www.elsevier.com/locate/apsusc

Applied Surface Science 253 (2007) 8050–8053

The influence of shielding gas in hybrid LASER–MIG welding

Giovanni Tani, Giampaolo Campana *, Alessandro Fortunato, Alessandro Ascari

Department of Mechanical Construction Engineering, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy

Available online 28 February 2007

Abstract

Hybrid LASER-GMAW welding technique has been recently studied and developed in order to meet the needs of modern welding industries.

The two sources involved in this process play, in fact, a complementary role: fast welding speed, deep bead penetration and high energy

concentration can be achieved through the LASER beam, while gap bridgeability and cost-effectiveness are typical of the GMAW process.

Particularly interesting, in this context, is the CO2 LASER–MIG welding which differs from the Nd:YAG LASER–MIG technique for the high

powers that can be exploited and for the good power/cost ratio of the process.

This paper is a part of a wide study on the hybrid CO2 LASER–MIG welding and investigates the influence of the shielding gas both on the

stability of the process and on the dimensional characteristics of the weld bead. Two different parameters have been taken into consideration in

order to develop this analysis: the shielding gas composition and the shielding gas flow.

The experiment, performed on AISI 304 stainless steel plates, has been planned exploiting design of experiment techniques. The results have

been analyzed through a statistical approach in order to determine the real influence of each parameter on the overall process.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Hybrid welding; Shielding gas; GMAW; Laser; Stainless steel welding

1. Introduction

Hybrid Arc-LASER welding technology has been thoroughly

studied and successfully applied [1–3] in the last few years. In

particular hybrid LASER Nd:YAG–GMAW process meets the

needs of automotive industry thanks to its easy implementability

on anthropomorphic robots, while LASER CO2–GMAW is

suitable for shipbuilding industry, but also for transport and

aerospace industry applied to panels manufacturing, thanks to its

characteristic high powers. On the other side the two welding

sources, coupled to perform an hybrid welding process, require a

fine tuning of both sets of technological parameters in order to

obtain a stable, repeatable and productive process.

According to this, many studies have been carried out

regarding power-related parameters [4,5] such as coupled arc

voltage and LASER beam power and on source positioning

related ones [6] such as defocus position and distance between

the sources, in order to trace out the basics regarding the

applicability of the process. More specific studies have been

carried out considering plasma interaction [7] and molten pool

* Corresponding author. Tel.: +39 0512093456; fax: +39 0512093412.

E-mail address: giampaolo.campana@unibo.it (G. Campana).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.02.144

fluid dynamics [8] with the aim of tuning the complex

equilibrium which stands behind this kind of processes.

The aim of this work is therefore to investigate the influence

of shielding gas composition and flow on the whole process. In

order to accomplish this goal the planning of the experiment

and the analysis of the results have been performed by means of

design of experiment techniques. This approach allowed to

clearly underline the real influence of the studied parameters on

the stability of the process and on weld bead geometry.

2. Experimental setup

The equipment used to carry out the experimental tests is

based on an EL.EN. C3000 FAF 3 kW CO2 laser source and on

a CEBORA Sound MIG 3840/T Pulse 380 synergic pulsed

GMAW generator. In order to couple the two sources a Binzel

ABIMIG automatic torch has been exploited and assembled on

the laser head of the CNC cell.

The experimental activity was carried out using AISI 304

stainless steel. The test specimens are couples of 15 mm �10 mm � 120 mm sticks tack welded on each end in a zero-gap

butt joint configuration. Every stick has been obtained by sawing

from an 8 and 10 mm thick plate in order to simulate a poor

G. Tani et al. / Applied Surface Science 253 (2007) 8050–8053 8051

quality edge preparation. Every specimen has been then welded

without any restraint or jigs in order to maximize the effect of

weld distortion.

Several parameters have been kept constant during the

whole testing:

� G

MAW torch inclination: 658. � W elding speed: 1 m/min.

� W

elding direction: MIG trailing.

� G

MAW source setting: synergic pulsed.

� L

aser power: 3 kW.

� A

ISI 308L 1.0 mm filler metal wire.

