Post on 17-Jan-2023
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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).
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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
)
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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)