The effect of shielding gas composition on welding performance and weld properties inhybrid CO 2 laser–gas metal arc welding of carbon manganese steelAnna Fellman and Veli Kujanpää Citation: Journal of Laser Applications 18, 12 (2006); doi: 10.2351/1.2164481 View online: http://dx.doi.org/10.2351/1.2164481 View Table of Contents: http://scitation.aip.org/content/lia/journal/jla/18/1?ver=pdfcov Published by the Laser Institute of America Articles you may be interested in Mechanical and microstructural characteristics of the dissimilar materials butt joints by hybrid CO 2 laser-gasmetal arc welding J. Laser Appl. 23, 012002 (2011); 10.2351/1.3549591 Effect of laser and laser hybrid welding on the corrosion performance of a lean duplex stainless steel J. Laser Appl. 22, 150 (2010); 10.2351/1.3533146 Review of laser hybrid welding J. Laser Appl. 17, 2 (2005); 10.2351/1.1848532 High-speed simultaneous observation of plasma and keyhole behavior during high power CO 2 laser welding:Effect of shielding gas on porosity formation J. Laser Appl. 12, 245 (2000); 10.2351/1.1324717 Welding of ship structural steel A36 using a Nd:YAG laser and gas–metal arc welding J. Laser Appl. 12, 185 (2000); 10.2351/1.1309549

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The effect of shielding gas composition on welding performance and weldproperties in hybrid CO2 laser–gas metal arc welding of carbonmanganese steel

Anna FellmanLaser Processing Laboratory, Department of Mechanical Engineering, Lappeenranta University ofTechnology, FIN-53850 Lappeenranta, Finland

Veli KujanpääLaser Processing Laboratory, Department of Mechanical Engineering, Lappeenranta University ofTechnology and VTT Industrial Systems, FIN-53850 Lappeenranta, Finland

�Received 2 November 2004; accepted for publication 13 June 2005�

Metals industries producing large structures currently have a particular interest in hybrid laserwelding processes, which possess advantages compared with conventional methods of welding. Onemajor benefit is a reduction in deformation that enables the amount of postweld finishing work to bereduced. Assembly then also becomes simpler because of the greater accuracy that may be achieved.Larger joint tolerances may be accommodated compared with laser welding alone. By usingappropriate filler metal, the weld metal composition may be controlled to meet metallurgical criteria.The hybrid CO2 laser–gas metal arc �GMA� welding process was investigated in this study; the aimbeing to clarify the effects of process gas composition on welding performance, weld cross section,quality, and mechanical properties, when welding carbon manganese steel. Helium, argon, andcarbon dioxide were used in varying proportions as shielding gases for welding I-butt and T-buttjoints. The composition of the shielding gas was found to affect welding performance, weld quality,and weld cross-section geometry. The best results were obtained by using argon, with a heliumcontent of 40%–50% and a carbon dioxide content of 2%–5%; the exact composition depending onthe groove edge quality. Hybrid laser welds of high quality may be produced in carbon manganesesteel with a variety of shielding gas compositions, but to maximize productivity, reliability, andquality, while minimizing gas cost, the composition must be selected carefully. © 2006 LaserInstitute of America.

Key words: hybrid CO2 laser–GMA welding, hybrid welding, shielding gases, weld quality

I. INTRODUCTION

Hybrid welding processes comprising a laser beam andan electric arc were first investigated in the late 1970s,1 butthe benefits for industrial production have only recently beenrealized. Hybrid processes combine the high productivity, ac-curacy and deep penetration of laser keyhole welding withthe larger fitup tolerances of electric arc welding, while mini-mizing the energy input. The hybrid CO2 laser–gas metal arc�GMA� welding process is the most popular variant, nowbeing used in car body manufacturing, the pipeline and con-tainer industries, and in shipbuilding.

A characteristic of laser hybrid welding is the large num-ber of parameters that control the quality of the weld. This isconsidered to be a drawback because the amount of trial anderror testing required to specify a welding procedure is large,which complicates the introduction of the process. However,the flexibility in parameter selection enables hybrid laserwelding to be used in a large number of applications.

