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Interchangeable metal transfer phenomenon in GMA welding: Features, mechanisms, classification
Accepted Manuscript Title: Interchangeable metal transfer phenomenon in gma welding: features, mechanisms, classification Author: Am´erico Scotti Vladimir Ponomarev William Lucas PII: DOI: Reference:
S0924-0136(14)00197-6 http://dx.doi.org/doi:10.1016/j.jmatprotec.2014.05.022 PROTEC 14009
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
7-8-2013 16-5-2014 18-5-2014
Please cite this article as: Scotti, A., Ponomarev, V., Lucas, W.,Interchangeable metal transfer phenomenon in gma welding: features, mechanisms, classification, Journal of Materials Processing Technology (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.05.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Manuscript
INTERCHANGEABLE METAL TRANSFER PHENOMENON IN GMA WELDING: FEATURES, MECHANISMS, CLASSIFICATION
a
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Américo Scottia, Vladimir Ponomarevb, William Lucasc
Federal University of Parana, Department of Mechanical Engineering, 80060-000, Curitiba,
b
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PR, Brazil, [email protected]
Federal University of Uberlandia, Faculty of Mechanical Engineering, Av. Joao Naves de
Independent Consultant, Cambridge, CB25 0HF, UK, [email protected]
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c
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Avila, 2121, CEP: 38400-902, Uberlândia, MG, Brazil, [email protected]
Corresponding author: Vladimir Ponomarev; Federal University of Uberlandia, Faculty of
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Mechanical Engineering, Av. Joao Naves de Avila, 2121, CEP: 38400-902, Uberlândia, MG, Brazil; [email protected]; tel.: +55 34 3239 4483, +55 34 9161 2050, fax: +55 34
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Abstract
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3239 4482
Metal transfer modes in arc welding processes have previously been classified as Natural or Controlled Metal Transfer. Modern laboratory techniques have helped to establish
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a new transfer classification mode in GMAW of carbon steels, which has been termed Interchangeable Metal Transfer. In order to characterise the new mode, a series of specimens was welded at different combinations of welding current (wire feed speed), arc voltage and gas composition. Laser backlighting techniques and high speed filming were employed to study metal transfer. The video was synchronized with the welding current and arc voltage signals to aid the understanding of the transfer behaviour. The results showed that this new Interchangeable Metal Transfer class is distinguished from the Natural or Controlled Metal Transfer class because of its unique characteristic of periodical changes in
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the transfer mode induced by changes in welding parameters (a self-sustained behaviour). The characteristic feature of the Interchangeable Metal Transfer class was shown to comprise of two or more natural transfer modes occurring in a regular repetitive sequence. The metal transfer sequence occurs without interference from the operator or the adaptive control system of the power source. Phenomenological explanations based on arc physics
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are given to justify the main governing factors for the particular metal transfer characteristics.
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Key words: Welding; GMAW; Metal Transfer.
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1 – Introduction
Gas Metal Arc Welding (GMAW) is a widely used process in the metal fabrication
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industry. Welds are produced by using an arc to melt a wire electrode. Metal from the melting wire is transferred to the joint in the form of droplets that detach from the electrode tip. The
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performance of this process is governed by the metal transfer mode that is the way in which the metal droplets are detached from the wire electrode and transferred to the weld pool.
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Mode (a short for metal transfer mode) defines the characteristic behaviour of the droplets transferring from the wire to the weld pool. For example, the “globular” mode describes large drops being detached from the wire and transferring under gravity to the weld
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pool whilst the “spray” mode describes small droplets being projected from the wire tip to the weld pool. Group of modes refers to a number of modes that have similar characteristics. Several characteristic transfer modes have been described in current literature. The first classification, established more than 30 years ago by the International Institute of Welding (IIW), as seen in the IIW Doc. XII-636-76 (1976), is still used by several researchers. Despite its merit, this classification is applicable to natural transfer modes only and neither encompasses recent controlled transfer types nor the metal transfer modes recognizable only when using sophisticated measurement techniques. Natural modes are defined here as a mode with transfers not forced by additional electrical parameter or wire feeding control, in
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contrast to controlled transfer modes, as explained by Scotti et al. (2012) in another publication. Scotti et al. (2012), when using a laser shadograph system with synchronized electrical signals and high speed filming, observed some new modes and described their particular characteristics. These authors proposed a revised classification for metal transfer specifically for use by scientific personnel (researchers, scholars and students). However,
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there is no one best mode covering all applications as there is a preferred mode for a specific application. As a consequence, a better understanding of the metal transfer phenomenon is
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important for improvements in the quality and productivity of GMA welding.
