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Characterization of GMAW arc instability phenomenarelated to low oxidation potential shielding gasesMaria Celeste Monteiro de Souza Costa a , Cícero Murta Diniz Starling b & Paulo JoséModenesi ca Federal Centre of Technological Education of Minas Gerais, Coordination of Mechanics,Belo Horizonte, Minas Gerais, Brazilb Department of Materials Engineering and Construction, Federal University of Minas Gerais,Belo Horizonte, Minas Gerais, Brazilc Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais,Belo Horizonte, Minas Gerais, BrazilPublished online: 09 Feb 2010.

To cite this article: Maria Celeste Monteiro de Souza Costa , Cícero Murta Diniz Starling & Paulo José Modenesi (2010)Characterization of GMAW arc instability phenomena related to low oxidation potential shielding gases, Welding International,24:3, 214-221, DOI: 10.1080/09507110902843859

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Characterization of GMAW arc instability phenomena related to low oxidation potentialshielding gases

Maria Celeste Monteiro de Souza Costaa1, Cıcero Murta Diniz Starlingb2 and Paulo Jose Modenesic3

aFederal Centre of Technological Education of Minas Gerais, Coordination of Mechanics, Belo Horizonte, Minas Gerais, Brazil;bDepartment of Materials Engineering and Construction, Federal University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil;cDepartment of Metallurgical and Materials Engineering, Federal University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil

(Received 11 July 2007; final version received 20 August 2008)

Characterizing instability phenomena in arc welding may be a complex task, since they depend on a number of interrelatedfactors. Such phenomena may manifest themselves in various ways, acting on the characteristics of the arc, the metaltransfer, the amount of splatter and fumes formed, the arc format, the geometry of the bead and other aspects of welding. Theliterature on the subject reports diverse forms of instability associated with arc welding with solid wire and gas shielding(GMAW). This work aimed to define the instability phenomena associated with the GMAW process in welding with gaseswith a low oxidation potential. To do so, the welding current and voltages were monitored and metal transfer was filmed athigh speed. The welding tests were carried out on plates of common carbon steel with a source operating at a constantcurrent and an electrode with positive polarity. By synchronizing the filming with the welding current and voltage signals,the results showed that, during the periods of highest voltage, the transfer tended to be globular repulsive, as opposed tospray transfer during the lowest voltage periods. The synchronization of the electrical signals with the optical sensorindicates that, during periods of unstable operation, the arc is more luminous. It also showed that more fumes were generatedduring these periods.

Keywords: GMAW; arc instability; arc physics; shielding gases

1. Introduction

In welding with a consumable electrode, particularly in

GMAW welding, the behaviour of the arc root has a

significant effect on the stability of the process. In welding

steels, in general, when oxidizing gases are added to an

initially inert shielding gas, a reduction in the mobility of

the arc root is seen, which improves the stability of the

process1. This effect is associated with recomposition of

the layer of oxide close to the arc root, which is continually

destroyed by the electrons being emitted by the arc2.

Obviously, various other aspects of GMAW welding, such

as the conditions for transferring the addition metal,

exercise an important influence on arc behaviour.

In the literature on the subject, there are a number of

studies related to instability in the GMAW process,

focussed on the changes in metal transfer during welding.

In 1975, Lucas and Amin confirmed that there were

alterations in the transfer method associated with the level

of deoxidation of the steel wired. When these were less

deoxidized, the gases generated (possibly CO and CO2)

caused explosions in the drops of liquid metal during the

transfer, which would not occur with wires with a higher

level of deoxidation3. In 1981, Agusa et al. studied the

effect of adding cerium to the wire, with the aim of

improving the stability of the arc in welding carbon steel

with pure argon shielding gas. The authors confirmed that

repulsive globular transfer occurred in conditions where

only spray transfer would be expected4. Roswell, in 1985,

in a study of carbon and stainless steels, looked into

disturbances in the arc, including sudden changes in their

length, in the welding current and variations of the

geometry of the weld bead in the GMAW process

operating with spray transfer. These were associated with

the occurrence of contamination on the surface of the wire

and in the contact tip5. In conditions of GMAW welding

with short circuit transfer, the process usually presents

lower stability than with spray transfer. For this condition,

various studies were made about the stability level of the

process with variations in electrical signals6 – 8.