During hybrid CO2 laser–GMA welding, synergistic ef-fects of the laser keyhole and GMA welding processes areobserved. Plasma induced by the laser beam facilitates igni-tion of the electric arc, making it more stable. This is be-lieved to be a result of increased ionization of the plume

generated above the workpiece, which partly constricts thearc, enabling the power density to increase rapidly, thus pro-ducing a greater penetration depth.2 However, a significantamount of far infrared CO2 laser beam energy may be ab-sorbed and scattered by the plasma. Plasma formation bothreduces the beam power incident on the workpiece and ef-fectively defocuses the beam by reradiating energy. The for-mation of a deeply penetrating vapor keyhole is thus hin-dered, which reduces the efficiency of the process. It istherefore essential to minimize plasma formation if penetra-tion is to be maintained.3

Helium is often used to prevent plasma formation whendeveloping laser welding procedures. In contrast, in this in-vestigation the GMA shielding gas alone was used forplasma suppression. The effects of different shielding gasmixtures on weld quality, weld geometry, and the stability ofhybrid CO2 laser–GMA welding of carbon steel aredescribed.

II. THE ROLE OF PROCESS GAS IN LASER, ARC,AND HYBRID WELDING

Process gas has two principal roles in laser welding: toshield the weld keyhole and solidifying molten metal from

JOURNAL OF LASER APPLICATIONS VOLUME 18, NUMBER 1 FEBRUARY 2006

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oxidation, thus avoiding porosity and oxide inclusions; andto ensure efficient transmission of the laser beam as it isfocused to provide high penetration, by minimizing beamexpansion and scattering caused by vapor and gas around thekeyhole.3 In arc welding the shielding gas protects the mol-ten metal which results in a large melt pool and the genera-tion of fume during welding. The arc becomes more unstablewhen the carbon dioxide content rises. As a consequence, theamount of spatter increases.4–6

However, a small amount of carbon dioxide mixed withargon stabilizes the arc, improving metal transfer and lower-ing the surface tension of the melt. A reduction in surfacetension creates a smoother weld bead face, and reduces theheight of the weld bead, the likelihood of undercutting, andthe amount of spatter generated during welding by increasingthe fluidity of the melt. In practice, a carbon dioxide contentof 0.5%–1% is sufficient to lower the surface tension of themelt. The optimum amount of CO2 in inert shielding gas is,depending on the work piece surface condition, the joint ge-ometry, the welding position or technique, and the base metalcomposition. Generally, in arc welding 8%–10% CO2 is con-sidered a good compromise to cover a broad range of thesevariables.4,7

In hybrid CO2 laser–GMA welding, two very differentprocesses must be shielded. Individual shielding of each pro-cess may be arranged via separate nozzles. However, thecombination of two separate gas flows in a very small areamay cause turbulence, which reduces the shielding effect ofthe melt. In this work, gas flows from both processes werecombined such that helium process gas needed for CO2 laserwelding was mixed into the argon–carbon dioxide shieldinggas used in the GMA welding process. It was confirmedvisually that the shielding gas for GMA welding was suffi-cient to prevent excessive plasma formation, thus enablingfull penetration to be achieved. It was therefore possible toeliminate the need for a separate plasma control gas nozzlefor CO2 laser welding.

III. EXPERIMENT

A 6 kW Rofin Sinar RS6000 CO2 laser was used to-gether with Esab Aristo LUD 450V conventional multipur-pose welding equipment equipped with an Esab MEK 44Cfiller wire feed unit for hybrid laser welding.

Welding trials were performed using low-alloyed carbonsteel RAEX 275 MC Laser �S275J2 according to structuralsteel standard EN 10025-2:2004�, which is manufactured byRautaruukki Oy and designed to exhibit good laser cuttingperformance. I-butt and T-butt joint configurations usingplates of thickness 6 mm with laser-cut edges were welded

in a single pass. The filler wire used was Esab OK Autrod12.51 �SFA/AWS A5.18, ER 70S-6, EN 440 G3Si1�, of0.8 mm diameter. Table I gives a chemical analysis of thesteel used and the alloying elements in the filler wire. Thetest plates for I-butt joints were 350 mm in length and150 mm in width. The plates were tack welded such that inthe first two sets of trials no gap existed in the joint, whereasin a third set of trials a gap of 0.8 mm was introduced be-tween the plates. The dimensions of the flange of the T-buttjoints were 350�150 mm �length�width�, with a verticalweb of 150 mm height. The plates were tack welded suchthat the web was inclined at an angle of 10° relative to thenormal to the flange, producing a groove angle of 10°. Thefixturing geometry for both I-butt and T-butt joints is shownin Fig. 1.

