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Influence of GMAW Shielding gas in productivity and gaseous emissions

I. Pires*, T. Rosado, A. Costa, L. Quintino

*Instituto Superior Técnico - Lisboa

Address: Av. Rovisco Pais, 1049-001 Lisboa – Portugal

Email: inespires@ist.utl.pt

ABSTRACT

The development of shielding gases for GMAW applications has been of

increasing interest and importance for three mainly reasons: to improve the

productivity of the process, to increase weld integrity and quality, and to

reduce the healthy and safety problems due to fume and particle emissions.

The present paper outlines the influence of seven shielding gas mixtures

(Ar+2%CO2, Ar+8%CO2, Ar+18%CO2, Ar+5%O2, Ar+8%O2,

Ar+3%CO2+1%O2) on the weld bead profiles, which are directly related to

the productivity, using fillet joints.

Fume emissions, where analysed for one of the most common shielding gas

used by industry, 82%Ar+18%CO2 with three wire diameters in bead-on-

plate deposits.

Keywords: Shielding gas mixtures, productivity, welding fumes

INTRODUCTION

The Gas Metal Arc welding (GMAW) process has been of great importance

for welding construction all over the world. This fact is related to its high

flexibility, which allows the welding of different materials and thickness, and

to its considerable potential for automation and robotization [1].

The development of welding shielding gas mixtures in recent years has been

based on the need to establish a stable arc, to obtain a smooth molten

metal transfer and to reduce fume emissions; this will improve process

performance, productivity and control, and will reduce the risk of fusion

defects. The most common defects in GMAW, which affect both productivity

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and quality, essential factors when considering automation and robotization,

are strongly dependent of the shielding gases and mixtures used.

Improvement in productivity of welding processes is one of the growing

tendencies nowadays in welding technology. It has a direct impact on the

reduction of production costs, particularly in enterprises for which welding

processes are the main method of production. Producers aim at the

assurance of favourable economic rates, which influence both production

profitability and the competitively. In parallel with constantly increasing

mechanization and automation rate, there is an increase in the application

of modern welding consumables (filler metals and shielding gases), for

welding speed increase or for general effectiveness of welding improvement

[2,3].

Shielding gases present different physical and chemical properties, such as

thermal conductivity, ionization energy and chemical activity, which affect

the arc behaviour, and consequently the weld bead profiles. It is well known

the important role of oxygen and carbon dioxide in the reduction of the

surface tension of the molten metal, promoting a good wetting of the parent

metal. This behaviour is due to the formation of oxide films which have

lower values of surface tension (0.2 – 0.26 Nm-1) comparatively to the

parent metal (1.7 – 1.9 Nm-1). The welds made with these shielding gases

present good profiles and smooth surfaces, giving a high fatigue strength to

the joint [4].

The surface tension gradients T∂∂γ , where γ is the surface tension of the

molten metal, and T the temperature, have an important role on the

amount of heat transmitted to the liquid/solid interface, and hence on the

shape of the weld bead and penetration. Heiple and Raper [5] suggested

that the gradient signal influences the molten metal flow direction. When

the surface tension gradient is negative, the correspondent flow is

conducted towards the lateral walls of the welding pool (figure 1a).

However, the addition of oxidizing components can invert the gradient

signal thus changing the molten metal flow in the weld pool, in such a way

that the flow at the surface is directed to the centre of the weld pool (figure

1b), which gives rise to deeper welds.

The penetration depth was found to be related with the “arc strength”,

which is defined by the following equation [4]:

3

21

251057.3

arc

arcl

IxxF −= Equation 1

Where I is the current intensity and larc is the arc length.

Figure 1. Convection in the weld pool with a) Negative surface tension gradient, b)

Positive surface tension gradient [4].

Despite the advances in welding automation and control technology, welders

are still exposed to the welding fumes and hazardous gases. Furthermore,

usually the amount of fumes generated during welding increases with the

increase of productivity.