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Heald et al. (1994) showed that the groups of modes, and respective transfer modes, are related to welding process parameters and shielding gas types, usually represented
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through diagrams, which are often referred to as “transfer mode maps”. Scotti (2000) presented different versions for them, having similar content, yet using different approaches,
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as illustrated in Fig. 1. Arc voltage (Ua) plotted against welding current (Iw) is the most conventional way of representing a transfer map. A second version would use “arc length”
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(La), or, more precisely, the “arc gap extension”, instead of arc voltage, since arc gap is considered to describe the influence on transfer behaviour more realistically. It is important to
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point out that the arc voltage and the arc length are in some cases incorrectly used as synonyms. A direct relationship between the arc length and the arc voltage is widely known (the higher the arc length, the higher the arc voltage), but it is valid only for a given current.
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When the correlations are established as a function of the current, as the present case, the arc length can be maintained constant for different current values, because they are independent of each other. On the other hand, the voltage will increase as the current is augmented for any arc length, since the voltage is dependent of the current for a given arc length (static characteristics of arcs). As a result, the two drawings can take slight differences in shape.
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Fig. 1. Schematic maps of the main natural metal transfer modes occurring in GMA welding
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as a function of the welding current (Iw), represented by either the welding voltage setting, on
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the left, or the arc length, on the right (after Scotti et al., 2012).
It is also important to mention the transition zones between adjacent transfer mode
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fields. In the transition zones, droplet detachment becomes intermediate between, for example, larger droplets of globular transfer and the smaller droplets of the spray transfer
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modes. This phenomenon can be explained by using a model proposed by Watkins et al. (1992), based on the Shaw’s model for water droplet growth and detachment. Shaw had observed that droplets flowing from a faucet detached at periodic intervals for low flow rates. As the flow rate increased, the flow rate changed from periodic and predictable to an
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aperiodic quasi-random pattern of behaviour. Haidar & Lowke (1996) used a theoretical approach for the prediction of the droplet formation. A two-dimensional time-dependent model, accounting for the effects of surface tension, gravity, inertia and magnetic pinch forces in the droplet, was used. The wire feed speed and gas flow rate were also incorporated into the predictions. They also predicted the presence of both small and large droplets (alternately) at the transition zone between globular and spray modes, in agreement with the above-mentioned work. Similar droplet flow characteristics obtained by the above models were experimentally
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detected by Clark et al. (1989) and Johnson et al. (1992) in similar conditions (GMAW, Ar2%O2, 0.89 mm electrode wire). Johnson et al. observed an electrode extension increase during the detachment of large droplets, justified by a slower melting rate than expected. After a series of small droplet detachments, the electrode extension decreased, since these small droplets melted off faster than the average rate. According to this author, this cycle
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sometimes repeated itself several times. For example, one or two large droplets may be
followed by a series of small droplets but then followed by other one or two large droplets.
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Madigan et al. (1992) also observed electrode extension changes during metal transfer.
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Working in the droplet – spray transition zone, with a constant current power source, they observed an electrode extension increase (arc length decrease) just before droplet
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detachment. These authors considered the electrode extension to be the sum of the solid cylinder and the droplet diameter.