The shielding gas parameters have been varied:

Fig. 1. Characteristic dimensions of a weld bead obtained exploiting a 40% He

� T hree gas flow rates: 10, 30, 45 l/min. mixture and a 30 l/min flow rate. � T hree different gas compositions:

� 30% He–67% Ar–3% O2;

� 40% He–57% Ar–3% O2;

� 60% He–37% Ar–3% O2.

Previous experiments, carried out by the authors, allowed to

evaluate the following setup parameters which permit to

achieve good bead penetrations and geometry [9]:

� A

rc voltage: 20 V.

� D

istance between sources: 3 mm.

� L

aser beam focal position: 7 mm below the upper base metal

surface.

Fig. 2. Comparison between mean bead penetration depths as a function of gas

flow rate and composition.

3. Experimental

The experiments were split into two different phases: a

preliminary and a main one.

The first preliminary stage was aimed to investigate the

minimum helium percentage necessary to achieve a stable

process. Several tests have been carried out exploiting 10% and

20% He mixtures at different flow rates. In these cases a

consistent plasma plume above the molten pool caused the

absorption of a large portion of the laser energy with a negative

effect on bead penetration depths and on process efficiency.

Starting from 30% He gas mixtures the working condition

changed radically. The high ionisation energy of helium

together with the higher percentage of the gas in the mixture

allows to limit the plasma plume formation and consequently

the absorption of the laser beam. According to this outcome, in

the following part of the experiment 30%, 40% and 60% He gas

mixtures were exploited.

During the main experimental stage every seam was

produced exploiting a shielding gas environment characterized

by a specific flow rate/composition match. Every single trial has

been repeated twice in order to evaluate the repeatability of the

process and to free the results from external disturbance factors

as much as possible.

The average length of the obtained seams is 100 mm and

every bead has been cross-sectioned in correspondence of its

normal symmetrical plane in order to investigate the shape of

the melted zone. Every cross-section has been polished and

etched for observation with optical microscope and the bead

geometry has been characterized by measuring the following

parameters: penetration depths (D), widths (W) and reinforce-

ments (R), as shown in Fig. 1.

A two-way analysis of variance has been carried out on the

measured dimensions in order to get information regarding the

influence of the controlled process parameters and of the

external disturbance factors.

4. Results and discussion

Figs. 2 and 3 concern bead penetration depths and widths,

respectively. The plotted lines show the mean values between

the two repetitions for each helium percentage analyzed. These

graphs show that bead depth increases if helium percentage and

gas flow increase, as well as bead width.

Figs. 4–6 show the values related to bead depths for both

repetitions together with the respective mean value. By

comparing these graphs the evidence is that at higher helium

percentages, 60%, the dispersion of results becomes more

important. This is, probably, due to the destabilizing effect of

helium on the GMAW arc especially in a pulsed mode.

Fig. 3. Comparison between mean bead widths as a function of gas flow rate

and composition.

Fig. 5. Graph of bead penetration depths behaviour exploiting a 40% He gas

mixture.

Fig. 6. Graph of bead penetration depths behaviour exploiting a 60% He gas

mixture.

Table 1

Analysis of variance of bead penetration depth results

G. Tani et al. / Applied Surface Science 253 (2007) 8050–80538052

The two-way ANOVA summarized in Table 1, puts in

evidence that the role of disturbance and uncertainty factors

is not negligible if compared to the role of the investigated

parameters. In fact both the calculated F values for gas flow

(F = 1.41) and composition (F = 4.23) are lower than

Fcrit = 9.55, which is the critical value of this experiment.

This means that the null hypothesis cannot be rejected and

that the variations of bead width and depths could be due

only to external factors rather than to the controlled

parameters. Regarding bead width a similar conclusion

can be drawn, in fact data shown in Table 2, confirm the same

results obtained for bead penetration. The above-mentioned

considerations underline that, in hybrid Laser CO2–GMAW

welding, the shielding gas plays an important role on the

feasibility of the process since a minimum helium percentage

– equal to 30% – is needed to obtain a suitable welding

environment. On the other hand, the experiments demon-

strate that higher helium content mixtures as well as high

flow rates yield to progressively unstable GMAW arcs and, in

particular, they do not affect significantly bead geometry and

bead depths. This experimental outcome is rather positive in

terms of process cheapness since helium is an expensive

component.