The amount of fumes produced during welding depends on the welding

procedure, chemical composition of the shielding gas, filler metal and base

material, presence of coatings, time and severity of exposure and

ventilation [6,7]. Therefore, it is necessary to study the influence of welding

parameters on the fumes released, so welders can work in a healthier

environment.

Any material is a potential source of fume when heated to a high

temperature. The fumes released during GMAW are the result of the hot

welding wire, the droplets that are transferred from the wire tip to the weld

pool and the weld pool itself [8]. Welding fume consists of metal oxide

particles that are formed during welding. These particles are small enough

to become and remain airborne and are easily inhaled. Steels contain

alloying elements that, in their pure forms, could be hazardous to worker

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health if inhaled or ingested. Steels contain manganese, element which

could influence the Parkinson disease [9].

EXPERIMENTAL PROCEDURE

Weld bead profiles

In order to study the influence of seven shielding gas mixtures (Ar+2%CO2,

Ar+8%CO2, Ar+18%CO2, Ar+5%O2, Ar+8%O2, Ar+3%CO2+1%O2) on the

weld bead profile and consequently on the productivity of the GMAW

process, welds were made using, for each shielding gas, two different wire

feed speed. The parameters used during the tests are illustrated in Table 1

PARAMETERS

Electrode AWS E 70 S-6 Electrode diameter (mm) 1.2 Electrode extension (mm) 16 Gas flow (l/min) 15 Parent metal thickness (mm) 6 Welding speed (mm/min) 150 Wire feed speed (m/min) 6, 7

Table 1- Tests conducted during experimental work

A conventional power supply, ESAB LAN 400 was used to conduct the study.

The torch was hold by simple mechanised system.

A computer equipped with an analogue-to-digital (A/D) conversion board

was used to sample the current, the voltage and the wire feed speed during

welding (figure 2).

5

D A T AA C Q U IS IT IO N

S Y S T E M V

I

W .F .S

P O W E RS U P P L Y

W IR E F E E DS P E E D U N IT

Figure 2- Scheme of the welding monitoring system [1].

Fume emissions

Before analysing the influence of shielding gas mixtures on the fumes

produced during welding, it was firstly conducted a study on the influence of

the wire diameter and current intensity on fume formation rate, with a

92%Ar+18%CO2 shielding gas mixture, in order to understand the

importance of these factors on the amount of fume generated. This gas was

chosen for the first set of trials as it is a very commonly used in industry.

The results obtained set values for comparison with other gas mixtures.

For this purpose several beads on plate were made using a mild steel wire,

with three different diameters, on 8mm thick plates. A summary of the

parameters used for these tests is indicated in table 2.

Filler wire diam. [mm] 1,6mm Welding current [A] 150 220 300 400

Filler wire diam. [mm] 1,0mm

Welding current [A] 150

220

300

Filler wire diam. [mm] 0,8mm Welding current [A] 80 150 220

Table 2 - Welding parameters used for measuring the fume formation rate

Fume formation rate (FFR) was measured using the standard procedures

contained in EN ISO 15011-2. For this, a fume chamber was built (Figure 3).

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A turntable was used, upon which the plates were fixed. The extraction rate

used was of 100 cm3/h. Three tests were executed for each trial condition,

and the average value calculated.

Figure 3 - Fume chamber used in the experimental procedure, where: 1. Fume box; 2. Guided welding gun fixture; 3. Air flow probe; 4. Extraction; 5. Sampling tubes; 6. CO analyser; 7. NO-NO2 analyser; 8. Micromanometer; 9. Recorder. [10]

The fumes emitted were collected on pre-weighted glass fibre filters

(Whatman GF/A), which were then reweighted to give the total weight of

fumes produced. The weight was then used along with the arc time to

calculate fume formation rate (FFR). In these experiments, arc time

employed was 60 seconds. For the purpose of this work, the FFR is defined

as the weight of fume generated per unit of arc time and is quoted in g/min.

RESULTS AND DISCUSSION

Weld bead profiles and Productivity

The productivity of the GMAW process, as well as the mechanical properties

of the welded joint are related and influenced by the shape of the weld

beads, with deeper welds enhancing the productivity, through the use of

higher welding speeds.