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Despite the evidence that there might be some distinctive metal transfer modes happening in the transition zone between two adjacent transfer mode fields, most
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researchers describe them as transfer mode instability of a chaotic character. However, Scotti (2000) reported that, under certain welding conditions, two or more natural-like transfer
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modes can happen in a periodic sequence (without any interference of the operator and/or a control system). He also showed in his results that this periodic pattern of changes in the metal transfer mode is not restricted to the transition zones between adjacent fields but may
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also occur in different combinations. For example, Short-circuiting - Projected Spray, Shortcircuiting - Streaming Spray, Globular-Projected Spray, Globular-Streaming Spray, GlobularShort-circuiting - Streaming, Spray-Globular, etc. were observed. These patterns of transfer have not been widely commented on in the literature, most likely because the related transfers are difficult to identify using ordinary laboratory techniques. Moreover, they are easily confused with temporary transfer instability which may occur for example when operating within a transition operational envelope between two adjacent natural transfer modes. Despite the above mentioned reports, there is little published information on multi-
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mode metal transfer. Thus, the objective of this work was to study more consistently the existence of the above mentioned metal transfer class of modes, hereafter referred as “Interchangeable metal transfer”.
2 -Experimental Procedure
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A series of experiments was carried out with the aim of reproducing welding
conditions that would lead to differing modes of interchangeable metal transfer in GMA
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welding. Bead-on-plate welds were carried out on carbon steel plates using a 1.0-mm-
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diameter wire of the AWS ER70S-6 class with DCEP. The approach was to select a different shielding gas (Ar + 2% or 5%O2) and then set a combination of inductance, welding current
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and arc voltage to produce the desired droplet transfer mode. An electronic constant voltage output characteristic power source was used in these experiments.
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The main methodological approach applied was based on a system for metal transfer visualization as used by Lin et al. (2001) and Bálsamo et al. (2000), among others. The
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experimental rig was set up as shown in Fig. 2. A shadow of the non-transparent components (contact tube, electrode, metal drops, weld pool and plate) of the arc region was
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projected onto the lens of a camera, a technique known as backlighting. A high-speed digital camera working at 2,000 fps and a 632.2 nm He-Ne laser were used. To enable the arc to be seen also, optical filters of different intensity were employed. The electric signals were
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synchronised with the film frames to correlate the variations in arc voltage and welding current with the formation and detachment of the droplets. Synchronization was carried out using a dedicated program built in a LabView® platform.
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8 7 6 5
5
9 Synchronization signal
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4
Ua
3 A
400
Iw
300 200 100
Ua
0 V
20
11
0
Iw 10
0
10
20
30
40
t, ms
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1
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2
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Fig. 2. Details of the optical laser system used for metal transfer visualization. 1, light source (laser); 2, neutral filters; 3, divergent lens; 4, convergent lens; 5, protection glass; 6, band-
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pass and neutral filters; 7, high-speed video camera; 8, monitor; 9, image recording unit; 10,
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3 - Analysis of Results
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computer; 11, current hall probe (after Scotti et al., 2012).
Different types of interchangeable metal transfer modes (two or more transfer naturallike modes happening in a periodic sequence) were generated. The characteristics and the
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reason for the occurrence of each interchangeable metal transfer mode are described and discussed as follows.
3.1 - Interchangeable “short-circuiting – spray” mode The two natural transfer modes during this type of the interchangeable metal transfer are the short-circuiting mode and the streaming/projected spray one, as illustrated in Fig. 3. The welding conditions (arc voltage and welding current instant values) initially favour the natural short-circuiting transfer which includes the droplet growth and short-circuit stages.
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However, during the post short-circuit period, a higher mean current level leads to a high post short-circuit current, which remains temporarily above the transition current level. Due to this augmented current, the electrode melting rate becomes momentarily higher than the wire feed speed (WFS) and the arc length progressively increases. This has the effect of preventing short-circuiting transfer as the process has sufficient time to enable more than
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one tiny droplet to detach sequentially. With a constant voltage power source, as a longer arc makes the welding current decrease, the electrode melting rate also falls gradually. As
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electrode melting rate becomes less than the WFS, the wire tip returns to approaching the
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weld pool. The combination of a low current and a short arc reinstates the conditions required for short-circuiting to occur. Normally only one drop is transferred before a new
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cycle is initiated.