Modenesi and Nixon, in 19949, described, for GMAW

welding of low carbon steels, a phenomenon of instability

associated with the oxidation potential of the shielding

gas. This phenomenon was shown by disturbances in the

form of the arc, in the current and welding voltage levels

and in the metal transfer which change from spray to

repulsive globular. This will depend on various factors,

such as the arc length, the composition of the shielding gas

and the characteristics of the welding source, and also

seems to depend on the welding time. In welding with

constant voltage sources, when the process began, the

ISSN 0950-7116 print/ISSN 1754-2138 online

q 2010 Taylor & Francis

DOI: 10.1080/09507110902843859

http://www.informaworld.com

Welding International

Vol. 24, No. 3, March 2010, 214–221

Selected from Soldagem & Inspecao 2008 13(3) 181–189

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operation was unstable, with repulsive globular transfer

and a high degree of splash formation and then became

stable, after a period of time that would depend on the

welding conditions. Major fluctuations in the arc length

and in the current levels were seen during the operations’

transition from unstable to stable, resulting in changes that

can be seen easily in the process voltage and current

oscillograms (Figure 1).

The phenomenon was also studied with constant

current sources, changing the welding conditions by

varying the contact tip to work piece distance (CTWD).

Note that instability occurred for lower CTWD values

(which would correspond to a shorter arc length). In this

condition, along with alterations in the transfer method and

in the appearance of the arc, there were wide variations in

the welding voltage, with more frequent voltage peaks in

conditions of the greatest instability, Figure 2. The voltage

peaks were reduced when the stability of the process was

improved, for example by increasing the arc length or using

a gas with a higher oxygen level. To explain these and the

characteristics of the phenomenon, Modenesi and Nixon9

proposed that it was associated with changes in the

mechanism by which Eh electrodes were emitted from the

cathode. As a result, in the present work, this form of

instability will be known as cathodic mechanism instability.

Although the phenomena referred to have a strong

influence on the operating conditions for GMAW welding,

they have hardly been studied since their initial description.

This can, in part, be explained by the use in welding carbon

steel of shielding gases with a sufficiently high oxidation

potential to prevent instability being shown in welding with

spray transfer. However, recent studies10 – 12 indicate that

this instability may occur in other forms of transfer, even

with shielding gases, generally used in welding common

steels and that it can also occur in welding with stainless

steel and aluminum. In this context, this work forms part

of a project that is intended to complete analysis of the

aspects of this phenomenon. Its specific objective is to

prove that this phenomenon occurs, using the most

advanced techniques of controlling welding processes.

2. Materials and methods

The test pieces to be used, which were 50 £ 250 mm,

were removed from SAE 1020 carbon steel plates that

were 12.5 mm thick. The layer of build-up was removed

manually and, prior to the start of welding, each test piece

was brushed with a rotary brush and cleaned with acetone.

The tests were carried out by depositing weld beads on

the plates, using a mechanized GMAW process with a

computerized welding source linked to a secondary source

supplying a constant current. The addition metal used was

AWS ER 70S6 wire, 1.2 mm in diameter. Pure argon and

Ar þ 1% O2 were used as the shielding gases. To carry

out the tests, alterations were made to the CTWD in order

to verify the influence of the arc length on instability in

cathodic mechanisms. In all the tests, the voltage, welding

current and, in some tests where pure argon was the

shielding gas, the variation in luminosity were recorded by

using a digital data acquisition system and processed with

the SINAL programme (LABSEND–UFMG). The lumin-

osity of the arc was measured with a photodiode placed

around 40 cm from the arc, using the feature proposed by

Miranda13. Tables 1 and 2 set out the adjusted parameters

used to carry out the tests. It can be seen that the CTWD

values used were different for the two shielding gases.