After the metallographic analysis of the fillet welds the following parameters

were measured: wet angle, reinforcement, penetration and dilution rate, for

each shielding gas mixture and for short-circuit and spray transfer. These

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factors, in combination, are a measure of productivity. To achieve higher

productivity it is desirable to maximise weld penetration, as well as the

amount of filler metal that has been deposited in the joint.

Reinforcement and wet angle

Surface tension has a significant influence on the wet angle and weld

reinforcement, and it decreases with the increase of the oxidant potential of

the mixture (which reflects the amount of oxygen present in the mixture).

Shielding Mixture Oxygen equ. (%)

Ar+2%CO2 0.8

Ar+8%CO2 3

Ar+18%CO2 7.2

Ar+5%O2 5

Ar+8%O2 8

Ar+3%CO2+1%O2 2.2

Ar+5%CO2+4%O2 6

Table 3 - oxygen equivalent for each shielding gas mixture

In figure 4, which represents the evolution of the reinforcement with

different shielding gas mixtures, with short-circuit and spray transfer, it can

be seen that the reinforcement decreases with the increase of the oxidant

content of the mixture, for both transfer modes, with the Ar+8%O2 mixture

presenting the lower values. This fact is related, as explained above with the

decrease of the weld pool surface tension, promoting a more fluid weld pool.

This is also the reason why the lower wet angles are obtained with more

oxidant shielding mixtures.

nfo

rcem

ent

(mm

)

8

Figure 4 – Evolution of the reinforcement with different shielding gas mixtures, with

short-circuit and spray transfer.

It should be noted that the welds obtained with spray transfer show a

concave shape, with an abatement of the weld (i.e. welds with negative

reinforcement). These results are related to the decrease of the weld pool

surface tension, as a consequence of an higher arc temperature related to a

higher heat input (the surface tension decreases with the increase of

temperature), a more fluid weld pool and to the transfer mode itself (the

droplets collide into the weld pool at a higher speed). The same trends were

observed when analysing the wet angle (see figure 5).

Figure 5 – Evolution of the wet angle with different shielding gas mixtures, with

short-circuit and spray transfer.

Weld penetration

shielding gas

wet

angle

(º)

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In what concerns weld penetration, it can be seen (figure 6) that it

increases with the increase of CO2 content in the mixture, both in binary and

ternary shielding mixtures. This behaviour can be explain by the arc force

(eq. 1), which relates the deep of penetration, with the arc length and

current intensity.

For the same current intensity, this force increase with the decrease of the

arc length. Previous work done by the authors [1] allowed to conclude that

for the same current intensity, the arc length decreases with the increase of

CO2 content in the mixture. Hence, mixtures with higher amounts of CO2 will

lead to higher arc force and consequently deeper penetrations.

In opposition, with the binary mixtures with O2 it is observed a tendency for

decrease of penetration with the increase of this element in the mixture.

The reason of this behaviour is related to the higher arc temperature

associated to the Ar+8%O2 mixtures when compared to the Ar+5%O2,

leading to a higher arc length and consequently to a lower arc force.

Figure 6 – Evolution of the penetration with different shielding gas mixtures, with

short-circuit and spray transfer.

It should be noted that argon oxygen mixtures present finger tip

penetrations, especially with spray transfer (figure 8). The reason for this

phenomenon is best understood with an analysis of the arch shape and

approximate distribution of temperatures (figure 7).

shielding gas

Penet

ration (

mm

)

10

Figure 7 – Temperatures distributions (approximation) in an Argon/oxygen arc

(left) an in a argon/carbon dioxide (right)

Figure 8 – Fillet weld obtained with an Ar+5%O2 shielding mixture (heat

input=38.56J/mm)

For the binary mixtures Ar+CO2 it is also observed an increase of the lateral

penetration with the increase of CO2 content (figures 9 and 10), which is

noted for both spray and short circuit transfer. This increase is related not

only to a more uniform temperature distribution (figure 7), but also to the

increase of the surface tension gradient.