Streaming spray
Short-circuiting
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Transition current
Ua
10
Iw
20
30
40
Short-circuiting
50
t (ms)
Projected Spray
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(A) 400 300 200 100 0 20 (V) 0 0
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Short-circuiting
(A) 400
Iw
300 200 100
Ua
0 20
(V) 0
0
10
20
30 t (ms)
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Fig. 3. Examples of an Interchangeable Metal Transfer mode of the type “short-circuiting – spray” (above “streaming” and below “projected” spray) and the correspondent arc voltage (Ua) and welding current (Iw) traces: mean Ua = 23.5 V; mean Iw = 170 A; set WFS = 7 m/min; travel speed = 36 cm/min; contact-tube to work distance (CTWD) = 18 mm; shielding gas =
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Ar + 5%O2.
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As can also be seen in Fig. 3, switching of the natural metal transfer modes is cyclical
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which mainly depends on the inductance of the power source (dynamic response of the current, i.e., current rising and falling rates), arc length and the combination of the electrode
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and the shielding gas which influences the surface tension. The latter determines the transition current level and the others act together to determine the short-circuit duration and
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indirectly, the short-circuiting current level. These preconditions substantiate the reason for
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occurrence of the "short-circuiting - spray" interchangeable metal transfer mode.
3.2 - Interchangeable “globular - spray” mode
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As seen in Fig.4, globular and spray natural transfer modes are interchanging giving rise to an interchangeable transfer mode. It is considered that the reason for this mode is that when using shielding gas mixtures with less than 12% CO2 and a carbon steel wire, the
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electrical resistivity of the droplet becomes higher than that of the arc column. During a globular transfer under such conditions in which the droplet resistivity is greater than the arc column resistivity, the growth of the droplet overcomes the effect of the shortening of the arc regarding the resistance variation. The increase in the summation of the electric resistances consequently reduces progressively the welding current and resulting in a reduction of the wire melting rate. Thus, even though the arc length became shorter, the voltage measured between the contact tip and the work piece increases, as illustrated in Figs. 5 and 6. Due to a progressive reduction of the electrode melting rate, the electrode tip with a globular droplet attached approaches the weld pool, sometimes causing incipient short-
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circuits, as shown in Fig. 7. Together with an increase in the electrode extension, the total electric resistance starts to reduce so that the lower resistivity of the wire dominates the resistivity of the arc column and the welding current starts to increase again. Thus, the welding current can reach values above the transition current which is low for these low CO2 Ar based gas mixtures. This results in a projected (see Fig. 4) or even streaming spray
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transfer (see Fig. 7). The resulting high electrode melting rate coincident with this high
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re-established for the globular transfer and a new cycle sets in.
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current causes the arc length to increase and the current to reduce. The conditions are now
Range of the electrode tip oscillation
Projected spray
Globular
620 700
726
751
752
29 27 25 0.45
Ua 0.5
(Ohm)
0.55
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0.22 0.20 0.18 0.16 0.14 0.12 0.45
794 804 805 812
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(V)
781 783
843 852 950 1035 1058 1059 1082 1090
(A)
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Transition current
760
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Globular
200 180 160
Iw
140 120
0.6
0.65
0.7
0.75
0.8
0.85
0.9
t (s)
0.6
0.65
0.7
0.75
0.8
0.85
0.9
t (s)
Ra
0.5
0.55
Fig. 4. An example of an interchangeable metal transfer mode of the “globular – spray” type and the correspondent Ua, Iw and instantaneous arc resistance (Ra) traces: mean Ua = 27.9 V; mean Iw = 166 A; WFS = 6.3 m/min; travel speed = 30 cm/min; CTWD = 18 mm; shielding gas = Ar + 5%O2.