The GMA welding torch was located close to the focalpoint of the CO2 laser beam; the distance between the fillerwire and the laser beam was approximately 3 mm. The torchwas inclined at a slope angle of 55° in relation to the surfaceof the workpiece. Welding of T-butt joints was performed inthe horizontal-vertical position, �see Fig. 1�. The rotationalangle of the torch was 50° and the laser beam made a rota-tional angle of 15° in relation to the vertical. The torch anglewas leading in terms of the welding direction. The orienta-tions of the welding heads are shown in Fig. 2.

Welding parameters were obtained experimentally, basedon the need for full penetration and a visually acceptableweld. Tables II and III give the principal processing param-eters used. The arc current and voltage were measured at theoutput of the GMA welding machine; the machine gives theaverage values of current and voltage during welding.

In this study, the synergy lines of the arc source wereused. For Aristo, the filler wire type and diameter and shield-ing gas composition are selected to get the right synergy lineused in welding. This synergy compensation of the GMA

TABLE I. Chemical analysis �wt % � of RAEX 275 MC laser steel and a nominal composition of OK 12.51 informed by the filler wire manufacturer. Thecarbon equivalent of the steel used is 0.32.

wt % Fe C Si Mn S P Al Nb V N Mo Cu Cr Ni

RAEX275 MC

Bal. 0.08 0.01 1.39 0.004 0.008 0.028 0.003 0.006 0.005 0.008 0.021 0.03 0.04

OK12.51

Bal. 0.1 0.9 1.5 — — — — — — — — — —

FIG. 1. Fixturing geometry of I-butt �left� and T-butt �right� joints.

13J. Laser Appl., Vol. 18, No. 1, February 2006 A. Fellman and V. Kujanpää

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welding machine adjusts the welding parameters duringwelding to maintain a constant weld quality. However, nosuch synergy lines have been developed for the shielding gasmixtures used in the experiments. The same synergy linesetup was therefore used for every experiment, based on thatfor carbon steel, a filler wire diameter of 0.8 mm, and ashielding gas mixture of argon and 2% carbon dioxide, de-spite the fact that the shielding gas mixture was changedbetween trials. Since the welding machine controls the arc,adjustments are automatically made to the process accordingto the condition of the arc. Thus changes in shielding gascomposition affect the arc voltage and current.

The only process gas used was a shielding flow in theGMA welding torch—no separate plasma/shielding gas wasused for the CO2 laser. The shielding gas mixtures used com-prised helium, argon, and carbon dioxide. The gas mixtureswere produced in a Witt-Gasetechniks KM 100-3 ME gasmixer. The gas mixer is able to handle compositions in therange 0%–100% helium, 0%–100% argon, and 0%–25%CO2. The flow rate was set at 25–30 L/min, as measuredwith an argon-calibrated rotameter.

The test specimens were checked visually, and a selec-tion of the butt joints radiographed. One batch of butt jointswelded with different shielding gas compositions was testedin accordance with the procedure outlined in the standard ENISO 15614-11.8 Some of the parameters used in welding ofspecimens for testing procedure are presented in Table IV.Other parameter set-ups were the same as the ones men-tioned in Table II. All other parameters were kept constantexcept the shielding gas composition and the tests with allshielding gas compositions tested were made for both gaps�0 and 0.8 mm�. Weld testing included both nondestructiveand destructive techniques. Nondestructive testing comprisedvisual and radiographic examination. Destructive testing in-cluded hardness measurement, bend, and transverse tensile

testing and impact toughness testing. Weld cross sectionswere established from the specimen used for macroscopicexamination.

IV. RESULTS

A. Welding performance

An additional helium plasma control gas flow was foundto be unnecessary if a minimum of 30% helium was presentin the shielding gas during GMA welding. To avoidexcessive plasma formation �from visual observation� duringhybrid CO2 laser welding with a helium content of 30%, arelatively high flowrate of shielding gas is required, whichmay create turbulence, disturbing the shielding of the weld.A helium content of 40%–50% with a relatively low flowrate produced acceptable welds. In this case, the processremains stable and the laser beam created a full penetrationinto the material. In addition, the gas flow required with thehigher composition was smaller in comparison. With ahelium content above 50%, the process became unstable:more noise was generated and the amount of spatterincreased, as illustrated in Fig. 3.