This was due to the different instability tendencies

associated with these gases. As the process with the

Ar þ 1% O2 mixture had a lower tendency to be unstable,

Figure 2. Voltage levels for different CTWD distances obtainedin the tests with the source operating at a constant voltage (pureargon shielding gas and a current of 255 A)9.

Figure 1. Typical result of tests with the source operating at aconstant voltage: t0, moment of start of test and t1, transition fromunstable operating mode to stable9.

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it was necessary in this case to use shorter CTWD values

and therefore produce shorter arcs to cause instability to

appear.

Some of the tests were filmed at high speed, using a

camera with a capacity to film for up to 13 s when running

at 2000 frames per second. In these tests, the weld region

was shaded with a He–Ne laser and the welding voltage

and current signals were synchronously recorded on film

using a system developed by LAPROSOLDA–UFU14 – 16.

This synchronization between the current and voltage

signals and the frames from the high speed filming enables

the relationship to be detected between the changes in the

electronic signals from the process and the changes seen

in the arc, in the transfer methods and in other aspects of

the process.

3. Results and discussion

Figure 3 shows, for two shielding gases (pure argon and

Ar þ 1% O2) and for the different CTWD values, typical

oscillograms of the welding voltage obtained in the tests. It

can be seen that, for both shielding gases and with shorter

CTWD values (shorter arc lengths), there were two voltage

levels with a difference of around 10 V between them,

typical of operations with unstable cathodic mechanisms9.

As expected, an increased in the arc length (obtained by

increasing the CTWD) led to a reduction, and eventually a

disappearance, in the periods of higher voltage and, at the

same time, a reduction in the occurrence of typical

operating conditions (repulsive globular transfer and a

high level of splatter and fumes) for this type of instability.

During the various tests carried out, it was seen,

qualitatively, that when the process had periods of

instability, more fumes were formed. Figure 4 shows

photographs that illustrate this effect for two tests carried

out with pure argon shielding gas and different values for

the CTWD. Note the reduction in the level of fumes

generated during welding, when passing from a condition

of high instability (Figure 4(a)) to a more stable condition

(Figure 4(b)). The rapid alternation between the periods of

higher and lower voltage makes it difficult to show a

direct association between the unstable operating con-

dition and the periods of higher voltage, although the

existence of this association was expected from indirect

evidence (the relationship between the CTWD value, the

Table 2. Parameters regulated for the tests carried out with thelight sensor.

Welding parametersCTWD ¼ 22 mm Data acquisition

frequency ¼ 5 kHzShielding gas ¼ pure argon No. of points

acquired ¼ 15,000Welding speed ¼ 25 cm/min Acquisition time ¼ 3.0 s

Figure 3. Oscillograms for voltages obtained in tests with the source operating at a constant voltage for different contact tip work piece(CTWD) distances. Welding current 255 A.

Table 1. Parameters regulated for welding tests.

Parameters variedShielding gas CTWD (mm)Pure argon 24 and 29Ar þ 1% O2 20 and 26

Other parametersRegulated current ¼ 255 A Gas flow: 16 L/minWire feed speed ¼ 7.8 m/min Nozzle diameter: 52 mmWelding speed ¼ 25 cm/min

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level of instability and the shape of the oscillograms, and

the results of the tests, not described in the present work,

when operating at a constant voltage).

Another effect that was suggested during testing with a

source operating at a constant voltage was an increase in the

arc luminosity during the periods of instability. Figure 5

shows, in a tests carried out in conditions of instability

and when operating at a constant current, a simultaneous

variation in the welding voltage and the arc luminosity.