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55
59
60
61
76
80
95
103
109
115
120
123
124
125
U (V)
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31 30
cr
29
27 0.3
0.35
0.4
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28 0.45
0.5
0.55
t (s)
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Fig. 5. Voltage trace showing the arc voltage variation as a function of the droplet growing and detachment (globular transfer): Iw = 182 A; WFS = 6.7 m/min; CTWD = 20 mm; shielding
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gas = Ar + 2%O2
Ar+10 … 12% CO2
U
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Ar+1 … 5% CO2
U
U
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t
t
Iw
t
Iw
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Iw
Ar+20 … 100% CO2
Droplet resistivity > Arc column resistivity
Droplet resistivity ≈ Arc column resistivity
Droplet resistivity < Arc column resistivity
Fig. 6. Schematic illustration of the alteration of the ratio between the droplet and arc column electric resistivities as a function of the CO2 content in an argon based gas mixture. The droplet and arc column electric resistivities are illustrated by lines, where the thicker line means lower resistivity
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Range of the electrode tip oscillation
750
800
Short-circuiting
850
900
950
958
959
Projected Spray
962
964 970
Streaming spray
980 985 990
Iw
Transition current (A)
45
35
Ua
1350 1400
1450
1.05
1480 1485
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1200 1250 1300
0.95
1.15
ed
0.85
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15
5 0.75
175 150 125 100
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25
200
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Iw
1487
1490
75 50 25 0 -25
1.25
1.35
t (s)
1550 1600 1650 1700 1750 1800 1850 1900
Ra
0.85
0.95
1.05
1.15
1.25
1.35
t (s)
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0.24 0.2 0.16 0.12 0.08 0.04 0 0.75
995 1000 1050 1100 1150
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Ua (V)
(Ohm)
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Globular
Fig. 7. An example of an interchangeable metal transfer mode of the “Globular - Streaming Spray” type: mean Ua = 28.4 V; mean Iw = 177 A; WFS = 6.5 m/min; travel speed = 36 cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
Thus, the reason for a "globular - spray" interchangeable metal transfer mode is a lower specific resistance of the arc column compared to that of the metal droplet, which is conditioned by the use of shielding gas mixtures with less than 12% CO2.
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Then, the question is whether shielding gas mixtures with more than 12% CO2 could promote or not an interchangeable transfer “globular – spray” as the specific resistance of the arc column is now higher than that of the droplet, as illustrated by Fig. 6. During the globular transfer stage, as long as the droplet is growing and, consequently the arc length is reducing, the arc voltage reduces as well. The reason is that the reduction of the total arc
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column voltage drop is more significant than the voltage drop in the growing droplet which when using constant voltage power sources, causing a current increase. Then, if,
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hypothetically, the current exceeds the transition current level, the spray transfer mode could
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be established followed by an increase of the electrode melting rate and the arc lengthening of the arc. This behaviour could be accompanied by a rise in the arc column resistance,
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resulting in a reduction in the current. Finally, there might be a re-establishment of the globular transfer mode. A new cycle would be re-established. However, the higher the CO2
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content in a gas mixture, the higher the transition current value becomes, and, thus, it becomes more difficult to be exceeded. This is the main reason why the interchangeable
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metal transfer mode is usually not observed when using CO2 rich argon based shielding
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mixtures.
3.3 - Interchangeable “globular – short-circuiting - streaming spray” mode Fig. 8 illustrates the interchangeable mode "globular – short-circuiting – streaming
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spray". It is considered that, in this case, the conditions described in both sections 3.1 (high post short-circuiting current) and 3.2 (a lower specific resistance of the arc column and of the wire electrode compared to that of the metal droplet) are acting in combination. There was a gradual reduction of the current between frames 792 and 900, caused by an increase in the growing droplet resistance. Due to the consequent reduction of the electrode melting rate, the electrode tip with a droplet starts to move towards the weld pool, from frames 900 to 1028. Together with an increase in the electrode extension, the total electric resistance starts to reduce and the lower resistivity of the wire now starts to dominate over that of the arc column so that the current increasing again. Although the increment in the current was not
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sufficient to exceed the globular-spray transition, as there was a short-circuit, the metal transfer mode transition became possible due to a high post short-circuiting current. The reduction of the arc voltage as compared with the case shown in Fig. 7 was one of the favourable conditions for this mode to happen.