In GMA welding, a small addition �1%–2%� of carbondioxide to the shielding gas stabilizes the arc and reduces thesurface tension of the weld pool. This improves the fluidityof the weld and makes the transition between the weld andthe base metal smoother on both the bead face and root, thusreducing undercut.4 It was observed that in hybrid CO2

laser–GMA welding small additions of CO2 stabilize the arc,but above 10% the stability of the process suffers. Theaddition of carbon dioxide smoothed the transition of theweld metal with the base metal, but the amount needed mustbe minimized to maintain process stability.

When welding I-butt joints with a gap of 0.8 mm it wasobserved that with a carbon dioxide content of 0% or 2%,

FIG. 2. Orientations of the weldingheads in case of: �a� butt joints, �b�T-butt joints, front view, and �c� T-buttjoints, side view.

TABLE II. Principal processing parameters of the hybrid welding trails.

Laser power at the work piece 4.8 kW Stick-out 1.4 mmDiameter of focal point �0.6 mm Arc type Synergic pulsed arcFocal length 300 mm Slope angle of GMAW torch 55°Rotational angle of CO2 laser�T-butt joints�

15° Rotational angle of GMAWtorch �T-butt joints�

50°

Distance between the filler wireand the laser beam

�3 mm Diameter of gas nozzle 10 mm

Diameter of filler wire 0.8 mm

14 J. Laser Appl., Vol. 18, No. 1, February 2006 A. Fellman and V. Kujanpää

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the geometry of the weld pool fluctuated, resulting in shortlengths of undercut and root concavity particularly at the endof the weld. The problem was mitigated by raising thecarbon dioxide content to 5% or 10%, which produced asmooth weld bead transition with the base material �Fig. 4�and full penetration. An increase in carbon dioxide alsomade the transition of the weld metal with the base metalless abrupt when welding T-butt joints. However, process

instability increased above 10% CO2. With a content of20%, the process was very unstable, producing a lot ofspatter.

B. Weld cross section

The composition of the shielding gas affected the weldcross section as well as the performance of hybrid welding.

For close-fitting I-butt joints, when the helium contentwas increased the weld face became narrower, the rootwider, and the weld cross section smoother. This can be seenin the external view in Fig. 3 and in the weld cross sectionsin Fig. 5.

Figure 6 shows the above-mentioned effects in graphicalform. The differences in weld widths were larger when thefiller wire feed rate was higher; the higher feed rate the moremetal is present in the weld pool. With a gap of 0.8 mmbetween the plates, the differences in weld cross section withvarying amounts of helium were smaller. Similarly, the weldcross sections of the T-butt joints were not affectedsignificantly by changes in helium content.

When welding I-butt joints with shielding gascontaining varying amounts of carbon dioxide, trends similarto those with helium were observed, although the changeswere significantly more sensitive to carbon dioxide content�Fig. 7�. The changes in the width of the weld cross sectionscan also be seen from the specimens for macroscopicexamination in Fig. 8. In some cases, the increase in carbondioxide may increase the weld reinforcement. The amount of

TABLE III. Variable of the hybrid welding trials.

Butt jointsT-buttjoints

No gap Gap 0, 8 mm

Filler wire feed rate �m/min� 10 or 14 12 12Welding speed �m/min� 1·1 1.2 0.8Pulse current �mean� �A� 105–180 115–120 110–120Pulse voltage �mean� �V� 32–35 33–34 33–37

TABLE IV. Some of the parameters used in the welding of specimens fortesting according to EN-ISO 15614-11.

Gap 0, 0.8

Stick-out �mm� 15Filler wire feed rate �m/min� 12Welding speed �m/min� 1.1Pulse current �mean� �A� 116–122Pulse voltage �mean� �V� 33–34Shielding gas flow rate �1/min� 25

FIG. 3. Increase in spatter, notably at the weld root, with an increase inhelium content.

FIG. 4. Effect of carbon dioxide on weld smoothness in hybrid I-butt welds:2% CO2 left, 5% CO2 right; 40% He, balance argon. Photos taken adja-cent to weld termination.