By comparing the two signals it can be shown that, during

the periods of greater welding voltage, there was a

simultaneous increase in arc luminosity, when compared

with luminosity during the periods of lower voltage. This

correlation between the welding voltage (greater or lower

voltage) and the arc luminosity is illustrated in Figure 5(b).

This figure shows that there are two areas where the points

are concentrated (welding voltage £ luminosity) which

correspond to the two operating conditions. This result

reinforces the hypothesis that the higher voltage periods

are associated with the instability seen and suggests, along

with the variations in voltage, the development of different

processes within the electric arc. Changes in voltage, in

conditions of constant current and without alterations

to the shielding gas, are commonly associated with

variations in the arc length, but this was not seen in the

present case (at least, not to a sufficient extent to justify

variations of around 10 V). On the other hand, changes in

luminosity, without any alterations to the current or arc

length, may be caused by alterations in the composition of

the arc column, particularly by the presence of elements

that emit strongly in the visible light band, such as vapours

from metallic elements.

The oscillograms in Figure 5 show another relevant

effect. During the periods of highest voltage, it can be seen

that the arc luminosity frequently shows (in comparison

with the welding voltage) greater variability and a

tendency to suffer some reduction after the initial increase

(Figure 6). This effect may be associated with the increase

in size of the metal liquid drop at the tip of the electrode

due to repulsive globular transfer (and the consequent

reduction in the arc length) or it may be due to the welding

region being obscured by the more intense generation of

fumes. Frequently, arc luminosity increases after its initial

reduction, and so the first hypothesis seems to be more

likely. In this case, the later increase in luminosity would

indicate the detachment of the liquid metal drop and the

Figure 4. Photos of the region close to the welding area in predominantly unstable operating conditions, CTWD ¼ 24 mm (a) andstable, CTWD ¼ 29 mm (b) and shielding with pure argon. Note the difference on the amount of fumes formed.

Figure 5. Oscillograms for welding voltage and light and graph plotting voltage against light obtained from the tests, with the sourceoperating at a constant current. Welding current 255 A.

Welding International 217

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consequent increase in arc length. If this hypothesis is

correct, it would support the thesis that the phenomena

observed, particularly the increase in the arc voltage, are

associated with processes occurring in the regions of the

electrodes, in particular in the cathode region. As a result,

the formation, growth and detachment of the drop, despite

causing changes in arc length (and, as a result, in its

luminosity) will have a much weaker effect on the welding

voltage, since a component responsible for any noticeable

increase in its value would not be located in the plasma

column.

Some authors (Scotti) took the view that the growth in

the drop size and its detachment caused variations in

the welding voltage due to the drop’s greater electrical

resistance when compared to the electrode. In the present

case, these variations and those of the arc length (with a

voltage gradient of approximately 0.7–0.8 V/mm) could

not explain the almost instantaneous changes of around

10 V seen when instability becomes established.

This as well as a number of other points looked into

here and in previous work9 were explored more deeply in

this present work with the help of high speed filming

(‘Shadow Graphs’) and by synchronizing the film frames

with the voltage and current signals14 – 16. Figure 7 shows,

for one of the tests carried out with carbon steel and pure

argon shielding gas, a part of its voltage oscillogram and

the sequence of film frames synchronized with it, from the

start to the end of a period of operation with the highest

voltage, with a duration of around 0.15 s.

It can be seen in Figure 7 that this process is initially

operating with goticular transfer with elongation. The

liquid metal cylinder that leaves the electrode falls very

close to the weld pool, causing a number of shorts circuits

of short duration. These shorts cause a repulsion effect

on the liquid metal cylinder that, in normal conditions

(without instability occurring) re-establishes its former

configuration after around 1–5 ms. When instability does

occur, the original shape of the electrode tip is not re-

established. As a result, after a rapid short circuit (frame

2), a period of operation begins with a voltage around 10 V

greater than before, and the series of phenomena as set out

above starts to occur. Note the increase in arc length with

the change in transfer mode (from goticular to globular).

