow in different arc weldingprocesses. The principle of operation andthe experimental setup of the Schlierentechnique are described. The most impor-tant settings and their influence on thequality of the Schlieren images of GTA aredescribed so that the range of applicationand the limit of the Schlieren techniquecan be specified. The results of the gasflow analysis for GTA, GMA, and plasmaarc welding (PAW) are presented, wherethe influences of typical welding parame-ters on the gas flow are displayed.

Experimental Procedure

Physical Principle and Measuring System

By the Schlieren technique, differencesin density that cause changes in the re-fraction index n, in the propagation veloc-

ity c and in the direction of light propaga-tion direction, can be visualized in trans-parent media. The angle of refrac-tionrelates itself to the incident angle α

(1)

Thus each change in density of themedium causes a change in the directionof light propagation — Fig. 1.

The differences in density that are ob-served during the welding process arecaused, according to the ideal gas equa-tion, by differences in pressure, tempera-ture, and concentration.

In order to make differences in densityin transparent media visible, the interfer-ence and the shadowgraph methods canalso be used alongside the Schlierentechnique.

In the interference method, two lightwaves are superimposed so that an inter-ference pattern is generated. The interfer-ence image allows the reconstruction ofthe location and the intensity of the lightrefraction as well as the speed of the gasflow, the density, and the temperature.However, this measurement method re-quires high precision in the adjustment ofthe measuring equipment.

By the shadowgraph method, deflec-tion of the light can be made visible by

means of the generated intensity of illu-mination dispersion E, which is propor-tional to the second derivation of thedensity along path y (Equation 2).

(2)

This method enables conclusions to bedrawn about the density gradient, but notabout the direction. Compared to the in-terference method, a lower resolution andsensitivity can be reached (Ref. 6).

Due to a marginal overhead (the inte-gration of a knife edge), it is possible toseparate the deflected from the uninflu-enced light, in order to increase the reso-lution and sensitivity. Furthermore, withthe so-called Schlieren technique, it is pos-sible to determine the direction of themeasured density gradient. The change inintensity of illumination caused by thelight deflection is proportional to the firstderivation of density according to the po-sition (Equation 3).

(3)In contrast to the interference method, theSchlieren technique is a simple and robustmeasuring system. However, an exactidentification of gas flow characteristics isnot possible.

The experimental setup is carried outas a Toepler’s Z-Schlieren assembly withtwo concave mirrors — Fig. 2. This as-sembly is compact and avoids errors dueto chromatic aberration caused by the op-tical lenses.

The concave mirrors are axially para-bolic mirrors with a diameter of 150 mmand a focal length of 1200 mm. The diam-eter lies in the recommended area from D= f/6 to f/12 (Ref. 6). In the region be-tween both mirrors, parallel light is gener-ated. In this optical path, different weldingarcs (Schliere) are inserted, influencingthe propagation of the parallel light. In thefocus of the first mirror, an aperture isplaced to produce a point light source en-abling the production of parallel light bymirror 1.

The knife edge is placed in the focus ofmirror 2. The knife edge is used to im-prove the contrast by blocking the deflect-ed light. Images of the Schlieren aregenerated by a high-speed camera with a200-mm objective with a macrolens.

The exact position in which theSchliere is arranged between the two mir-rors has no influence on the measurementoutcome. The deflection level of the lighta in the Schlieren aperture depends onlyupon the angle of deflection and the focallength f of the mirror.

Δa = ε﹒f (4)

αβ= =

sinsin

cc

nn

2

1

2

1

ρΔ

∂

∂E

y

2

2∼

ρΔ

∂

∂E

y∼

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Fig. 1 — The law of refraction as foundation of theSchlieren optic.

Fig. 2 — Toeplersche Z-Schlieren assembly.

Fig. 3 — Schlieren images, used filter pairs: blue/yellow (left) and red/green (right) with a shielding gasflow of 30 L/min of argon.

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Requirements of the Schlieren Method toAnalyze Welding Arcs

Alongside the already described basicrequirements such as the positioning ofthe mirrors, the quality of the Schlierenimages of arc processes is above all deter-mined by the light source and the slit(knife-edge) or alternatively colored filterpairs, which induce colored shadows andinterferences — Fig. 3.