FIG. 5. Effect of helium on hybrid weld bead cross section width in close-fitting I-butt joints: 30% He �left�, 80% He �right�; 2% CO2, balance ar-gon. Plate thickness 6 mm.

FIG. 6. Hybrid weld bead cross section width as a function of depth inclose-fitting I-butt welds of varying helium content; 2% CO2, balance ar-gon. �1� and �2� indicate filler wire feed rates of 14 and 10 m/min,respectively.

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carbon dioxide had a similar effect on the weld crosssections of T-butt joints �Fig. 9�. The weld cross sectionwidened slightly in the middle of the cross section and thetransition of the root with the base metal became a littlesmoother �Fig. 10�.

C. Weld quality

All welds were inspected visually initially, and weldingparameters were selected to qualify the weld in class B�stringent, highest quality� of the European standards EN-ISO 13919-1 and EN 25817.9,10 The most common weldimperfections in hybrid I-butt welds were excess weld metal

and excess weld penetration. These are not serious defects instatic loading, and may be reduced by reducing the fillerwire feed rate during welding or eliminated by grinding afterwelding. If a gap was present in the joint, undercuts and rootconcavities were observed when the shielding gas had a lowCO2 content. When the CO2 content was increased to asatisfactory level, no defects were visible. The weld defectsmost common for the hybrid T-butt welds were undercut anda lack of fusion in the middle of the weld cross section. Lackof fusion is a very serious defect, decreasing performance inboth static and dynamic loading, and can be very hard todetect without destructive testing.

Some of the I-butt welds were inspected usingradiography. Mostly there was only a small amount ofporosity and undercut observed. Many welds met therequirements of class B.9,10 Variations in helium and carbondioxide content appeared to influence porosity in the welds.The amount of porosity reduces as the helium or carbondioxide contents increase.

One batch of welds was subjected to a complete weldingprocedure quality testing. The welds showed no considerabledefects when examined visually, but radiography revealedporosity and localized undercutting. In some welds theporosity and undercutting, or both, lay above the acceptancelimit for a class B weld; these were classified to be of classC according to the standards �Table V�. One of the weldshad some lack of fusion and therefore was classified as Dclass weld. The test pieces for transverse tensile tests allfailed in the base metal �see Table VI�. All the specimensfulfilled the minimum ultimate tensile strength requirement�360 N/mm2� for base material. The joint strengthened thebase metal a bit �increase in ultimate tensile strength5–20 N/mm2� and therefore the elongation to fracture wasdiminished. The specimens for bend tests were taken bothtransverse and traverse to weld joint. Both weld bead androot specimens were bent 180° around the bending block ofdiameter 20 mm. All the specimens withstood the bendingwithout cracking. For impact strength tests there were threeimpact test specimens prepared for each section of weld andtemperature tested. Impact tests revealed that the welds hada ductile-brittle transition temperature of at least −60 °Cwith the requirement of impact strength at least 18 J �testpiece dimensions 5�10 mm�. Apart from a couple ofdeviations, welds and HAZs fulfilled even the impact energyrequirements of base metal in temperature −60 °C �for basemetal tested with 10�10 mm specimen impact strengthshould be at least 40 J in −20 °C�. The results of the impacttesting are presented in Fig. 11. The best impact strength

FIG. 7. Hybrid weld bead cross section width as a function of depth forshielding gases of varying carbon dioxide content; 40% He, balance argon.A gap of 0.8 mm was present in the joint.

FIG. 8. Effect of carbon dioxide on hybrid weld bead cross section width:0% CO2 �left�, 10% CO2 �right�; 40% He, balance argon. A gap of 0.8 mmwas present in the joint. Plate thickness 6 mm.

FIG. 9. Hybrid weld bead cross section width as a function of depth forT-butt welds using varying amounts of carbon dioxide; 40% He, balanceargon.

FIG. 10. Effect of carbon dioxide on hybrid T-butt weld cross-section width:1% CO2 �left�, 20% CO2 �right�; 40% He, balance argon. Plate thickness6 mm.

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was achieved in welds which were welded by using theshielding gases 60%Ar+40%He and 50%Ar+40%He+10%CO2. Hardness testing revealed that values were wellbelow the accepted limit of 350 HV in all sections of theweldment �Fig. 12�. The lowest average hardnesses aroundweld area were achieved by using 50%Ar+40%He+10%CO2 as shielding gas. Welding procedure testing

indicated that the changes in shielding gas composition didnot have a significant effect on the properties of a hybridCO2 laser–GMA weld.