During this period, the arc length started to reduce again

and the welding voltage stayed relatively constant and

high, except between frames 10 and 11, when there was a

swift fall in voltage and a loss of arc quality, which

suggests that there was an alteration in its operating mode

(form unstable to stable). This change must have also

facilitated the detachment of the drop seen between frames

12 and 14.

It can also be seen that the frames become darker,

which was related to the greater generation of fumes

during this type of unstable operation (compare, e.g.

frames 1 and 7 or 8) and, as already mentioned, greater arc

luminosity. This is in fact visible in filming during

the period of highest voltage, despite the shading that the

laser casts over the image, which does not occur in the

initial period (frame 1). Also note a greater agitation of

the weld pool during the unstable period (compare frames

1 and 8–10).

The final relevant aspect is the major alteration in the

shape of the arc, which is not clearly visible in Figure 7

due to laser shading, but which can easily be seen in

images obtained without using this technique. Figure 8

shows images of an arc obtained by filming at normal

speed, but with a camera shutter speed of 1/10,000 s. In the

most stable operating condition (pure argon shielding gas

and a CTWD of 29 mm), the arc had a basically normal

Figure 6. Detail of the oscillograms for welding voltage andlight, encompassing the two operating periods with high voltage.Welding current 255 A.

Figure 7. Oscillogram and film sequence synchronized with thevoltage signal. Source operating at a constant current, carbonsteel, pure argon shielding gas, welding current 255 A,CTWD ¼ 24 mm.

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shape (approximately conical) for all images, with its

largest diameter close to the piece. In the unstable

operating condition (pure argon shielding gas and a

CTWD of 29 mm) this format changes greatly between

various frames. In this case, the arc seems to be more

concentrated (i.e. it has a smaller diameter) close to the

part. Since the magnetic forces responsible for generating

the plasma jet depend on the arc geometry, this change of

arc format, suggests that, in the unstable condition of

operation, this jet must occur in the opposite direction

from normal, i.e. during the periods of highest voltage, the

plasma jet must run from the piece to the electrode. As a

result, the weakening of the plasma jet or even its change

in direction must be fundamental factors in explaining the

change in transfer mode seen (from goticular to repulsive

globular).

To explain all these phenomena, Modenesi and Nixon9

proposed a model that looks as the occurrence of

alternative mechanisms through the emission of electrons

from the cathodic region of the arc I situations in which

the regeneration of the oxide layer on the surface of the

base metal is complicated. In normal GMAW welding

conditions, this layer is essential for electrodes to be

emitted, but it is destroyed in the process17. In the absence

of a sufficient amount of oxygen, recomposition of the

oxide layer is made more difficult and its removal tends

to bond it to the weld pool, causing the arc to deflect,

increasing its real length and complicating its operation.

According to the proposed model, when the real length of

the arc due to this deflection becomes longer than the

distance between the electrode tip and the weld pool (AC,

Figure 9), the arc may start operating from the weld pool,

in which an alternative mechanism for emitting electrons

comes into being. Studies carried out into arcs in low

pressure environments indicated the existence of alterna-

tive mechanisms for electrons to be emitted18. These

mechanisms are associated with the formation of jets of

vapour on the surface of the cathode and operate at higher

voltage levels than those seen in the emission of electrons

associated with films of oxide. Based on these studies,

Modenesi and Nixon9 proposed that, in the conditions set

out above, similar mechanisms would start to operate in the

weld pool during the periods of instability considered to

have caused the increase in voltage observed. As a result,

the alternation between the periods of higher and lower

voltage would result in competition between the two

mechanisms for emitting electrons. A suitable amount of

oxygen in the shielding gas would allow regeneration of

the oxide layer, making it difficult for an alternative

mechanism for electron emission to occur. In addition,

when welding with a longer arc length, the relative

difference between the real and apparent lengths of the arc

would be less significant and, once again, it would be

difficult for an alternative mechanism for electron

emission to occur.