Short-circuiting
Streaming spray
Globular
780
792
850
900
950
980
1028
1029
1040
1045
1050
1065
1090
1110 1140
(A)
an
750
us
cr
Globular
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Range of the electrode tip oscillation
300
Iw
Transition current (V) 30 20
0 0.74
0.78
0.82
(Ohm) 0.2
Ra
0.12 0.08 0.04
0.78
150
0.82
100 50 0
0.86
0.9
0.94
0.98
t (s)
0.86
0.9
0.94
0.98
t (s)
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0 0.74
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0.16
ed
Ua
10
200
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40
250
Fig. 8. An example of an interchangeable metal transfer mode of the “globular – shortcircuiting – streaming spray” type: mean Ua = 27.5 V; mean Iw = 170 A; WFS = 6.5 m/min; travel speed = 36 cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
3.4 - Interchangeable “projected spray - streaming spray” mode There are certain energy-related conditions which can make the current vary periodically generating the "projected spray - streaming spray" interchangeable metal transfer mode, Fig. 9. The intensive generation of metallic vapour in an arc under streaming
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spray transfer creates the potential to alter the plasma properties in such a way as to force the current to reduce due to a lower ionization potential and/or heat transfer losses. The transfer would, then, turn into projected spray with less metallic vapour generation, which would make the current increase again, and so on. An alternative reason for the current starting to reduce at the end of the streaming
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spray stage is that, when using a constant voltage power supply, a progressively increasing arc length occurs. Vice-versa, during the projected spray transfer, a low current causes the
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arc to shorten, inducing a current rise and moving the transfer mode transition towards the
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streaming spray mode. In either case, there is a clear manifestation of the interchangeable metal transfer mode fundamental principles, when variations of conditions due to a previous
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transfer mode give rise to conditions for the following mode to take place.
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Range of the electrode tip oscillation
(V) 30
Ua
895
899
Ac
29
855
903
904
906
950
1100
Spray projected
1206 1255
1258 1300
1350 1402 1480
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820
Streaming spray
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Spray projected
(A)
Iw
215 210 205 200 195
28
27 1.75
1.8
1.85
1.9
1.95
2
2.05
2.1
2.15
1.9
1.95
2
2.05
2.1
2.15
t (s)
(Ohm) 0.15
Ra 0.14 0.13 1.75
1.8
1.85
t (s)
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Fig. 9. An example of an interchangeable metal transfer mode of the “spray projected – streaming spray” type: mean Ua = 28.7 V; mean Iw = 207 A; WFS = 8.7 m/min; travel speed = 36 cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
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4 – Classification of Metal Transfer Modes
Although the interchangeable transfer modes occur under welding conditions
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between those for adjacent natural ones, they should not be confused with a transition
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transfer mode, because they are characterized by sequential periodic repeatability. It is not a phenomenon of occasional natural instability between two modes. The most important
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characteristic of Interchangeable transfer is that the following mode is a consequence of the previous one. In particular, the variation of current, electrode temperature and/or plasma
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status due to a transfer mode gives rise to conditions for the following mode to take place. An interchangeable metal transfer mode takes place only if all the necessary conditions are
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present, i.e., a combination of welding current, arc length, material and diameter of the wire, shielding gas, contact tube to work distance and favourable dynamics (inductance) of the
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power source. It is important to note that further research work is still required to establish the ranges for the conditions promoting each of the interchangeable transfer modes. The Interchangeable Transfer Mode cannot be attributed to either Natural Transfer
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Modes because its characteristic sequential periodic changing between two or even more natural transfer modes or to Controlled Transfer Modes because there is no in-line or off-line control. These types of transfer mode possess all characteristics of an individual class of modes which has been called Class of Interchangeable Transfer Modes, as summarized in Fig. 10.
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Fig.10. GMAW Metal Transfer Classification based on hierarchical order: classes, groups
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and modes
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Interchangeable metal transfer modes are not identifiable by welders and operators, even though characterized by a low frequency of metal transfer interchanging (3 to 5 Hz). If
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the transfer is interchanging from globular to spray, it is unstable as occurs with a globular transfer. But if it is interchanging from short-circuiting and spray, the welder may not feel the any difference in performance from the normal short-circuiting operation. In fact, modern power source manufactures are trying to develop controlled metal transfers interchanging
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from, for instance, pulsed transfer to short-circuit transfer, to satisfy special applications. Including the new “Interchangeable Metal Transfer” class in the overall classification
of GMA Metal Transfer Modes for arc welding completes the classification. It is now possible to describe all modes of metal transfer from the simple “Natural Metal Transfer” class to the often quite complex multi-mode type of transfer categorised in the “Interchangeable Metal Transfer” class which has been identified in this study.