V. DISCUSSION

A. Effects of helium content on weld properties

The hybrid welding trials indicate that heliumintroduced into the welding zone via the GMAW torch canbe used to suppress plasma formation by the CO2 laser beamwith steel plate of thickness 6 mm, using the processingparameters given. Therefore, a separate plasma suppressiongas is not needed, and the hybrid welding head can becompact, which improves accessibility. The omission of aseparate plasma suppression gas also reduces the totalamount of gas consumed. However, the lower the heliumcontent of the shielding gas, the larger the flow ratenecessary to prevent excessive plasma formation to achievefull penetration of the weld, and care must be taken not tointroduce turbulence into the weld zone.

TABLE V. The effect of shielding gas used into quality rating of the weld.

Shielding gasAirgap�mm�

Qualityclass Defects found in radiography

40%He+60%Ar 0 B evenly distributed porosity, local undercut40%He+60%Ar 0.8 D lack of fusion, evenly distributed porosity,

local undercut40%He+55%Ar+5%CO2 0 B evenly distributed porosity, local undercut40%He+55%Ar+5%CO2 0.8 C group of pores40%He+50%Ar+10%CO2 0 B evenly distributed porosity40%He+50%Ar+10%CO2 0.8 B evenly distributed porosity30%He+68%Ar+2%CO2 0 C evenly distributed porosity, local undercut30%He+68%Ar+2%CO2 0.8 C evenly distributed porosity, local undercut40%He+58%Ar+2%CO2 0 C evenly distributed porosity, local undercut40%He+58%Ar+2%CO2 0.8 B evenly distributed porosity, local undercut60%He+38%Ar+2%CO2 0 C evenly distributed porosity, local undercut60%He+38%Ar+2%CO2 0.8 B evenly distributed porosity, local undercut80%He+18%Ar+2%CO2 0 B evenly distributed porosity, local undercut80%He+18%Ar+2%CO2 0.8 B evenly distributed porosity, local undercut

TABLE VI. The results of the tensile tests made on welds made with dif-ferent shielding gas compositions.

Shielding gasRp0.2

�N/mm2�Rm

�N/mm2�A5

�%� Point of fracture

40%He+60%Ar 340 465 27 Base metal40%He+55%Ar+5%CO2 334 446 27 Base metal40%He+50%Ar+10%CO2 335 455 29 Base metal30%He+68%Ar+2%CO2 342 456 28 Base metal40%He+58%Ar+2%CO2 340 456 27 Base metal60%He+38%Ar+2%CO2 338 449 27 Base metal80%He+18%Ar+2%CO2 332 453 31 Base metal

FIG. 11. The effect of shielding gascomposition on impact strength.

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With a helium content of 40%–50%, the process isstable, the weld metal is well shielded, and the gas flow raterequired is relatively small. This is significant for theeconomics of welding: outside the United States helium isalmost five times more expensive than argon. As noted, ahelium content of 40%–50% is sufficient for steel ofthickness up to 6 mm. It may be possible that the heliumcontent should be higher for a greater plate thickness.

The larger the helium content of the shielding gas, themore energy can be contained in the energy source. Thismay explain why the weld cross section became narrower atthe weld face when using a shielding gas with high heliumcontent; a higher energy source is more penetrating. Theplasma-suppressing effect of helium also reduces theeffective size of the energy source, thus reducing thetendency for a “nail-head” weld bead profile. The smootherweld cross section is a result of the higher energy and higherarc pressure. The effects of the helium content on the widthof weld cross section are smaller when the filler wire feedrate is small in relation to the groove volume. This isbecause there is a smaller amount of weld metal in the weldpool.

Porosity was virtually eliminated when using a heliumcontent of 80%. The dynamics of the weld pool and a wideweld cross section formed facilitates the escape of gasbubbles from the weld pool. A hotter weld pool alsosolidifies more slowly, increasing the time available fordegassing. However, class B of the European workmanshipstandard could be achieved by using helium contents in therange 30%–50%.