The welding voltage, for operation with constant

current, can be described by the following formula

(ignoring the electrical resistance of the cables, the contact

tip and between this tip and the electrode):

U ¼ ðUC þ UAÞ þ E·la þ ke·ðh2 AOÞ; ð1Þ

where UC and UA are the falls for the cathode and anode,

respectively; E is the average electrical field in the arc

Figure 8. Images of the arc obtained without shading for operating conditions: (a) stable and (b) unstable.

Figure 9. Schematic representation of the arc region in theGMAW process.

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column; la is the arc length; ke is the specific resistance

of the electrode; h is the CTWD and AO is as given in

Figure 9. As a result, the variation in voltage during the

change in operation would be:

DU ¼ UUnstable 2 UStable

¼ ðUCÞUnstable 2 ðUCÞStable þ ðE·laÞUnstable

2 ðE·laÞStable: ð2Þ

The variation associated with the change in the column

and in the arc length – [(Ela)Est 2 (Ela)Ins] – must cause a

reduction in DU due to the reduction in the arc length

(from AC to AO), if there are no important changes in the

characteristics of the column (denoted by the electric field

of the column, E). One possible change in the column

would be an increase in the concentration of metal

vapours. However, it is not to be expected that this change

would cause an increase in E, since the metals tend to have

a low ionization potential and could not cause the heat

conductivity of the arc column to rise sharply, since they

specifically contain iron, manganese and other light metal

elements commonly found in steel, with an atomic weight

higher than that of argon. As a result, the increase seen in

the welding voltage may possibly be associated with a

change taking place in the cathodic region, as proposed,

and a change in the mechanism for emitting electrons.

The results obtained in the present work suggest that,

frequently, the start of unstable operation tends to occur

after a short circuit between the electrode and the weld

pool (frame 2 of Figure 7). These short circuits tend to be

very rapid (around 1 ms in duration) and are not often seen

in oscillograms depending on the speed of acquisition of

the data used. This suggests that, in general, the deflection

of the arc in a search for oxide films in the weld pool,

as proposed by Modenesi and Nixon9, is not the most

important factor in initiating unstable operations. This

would actually be facilitated by the vaporization of the

liquid metal bridge between the electrode and the weld

pool at the end of the short circuit and, possibly, the

bonding between the film oxide and the weld pool, in the

case of welding with a shielding gas with a low oxidation

potential, which would allow electrons to be emitted by

jets of vapour for longer periods of time.

4. Conclusions

For those conditions in which unstable cathodic mechan-

isms occur in the GMAW process, the results obtained in

this work indicated that:

. As postulated above, high speed filming synchro-

nized with the electrical signals from the process

shows that the instability is associated with the

periods of highest welding voltage, during which no

major changes were seen in the apparent length of

the arc. During these periods, the process is

characterized by globular repulsive metal transfer,

strong generation of fumes and an increase in arc

luminosity.. During these periods of instability, an intense

agitation in the weld pool can also be seen (much

greater than that seen during the periods of stable

operation).. In general, it was seen that the instability of cathodic

mechanisms tends to start at the end of a short

circuit between the electrode and the weld pool.

This observation suggests the need for alteration for

the model proposed in the above work for this type

of instability, particularly in regard to the initiation

of periods of unstable operation.

Acknowledgements

The authors would like to thank everyone who contributed to thiswork, in particular FAPEMIG for their financial support,LAPSOLDA – UFU for providing the laboratory and itstechnical and scientific staff for carrying out the high speedfilming and CEFET/MG for releasing Maria Celeste Monteiro deSouza Costa from her academic activities to help completethis work.

Notes

1. Email: mcelestemsc@deii.cefetmg.br2. Email: cicerostarling@ufmg.br3. Email: modenesi@demet.ufmg.br

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