5 – Conclusions
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Modern laboratory techniques, especially high speed video filming synchronized with welding parameters acquisition, brought out evidences that: - There is a new metal transfer class “Interchangeable Metal Transfer”, which with the well known Natural and Controlled Metal Transfer classes completes the classification
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of metal transfer for GMA welding of carbon steels;
- The interchangeable metal transfer mode is distinguished from the others classes of
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metal transfer because of its unique characteristic of periodical changes in the transfer
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mode induced from short temporal changes in welding parameters (a self-sustained behaviour);
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- The interchangeable metal transfer mode may comprise two or more natural transfer modes happening in a periodic repetitive sequence, one following the other, as a
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consequence of the previous one. There is no operator or adaptive control system interference;
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- The interchangeable metal transfer mode can only take place if all the necessary conditions are present, i.e., a combination of welding current, arc length, material and
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diameter of the wire, shielding gas, contact-tube to work distance and favourable dynamics (inductance) of the power source; - The interchangeable metal transfer mode does not occur when using shielding gas
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mixtures with more than 12% CO2.
6 – Acknowledgements
The authors would like to thank the Brazilian agencies for research and development (CNPq and Fapemig), which have provided the financial backing for the specialized equipment used in this work (high-speed camera, laser back-light system, synchronized frames-electrical signal data loggers).
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7 – References
Bálsamo, P.S.S., Vilarinho, L.O., Vilela, M., Scotti, A., 2000. Development of an experimental technique for studying metal transfer in welding: synchronised shadowgraphy. Int. J. Join. Mater. 12(1), 1-12, ISSN 0905-6866.
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Clark, D.E., Buhrmaster, C.L., Smartt, H.B., 1989. Drop Transfer Mechanisms in GMAW. In: 2nd Int. Conf. on Recent Trends in Welding Science and Technology, ASM, Gatlinburg,
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Tennessee, USA, pp. 371-375.
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Haidar, J., Lowke, J.J., 1996. Predictions of metal Droplet Formation in Arc Welding, J. Phys. D: Appl. Phys., 29, 2951-2960.
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Heald, P. R., Madigan, R. B., Siewert, T. A., Liu, S., 1994. Mapping the Droplet Transfer Modes for an ER100S-1 GMAW Electrode". Weld. J. 73 (2), 38s-44s.
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IIW (International Institute of Welding), 1976. Classification of Metal Transfer, IIW Doc. XII636-76.
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Johnson, J.A., Smart, H.B., Carson, N.M., Waddoups, M., 1992. Dynamics of droplet Detachment in GMAW. In: 3rd Int. Conf. on Trends in Welding Research, ASM,
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Gatlinburg, Tennessee, USA, pp. 987-991.
Lin, Q., Li, X., Simpson, S.W., 2001. Metal transfer measurements in gas metal arc welding. J. Phys. D: Appl. Phys., 34 (3), 347–353, doi: 10.1088/0022-3727/34/3/3172000.
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Madigan, R.B., Quinn, T.P., Siewert, T.A., 1992. Sensing Droplet Detachment and Electrode Extension for Control of Gas Metal Arc Welding. In: 3rd Int. Conf. on Trends in Welding Research, ASM, Gatlinburg, Tennessee, USA, pp. 999-1002. Scotti, A., 2000. Mapping the Transfer Modes for Stainless Steel GMAW. J. Sci. Technol. Weld. Join. 5 (4), 227-234, ISSN 1362-1718. Scotti, A., Ponomarev, V., 2008. MIG/MAG Welding: better understanding – better performance. Artliber Editora Ltda., São Paulo, Brazil, pp. 268 - 277 (in Portuguese). Scotti, A., Ponomarev, V., Lucas, W., 2012. A Scientific Application Oriented Classification for Metal Transfer Modes in GMA Welding. Journal of Materials Processing Technology,
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212, 1406-1413, doi:10.1016/j.jmatprotec.2012.01.021. Scotti, A., Ponomarev, V., Resende, A., 2006. The influence of the electrode materials and shielding gas mixture on the specific electric resistances of the drop/column of the arc in GMA welding, IIW Doc. XII-1909-06. Watkins, A.D., Smartt, H.B., Johnson, J.A., 1992. A Dynamic Model of Droplet Growth and
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Detachment in GMAW. In: 3rd Int. Conf. on Trends in Welding Research, ASM,
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Gatlinburg, Tennessee, USA, pp. 993-997.