B. Effects of carbon dioxide content on weldproperties

Benefits were obtained by adding carbon dioxide insmall quantities to the shielding gas. These originate fromthe ability of the gas to reduce surface tension and stabilizethe arc. A reduction in surface tension leads to a smoothertransition between the weld bead and the base metal. If thecarbon dioxide content is sufficiently high, weldreinforcement may be increased. Improved smoothness ofthe weld cross section is caused by an increase in the arc

pressure that occurs when carbon dioxide moleculesdissociate.6 An increase in weld reinforcement is normallyattributed to an increase in the feed rate of the filler fromatmosphere, but the type of process gas and its flow rate alsoaffect the arc characteristics, mode of metal transfer,penetration and weld bead, speed of welding, undercuttingtendency, cleaning action, and mechanical properties of theweld metal.4

Process gas influences the formation of ionized plasma,which reduces the energy of the laser beam incident on theworkpiece. Plasma formation occurs through interaction ofthe laser beam, heated metal vapor from the keyhole, and theprocess gas. The degree of absorption by the plasma formedabove the keyhole is dependent on the temperature and theionization potential of the process gas used.11 During CO2

laser welding, process gas may be introduced coaxially withthe laser beam or from a separate nozzle positioned to oneside of the laser beam. The advantage of coaxial gas deliveryis that the nozzle affords some protection to the optics fromspatter. The disadvantage of this shielding gas geometry isthat the plasma formed during welding may rise inside thenozzle, reducing the focusing ability of the laser beam. Awell-defined gas flow, delivered from the side of the beam, isan efficient means of preventing plasma formation becausethe process gas flow also assists in removing the plasmaformed.3

Traditionally, pure helium is used as the process gas inhigh power CO2 laser welding when the desired penetrationdepth is high and the welding speed is low, because deeper,narrower penetration may be achieved than, e.g., argon.12

Helium has a better ability to suppress plasma formationbecause its atomic weight is low and ionization potentialhigh �24.46 eV�. Helium can thus absorb more energybefore ionizing and promote unacceptable plasma formation.The high thermal conductivity of helium is beneficial forremoving heat during CO2 laser welding.3,13 Argon shieldsthe weld from oxidation efficiently, but a plasma formsrelatively easily because of the low ionization potential ofthe gas �15.68 eV�.3 Argon can be used as a component inthe process gas �thus reducing helium consumption� forautogenous CO2 laser welding in cases where the laser

FIG. 12. The effect of the shieldinggas composition on the hardness ofweld areas.

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power is limited or the welding speed is high. It has beennoted that the addition of small amounts of reactive gasessuch as carbon dioxide in the process gas may be beneficialthrough an improvement in weld interface geometry.12 Purecarbon dioxide is not recommended for use as a process gasin continuous wave �cw� CO2 laser welding since then avery dense plasma cloud is easily formed above thekeyhole.3

The main function of the process gas in GMA welding isto shield the molten weld metal from the atmosphere; henceit is often referred to as the shielding gas. Shielding isimportant because most metals have a strong tendency toform oxides or nitrides. In GMA welding, helium is usuallyused as a component of the shielding gas when weldingstainless steels. The use of helium enables a higher arcvoltage to be produced than with argon, and so the arc has ahigher energy. Thus helium is more effective for weldingthick materials than argon.7,5 When pure helium is used asthe shielding gas, the resulting weld bead has a low profile,and penetration is deep, wide, and parabolic in section.Recombination energy, i.e., the conversion of kinetic energyof the negatively charged electrons into heat at the positivepole, produces deeper penetration �or enables a higherwelding speed to be used�, and increases deposition rates.Reduced movement in the melt, a more energetic arc, and awide weld cross section facilitate escape of gases from theweld metal, and so the porosity of the resulting weld issmall. The use of helium also creates relatively large, hotweld pools, which extends the time for solidification anddegassing. In GMA welding, a higher helium contentenables the welding speed to be increased in comparisonwith shielding mixtures of argon and carbon dioxide.However, a high helium content destabilizes the arc,producing irregular metal transfer in large droplets andassociated heavy spattering. Additions over 70% in arcwelding result in unstable arcs and uncontrollable droplettransfer. Because helium has a high ionization energy, theuse of pure helium may cause arc ignition and arc stabilityproblems. Helium additions of less than 50% are insufficientfor the characteristic features of helium shielding gas to beeffective. Since helium has a low density, a 2–3-fold gasflow is required to ensure a similar level of shielding as withargon. The fact that helium is expensive outside the UnitedStates has resulted in a trend to reduce its use in shieldinggases.4,6,13