The knife-edge or the filter affects thesensitivity of the Schlieren apparatus,whereas the magnitude of the deflectedlight can be assigned to different dye byusing color filters. The applicability of aper-tures with horizontal or vertical slits, or aniris as well as two- and four-color filters wasinvestigated. In all experiments, the openarea of the slits was equal and the orienta-tion of the illumination and the Schlierenslit was always identical.

The two-color filters (blue/yellow andred/green) as well as a four-color filter,utilizing all four colors, were used. Thebest results were obtained using the two-color filters, by which the turbulencescould be visualized with very strong con-trast — Fig. 3. In comparison, using thefour-color filter, only marginal color nu-ances could be recognized. However, thelight intensity was reduced when coloredfilters were used. Thus, the exposure timehad to be extended whereby a strong

cross-fading due to arc radiation resulted.Analyses of the influence of the geom-

etry and the orientation of slits clarifythat good results can be achieved withslits oriented perpendicular to the work-piece — Fig. 4.

The hot gas above the workpiece was vi-sualized using apertures with a slit, whichwere oriented parallel to the workpiece.The iris can be used to visualize gas flow inall directions, but the images are character-ized by a lower brightness of the image.

By reducing the slit width of the knifeedge, less diffracted light, and conse-quently smaller differences in density,can be visualized (Ref. 6). At the sametime the influence of the radiation of thearc decreases. However, less light fromthe light source passes the knife edge es-pecially if the width of the knife edge isless than the focal diameter. The goal ofthe slit variation was to be able to visual-ize the turbulence and the density gradi-ent of the shielding gas flow in the free jetof the process gas in close proximity tothe arc individually. It was ascertainedthat in spite of a small slit width, the den-sity variation produced by the arc domi-nated — Fig. 5.

When using identical concave mirrorsin the geometry described above, it is rec-ommended that the shape of the lightsources used be equivalent to that of theslit opening. Therefore, elongated rectan-

gular light sources were used. Initially, the applicability of simple

light bulbs was tested. Only by the use ofhigh-luminosity light sources could the slitopening as well as the exposure time of thecamera be reduced, so that:

1) The complete area of the gas flowwas illuminated,

2) overexposure of the images due tothe arc radiation could be avoided, and

3) minor differences in density could bevisualized in the gas-free jet.

Beside the power, the light source mustgenerate a high light intensity on the knifeedge. The gas flow in the boundary regionof the process gas-free jet can be visual-ized well using halogen lamps.

However, with the light sources used asdescribed in Fig. 6, the area of the arc can-not be investigated in detail due to itsstrong brightness. Thus, further analysesemployed alternative light sources such asa plasma arc and laser beam.

The radiation energy of a plasma arc isapproximately 10 to 20% of the total power.Thus, the radiation emission of a 250-Aplasma arc with a voltage of 30 V is about1000 W. Using this kind of arc is further-more advantageous since the projection ofthe light source is rectangular, as the knifeedge is. Considering the solid angle of emis-sion, only 1% of the radiation reaches themirror. Nevertheless, even this amount oflight is sufficient to obtain a detailed flow

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Fig. 4 — Schlieren image of a 100-A gas tungsten arc with vertical (top)and horizontal (middle) apertures, and an iris (bottom).

Fig. 5 — Images of Schlieren setups with a 3 × 6 mm focus slit and a Schlieren aper-ture slit of 2 × 6 mm (left), 3 × 6 mm (middle), and 5 × 6 mm (right).

Fig. 6 — Schlieren images made by using 50-W automobile headlight (top, left), 150-W tungsten coiled filament lamp (top, right), 250-W tungsten coiled filament lamp(bottom, left), and 150-W halogen lamp (bottom, right).

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image in an area that was not recognizablebefore — Fig. 7.

Using a 20-mW continuous wave laserof wavelength 532 nm in combination witha neutral gray filter with a transmittanceof 1%, the radiation of the arc could becompletely faded out — Fig. 7. However,by using a laser (point light source), a “dig-

ital” Schlieren image without intensity gra-dations results.

Results and Discussion

In order to analyze the gas flow even atthe boundary region of the process gas-free jet, despite the intensive arc radiation,

a GTAW arc is used as a light source. Theorientation of the light source, as well asthat of the Schlieren slit, is verticallyaligned to the surface of the workpiece.