Although this last factor is also patent with the increase of O2 amount in the

mixture, its effect is little in comparison with the decrease of penetration

due to the increase of arc length and shape.

on (

mm

)

Short-circuit

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Figure 9 – Evolution of penetration with different shielding gas mixtures, with

short-circuit

Figure 10 – Evolution of penetration with different shielding gas mixtures, with

spray transfer

Figure 11 – Fillet weld obtained with an AR+8%CO2 shielding mixture (heat

input=41.4 J/mm)

shielding gas

Penet

ration (

mm

)

Spray

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The combination of Ar, CO2 and O2 proved to be very efficient, leading to

weld beads with deep penetrations and relatively flat shapes. With spray

transfer, positioning of the weld in the joint present higher difficulties, with

misalignments occurring in some cases (figure 12).

Figure 12 – Fillet weld obtained with an AR+5%CO2+4%O2 shielding mixture (heat

input=42 J/mm)

Dilution Rate

The effect of shielding gas mixture on dilution rate is similar to the

penetration (figure 13), i.e. the dilution rate increases with the increase of

CO2 content in the mixture, and decreases with the increase of O2 content in

the mixture.

Figure 13 – Evolution of the dilution rate with different shielding gas mixtures, with

short-circuit and spray transfer.

As the CO2 content in the mixture increases, it is observed an increase of

lateral penetration and decrease of the reinforcement. This results in a

dilu

tion r

ate

(%)

shielding gas

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higher amount of parent metal that has been melt, and consequentially in a

higher dilution rate (figure 14). For the Ar+O2 mixtures the opposite is

observed. Besides the penetration decrease with the increase of O2 content

in the mixture, this penetration has a finger tip shape, which means that the

amount of parent metal that has been melted is reduced, and

consequentially also the dilution rate (figure 15).

Figure 14 – Behaviour of the increase of CO2 content in the amount of parent metal

that has been melt.

Figure 15 – Behaviour of the increase of O2 content in the amount of parent metal

that has been melt

These results can be used to advice GMAW users relatively to best shielding

gas mixtures to apply. Mixtures with Ar+8%O2 should be avoided, because

they don’t assure the desire quality. The penetration is low, the welds

present under-cuts and the finger tip penetration makes the joint alignment

more critical.

The Ar+18%CO2 mixture give rise to higher penetration with a good

agreement with the parent metal. However has the disadvantage of being

more sensitive to parameters regulation and lead to more spatter.

The combination of O2 and CO2 in the ternary mixtures leads to flat welds

with good penetrations associated to lower spatter.

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Fume emissions

The reduction of welding fumes is necessary to improve the shop floor

conditions for welders thus reducing the sick leave (both short term and

long term) caused by welding fumes. This reduction is a complex problem

which involves the technological control of fume emissions at the source

(welding parameters, wire composition and diameter, shielding gas

composition).

Figure 16 illustrates the variation of fume formation rate with current

intensity for three different wire diameters.

0,000,020,040,060,080,100,120,140,160,180,20

0 100 200 300 400I(A)

FFR (g/m

in)

0.8 wire 1.0 wire 1.6 wire

FFR (1.0 mm) = 0,0268e0,0057(I)

FFR(1.6mm) = 0,0164e0,0051(I)

FFR (0.8 mm) = 0,019e0,0102(I)

Figure 16 - Variation of fume formation rate with current, using three different wire

diameters

The equations proposed trough are only valid within the range of

parameters tested, and will be used as reference for comparison of FFR with

other mixtures, in experiments undertaken with similar welding procedures

[2].

The pattern observed in the curves above (figure 16) is similar for all the

diameters; in general fume formation rate increases with the increase of

current intensity, as a result of a higher arc temperature.

On table 4, it can also be observed that for the same current intensity, the

lower diameters lead to higher fume formation rates, this is probably due to

the fact that, for the same current intensity, the current density increases

with the decrease of the wire diameter, leading to the formation of higher

amounts of fumes.