In GMA welding of mild steels, argon mixtures withcarbon dioxide or nitrogen are used as shielding gases. Therelatively high atomic weight and size of the argon atomproduce effective shielding of the weld. The resulting weldhas a lower penetration because argon has a lower thermalconductivity than helium. Less energy is required to ionizeargon in the arc, making it relatively easy to becomeelectrically conductive. Thus argon shielded arcs are easy toignite and are stable even at low voltages.6,13

Carbon dioxide may disassociate, resulting in anincrease in gas volume, which improves shielding of theweld. The arc pressure also increases, producing a widerweld cross section and higher weld bead. Afterdisassociation, atoms may recombine, generating energy that

may be used for welding. This additional energy input issuggested to increase the time for the gas bubbles to escapefrom the weld pool, reducing porosity in the weld. Althoughcarbon dioxide provides good shielding for the weld pool,the arc voltage is increased, using wire. Also, the gapbetween the plates affects the reinforcement: for the samefiller wire feed rate; if there is no gap, the level ofreinforcement is higher than when there is a small gap �e.g.,caused by the groove preparation method�.

When welding close-fitting I-butt joints, the amount ofCO2 in the shielding gas should be restricted to a fewpercent if a smooth weld bead transition is desirable. Processstability also suffers when the carbon dioxide content rises.This is because of the increase in arc pressure duringmolecular dissociation, which produces forces that throw themetal droplets away from the weld, creating spatter. Whenwelding I-butt joints containing a gap, the shielding gascontent should lie in the range 5%–10% to preventirregularities in the weld face, which cause undercut, and inthe root where concavities may form. The reduction insurface tension of the weld pool with an increase in carbondioxide content is the reason for this observation.

Porosity was observed to be lower with higher amountsof carbon dioxide. This may be a result of energy beingreleased during formation of CO2 molecules after theirdissociation. The conduction of heat into the weld metallengthens the cooling time and therefore helps the gasbubbles to escape before solidification.

When welding T-butt joints, the most common defectswere undercut and a lack of fusion. Undercuts were formedwhen the rotation angle of the GMAW torch was inclinedaway from the center of the angle between the plates. A lackof fusion occurred when the alignment of the laser beam wasincorrect.

VI. CONCLUSIONS

The following conclusions may be drawn from the re-sults of this investigation into hybrid CO2 laser–GMA weld-ing of C–Mn steel of plate thickness 6 mm in the I-butt andT-butt joint configurations:

�1� For maximum productivity, minimum gas cost, andhighest weld quality, attention should be paid to theshielding gas composition used.

�2� Providing that the GMA shielding gas contains a mini-mum of 30% helium, no additional plasma suppressiontechniques are required for the power levels and weldingspeeds used.

�3� The addition of about 5% CO2 to the shielding gas pro-duces a smoother transition from the weld reinforcementto the parent material, and also stabilizes the arc. Weldscan then be produced to meet the most stringent class Bof the current European workmanship standard for laserbeam welding of steels, even for joints containing asmall gap.

�4� The effects of variations in carbon dioxide content in theshielding gas on weld cross section are greater thanvariations in the helium content.

19J. Laser Appl., Vol. 18, No. 1, February 2006 A. Fellman and V. Kujanpää

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�5� Changes in the shielding gas composition do not seem tosignificantly affect the mechanical properties of thewelds.

ACKNOWLEDGMENTS

The authors would like to greatfully acknowledge finan-cial support provided by the National Technology Agency ofFinland �TEKES� and the Finnish metals industry. Thepeople at AGA are especially acknowledged for a loan of themixer and for providing information and support during theresearch. Thanks go also to Risto Laitinen from RautaruukkiOy for arranging mechanical testing at Rautaruukki and forproviding valuable advice during the research. Pertti Kokkoand Marko Junni are acknowledged for performing the weld-ing experiments in the Laser Processing Laboratory ofLappeenranta University of Technology �LUT�, and AnttiHeikkinen from the Metallography Laboratory of LUT forperforming metallurgical examination. Dr. John Ion is alsogreatfully acknowledged for checking the grammar and giv-ing valuable comments about the content of this article.

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20 J. Laser Appl., Vol. 18, No. 1, February 2006 A. Fellman and V. Kujanpää

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