The Schlieren technique was used tomake high-speed images of the GTAW,PAW, and GMAW processes.

GTAW

GTAW with differing shielding gases,flow rates, and currents was analyzed —Fig. 8.

The transition of the process gas-freejet to the atmosphere is especially good tovisualize using argon with an appreciablehelium percentage (50%) as shielding gas.However, it has to be assumed that heliumhas an essential influence on the arc geom-etry and, above all, on the gas flow.

The arc current influences the temper-ature of the arc and the temperature of theeffluent gas. From the Schlieren images, itcan be clearly seen that the arc moves upfarther on the tungsten cathode, that thecore of the arc is brighter, and that there isa stronger flux of hot gas above the work-piece. Despite the brightness, the edges ofthe arc can be clearly detected.

The Schlieren measurement method canbe used to detect the turnover from a lami-nar to a turbulent gas flow of the processgas-free jet in GTAW. Turbulences sur-rounding the arc and turbulences in the ef-fluent hot gas can be clearly distinguished atshielding gas flow rates of 30 L/min andmore.

PAW

Investigating plasma arc keyhole weld-ing was carried out by bead-on-plate welds(6-mm-thick, mild-steel plates). To ignitethe main arc between the tungsten cath-ode and the workpiece, a pilot arc betweenthe cathode and the copper nozzle(anode) must be initialized. The pilot arcserves as preionization of the arc gap be-tween the electrode and the workpiece —Fig. 9. The Schlieren method is excellentlysuited to image the gas flow of the pilotarc. An advantage is the low radiationemission of this plasma jet.

The Schlieren images of real keyholewelding trials were correlated with the re-spective welding results — Fig. 10.

Clearly visible at low shielding gas flowrates is that the fluid flow above the hotweld joint (left of the torch) is dominatedby thermal buoyancy. In contrast, abovethe cold steel sheet (right of the torch) anequal and laminar outflow can be seen.With higher shielding gas flow rates, thedifferences between the gas flows over thehot and the cold steel sheet are less pro-nounced. It is assumed that the highshielding gas flow counteracts the thermalbuoyancy as well as causing the outflowing

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Fig. 7 — Schlieren images made by using 250-A plasma arc (left) and 20-mW continuous wave laser (λ=532 nm) (right).

Fig. 8 — Schlieren images of GTAW as a function of current, shielding gas, and flow rate.

Fig. 9 — Schlieren image of a pilot arc (3 L/min plasma gas flow) where the hot plasma jet is clearly observ-able. The impinging hot gas on the workpiece and the effluent hot gas on the surface of the workpiece are vis-ible by a dark plateau. Stalls in the periphery are detected by means of eddies.

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gas to deviate from laminar flow. Usinglow shielding gas flows, a considerable for-mation of oxides can be determined, whichis due to contamination of the protectivecover. It must be concluded that the for-mation of a turbulent gas flow (15 L/minshielding gas) does not always lead to badgas protection cover of the weld pool. Asufficient gas flow is necessary in order tocounteract the thermal buoyancy abovethe hot workpiece.

GMAW

Gas metal arc welding is characterizedby a high radiation emission of the metalvapor plasma. Schlieren images of gasmetal arc welding processes are thereforeespecially difficult to create at high cur-rents. As part of the investigations,Schlieren images were taken of a short arc— Fig. 11. In the images, gas flow separa-tions at the shielding gas nozzle and thecontact tip are, in contrast to GTAW,clearly visible. A reason for that is thehigh, very hot contact tip located insidethe shielding gas nozzle caused heating ofthe shielding gas.

For the analysis of a pulsed arc or aspray arc, it is necessary to use powerfullight sources or to mitigate wavelengthswith special intensive radiation emissionof the arc by filters.

Conclusions

The Schlieren method was used to vi-sualize gas flows in welding processes. Themain conclusions are as follows:

1) The Topler Z-Schlieren configura-

tion enablescost-efficientand time-re-solved gas flowanalysis.

2) It was as-certained that apowerful tung-sten filament

lamp and arcs were especially appropriateas light sources. In contrast, inferior im-ages were obtained with widened laserbeams.