Fig. 4. Graph of bead penetration depths behaviour exploiting a 30% He gas

mixture.

In terms of process fine tuning easiness a low helium content

environment does not affect the stability of the pulsed-arc metal

depositions; this allows to exploit factory default synergic

curves on the GMAW source instead having to implement

customized ones.

Source DF SS MS F P

Composition 2 0.0481 0.0240500 1.41 0.294

Flow 2 0.1447 0.0723500 4.23 0.051

Interaction 4 0.1450 0.0362500 2.12 0.161

Error 9 0.1540 0.0171111

Total 17 0.4918

Table 2

Analysis of variance of bead width results

Source DF SS MS F P

Composition 2 0.44924 0.224622 4.43 0.046

Flow 2 0.12314 0.061572 1.21 0.341

Interaction 4 0.06272 0.015681 0.31 0.865

Error 9 0.45625 0.050694

Total 17 1.09136

G. Tani et al. / Applied Surface Science 253 (2007) 8050–8053 8053

5. Conclusions

Hybrid CO2 LASER–GMAW welding process yields a great

industrial interest thanks to its large applicability and

versatility. In order to obtain a stable and efficient process,

the role of the shielding gas must be taken into consideration:

� A

minimum helium content, equal to 30%, must be exploited

to limit plasma formation and consequently a low laser power

absorption.

� A

30 up to 40% helium content gas mixture allows to exploit

factory default synergic curves in the GMAW sources and it

grants a good process feasibility.

� H

elium content above 40% yields to unstable arc conditions

and does not lead to a significant increase of bead penetration

depth.

� A

shielding gas flow between 10 and 30 l/min is enough to

grant a suitable cost-effective welding environment.

References

[1] H. Staufer, Laser hybrid welding and laser brazing: state of the art in

technology and practice by the examples of the Audi A8 and VW Phaeton,

in: Proceedings of the 3rd International WLT Conference on Lasers in

Manufacturing, 2005, pp. 203–208.

[2] C. Fuhrmann, D. Petring, R. Poprawe, Recent results on high power CO2

and Nd:YAG laser-arc welding of thick steel plates, in: Proceedings of the

3rd International WLT Conference on Lasers in Manufacturing, 2005, pp.

185–192.

[3] U. Dilthey, S. Olshock, Modern beam welding technologies in advanced

pipe manufacturing, in: Proceedings of the 3rd International WLT Con-

ference on Lasers in Manufacturing, 2005, pp. 163–169.

[4] S. Katayama, Y. Naito, S. Uchiumi, M. Mizutani, Penetration and porosity

prevention mechanism in laser-arc hybrid welding, in: Proceedings of the

3rd International WLT Conference on Lasers in Manufacturing, 2005, pp.

193–198.

[5] M. El Rayes, C. Walz, G. Sepold, The influence of various hybrid welding

parameters on bead geometry, Weld. J. (2004) 147–153.

[6] Y.S. Park, H.S. Kim, Y.S. Lee, J.K. Kim, J.H. Shin, S.J. Ha, C. Kim, Study

on the welding variables according to offset and separation using hybrid

laser welding, in: Proceedings of the 3rd International WLT Conference on

Lasers in Manufacturing, 2005, pp. 199–202.

[7] M. Katsuna, L. Chen, Interaction of both plasmas in CO2 Laser-MAG

hybrid welding of Carbon steel, Proc. SPIE 4831, 2003, 341–346.

[8] A. Fellman, A. Salminen, V. Kujanpaa, A study of the molten filler material

movements during CO2–laser–MAG hybrid welding, in: Proceedings of the

3rd International WLT Conference on Lasers in Manufacturing, 2005, pp.

171–176.

[9] G. Tani, L. Tomesani, G. Campana, A. Fortunato, A. Ascari, The influence

of arc transfer mode in hybrid Laser-MIG welding, in: Proceedings of the

International Conference of AMPT 2006, Las Vegas, USA.