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Wire Diameter

I (A) CO (ppm)

CO2 (ppm)

NO (ppm)

NO2 (ppm)

NOx (ppm)

80 46 4000 1 0 1

150 88 4000 0 0,2 1 0,8 mm

220 140 4100 0 0,4 0

150 67 4200 0 0,2 1

220 119 4000 0 0,2 1 1mm

300 138 4100 1 0,9 2

150 63 4000 0 0 0

220 87 4500 0 0 0

300 117 4300 1 0,2 1 1,6 mm

400 125 3900 0 0 0

Table 4 - Influence of current intensity and wire diameters on the amount of Gases

formation (ppm) for AR+18%CO2 shielding gas mixture

In what concerns gas emissions the same trend is observed relatively to

fume formation rate. In general gas emissions increase with the increase of

current intensity and decrease with the increase of wire diameter, for the

reasons explained above.

CONCLUSIONS

This paper presents experimental data about the influence of the operating

parameters and shielding gas mixtures on the weld bead profile. The

influence of the wire diameter and current intensity on fume formation rate,

with a 92%Ar+18%CO2 shielding gas mixture was analysed.

From the obtained results it can be concluded that:

1- The weld shape becomes less favourable as the O2 content in the

mixtures increases, i.e. penetration decreases and under-cuts are more

frequent. The decrease in penetration is related to the higher arc length,

associated with O2 mixtures. The reinforcement also decreases, as result of

the surface tension decrease.

2- The welds made with O2 shielding gas have a finger tip penetration

profile, in spray transfer as result of a non uniform arc temperature

distribution.

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3 – The welds made with Ar+CO2 mixtures give rise to high lateral

penetrations that decreases with the decrease of CO2 content in the

mixture.

4 - The welds made with ternary mixtures presented good bead shapes

specially when using short circuiting transfer. Most of these mixtures

compared with CO2 binary mixtures needs to be analysed in a case by case

approach.

5 – Mixtures with higher contents of CO2 tend to increase GMAW

productivity. These mixtures could be more efficient when thinking of

robotization, but arc stability needs to be address also.

6- Fume formation rate and gases emissions increase with the increase of

current intensity and decrease with the increase of wire diameter. The

results indicate an exponential increase of FFR with current, being the

exponential factor higher for smaller diameter wires.

REFERENCES

[1] Pires I., “Analysis of the influence os shielding gas mixtures on features of

MIG/MAG”, MSc Thesis, Lisbon Technical University, 1996, (only available in

Portuguese).

[2] D1.5, “Report on solutions to increase the welding speed at GMAW,

ECONWELD, COLL-CT-2005-516336, Economically welding in a healthy

way”, 2007

[3] G. Wang, P.G. Huang, and Y.M. Zhang,” Numerical Analysis of Metal

Transfer in Gas Metal Arc Welding”, Metallurgical and Materials Transactions

B; Vol ( 34B), 2003

[4] Jonsson P.G., Murphy A. B., Szekely., “The influence of oxygen aditions

on Argon-shielded gas metal arc welding processes”, Welding Journal,

74(2), 1995

[5] Lancaster J.F., “The Physics of welding”, International Institute of

Welding, Pergamon Press, 1986

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[6] Voitkevich V., “Welding fumes – formation, properties and biological

effects”, Abington Publishing, 1995

[7] Pires I., Quintino L., R. Miranda; “Analysis of the influence of shielding

gas mixtures on the Gas Metal Arc Welding metal transfer modes and fume

formation rate”, Materials and Design 28, 2007, pp 1623–1631

[8] Pires I., Quintino L., R.M Miranda, Gomes J.;” Fumes Emission during

Gas Metal Arc Welding”; Toxicological and Environmental Chemsitry, Volume

88, Number 3 / July–September 2006, pp 385-394.

[9] IIW Statement on Manganese; International Institute of Welding; 2005

[10]EN ISO 15011-2 , Health and safety in welding and allied processes –

laboratory method for sampling fume and gases generated by arc welding –

Part 2: Determination of emission rates of gases, except ozone (ISO 15011-

2:2003)