3) It is possible to detect the transitionfrom a laminar to a turbulent gas flow in aprocess gas-free jet in GTAW by increas-ing the shielding gas flow from 10 to 30 L/min.

4) Through the Schlieren method, thegas flow of a nontransfer pilot arc can beexcellently visualized. During studies on aplasma arc keyhole welding process, it wasshown that high shielding flow rates, de-spite intensive turbulences, provide a bet-ter protection of the process andcounteract diffusions effects.

5) First investigation on GMAWprocesses showed that high torch temper-ature principally abets the Schlieren analy-sis of the process gas-free jets. Due to thehigh radiation emission of the arc, power-ful illuminants in combination with opticalfilters are necessary, especially in theanalysis of spray and pulsed arcs.

References

1. Schnick, M., Füssel, U., and Zschetzsche,J. 2006. Simulation and measurement of plasmaand gas flows in plasma arc welding and cutting.8th International Seminar — Numerical Analysisof Weldability, Graz, Austria.

2. Zschetzsche, J. 2007. Diagnostics of gasshielded arc welding processes. DresdnerFugetechnische Berichte. Band 14.

3. Toepler, A. 1906. Observations accordingto a new optical method. Ostwalds Klassiker derExakten Wissenschaften Nr. 158. Leipzig, Ger-many.

4. Garcia, G., McClure, J. C., Hou, H. and

Nunes, A. C. Gas flow observation duringVPPA welding using a shadowgraph technique.NASA-CR-204347.

5. Cooper, P., Godbole, A., and Norrish, J.2007. Modelling and simulation of gas flows inarc welding. Implications for shielding effi-ciency and fume extraction. Proc. on the 60thAnnual Assembly of the International Institute ofWelding, Dubrovnik, Croatia.

6. Settles, G. S. 2001. Schlieren and Shadow-graph Techniques. Springer; Berlin, Germany,ISBN 3-540-66155-7.

7. Schardin, H. 1934. Toepler’s Schlierenmethod: Basic principles for its use and quanti-tative evaluation. Forschungsheft 367 –— Beilagezu Forschung auf dem Gebiet des Ingenieurwe-sens Ausgabe B Band. July/August.

8. Speiseder, M., and Lang, A. 2006. Opti-mization of the MIG-welding process by the useof numerical simulation and PIV measurement.The electric arc — A technology with a non-ex-hausted potential. Dresdner FugetechnischesKolloquium, TU Dresden, Germany.

9. Zobel, T. W. 1936. Increase of the cuttingspeed while flame cutting by the use of a newnozzle geometry. VDI-Verlag GmbH, Berlin,Germany

10. Settles, G. S. 1998. Visualization of liq-uid metal, arc, and jet interactions in plasmacutting of steel sheet. 8th International Sympo-sium on Flow Visualization.

11. Kim, S. J. 2009. Fluid dynamic instabili-ties in plasma arc cutting. PhD dissertation.Minnesota, Faculty of the graduate school, Uni-versity of Minnesota.

12. Allemand, C. D., Schoeder, R., Ries, D.E., and Eagar, T. W. 1985. A method of filmingmetal transfer in welding arcs. Welding Journal64(1): 45–47

13. Ebert, L. 2007. Optimization of fume ex-traction of torch integrated fume extraction de-vices. TU Chemnitz. AbschlussberichtAiF-Vorhaben 14:436 BR.

14. Foucault, J. B. 1859. Annales de l‘Obser-vatoire Impérial de Paris.

15. Mach, E. 1889. Further ballistic-photo-graphic experiments. Sitzungsband Akad. Wiss.Wien. 98: 1303–1309.

16. Rheinberg, J. H. 1896. On an addition tothe methods of microscopical research, by anew way of optically producing coulour-contrastbetween an object and its background, or be-tween definite parts of the object itself. J. Roy.Microsc. Soc., Ser. 2, 16(8):373–388.

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Fig. 11 — Schlieren adaptor of a short arc (3 m/min wire feed).

Fig. 10 — Schlieren images of plasma arc welding (S235, 6 mm; welding speed, 20cm/min; PG-flow, 3 L/min; plasma gas three-hole-nozzle, 3 mm; torch distance, 5mm; shielding gas flow 5 L/min (top); 15 L/min (bottom).

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