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Figure Captions
Fig. 1. Schematic maps of the main natural metal transfer modes occurring in GMA welding as a function of the welding current (Iw), represented by either the welding voltage setting, on
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the left, or the arc length, on the right (after Scotti et al., 2012).
Fig. 2. Details of the optical laser system used for metal transfer visualization. 1, light source
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(laser); 2, neutral filters; 3, divergent lens; 4, convergent lens; 5, protection glass; 6, band-
an
computer; 11, current hall probe (after Scotti et al., 2012).
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pass and neutral filters; 7, high-speed video camera; 8, monitor; 9, image recording unit; 10,
Fig. 3. Examples of an Interchangeable Metal Transfer mode of the type “short-circuiting –
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spray” (above “streaming” and below “projected” spray) and the correspondent arc voltage (Ua) and welding current (Iw) traces: mean Ua = 23.5 V; mean Iw = 170 A; set WFS = 7 m/min;
ed
travel speed = 36 cm/min; contact-tube to work distance (CTWD) = 18 mm; shielding gas =
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Ar + 5%O2.
Fig. 4. An example of an interchangeable metal transfer mode of the “globular – spray” type and the correspondent Ua, Iw and instantaneous arc resistance (Ra) traces: mean Ua = 27.9
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V; mean Iw = 166 A; WFS = 6.3 m/min; travel speed = 30 cm/min; CTWD = 18 mm; shielding gas = Ar + 5%O2.
Fig. 5. Voltage trace showing the arc voltage variation as a function of the droplet growing and detachment (globular transfer): Iw = 182 A; WFS = 6.7 m/min; CTWD = 20 mm; shielding gas = Ar + 2%O2 (after Scotti et al., 2006).
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Fig. 6. Schematic illustration of the alteration of the ratio between the droplet and arc column electric resistivities as a function of the CO2 content in an argon based gas mixture. The droplet and arc column electric resistivities are illustrated by lines, where the thicker line
ip t
means lower resistivity (after Scotti and Ponomarev, 2008).
Fig. 7. An example of an interchangeable metal transfer mode of the “Globular - Streaming
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Spray” type: mean Ua = 28.4 V; mean Iw = 177 A; WFS = 6.5 m/min; travel speed = 36
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cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
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Fig. 8. An example of an interchangeable metal transfer mode of the “globular – shortcircuiting – streaming spray” type: mean Ua = 27.5 V; mean Iw = 170 A; WFS = 6.5 m/min;
ed
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travel speed = 36 cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
Fig. 9. An example of an interchangeable metal transfer mode of the “spray projected –
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streaming spray” type: mean Ua = 28.7 V; mean Iw = 207 A; WFS = 8.7 m/min; travel speed =
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36 cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
Fig.10. GMAW Metal Transfer Classification based on hierarchical order: classes, groups and modes (described in more details by Scotti et al., 2012).
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S0924-0136(14)00197-6 http://dx.doi.org/doi:10.1016/j.jmatprotec.2014.05.022 PROTEC 14009
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
7-8-2013 16-5-2014 18-5-2014
Please cite this article as: Scotti, A., Ponomarev, V., Lucas, W.,Interchangeable metal transfer phenomenon in gma welding: features, mechanisms, classification, Journal of Materials Processing Technology (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.05.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Manuscript
INTERCHANGEABLE METAL TRANSFER PHENOMENON IN GMA WELDING: FEATURES, MECHANISMS, CLASSIFICATION
a
ip t
Américo Scottia, Vladimir Ponomarevb, William Lucasc
Federal University of Parana, Department of Mechanical Engineering, 80060-000, Curitiba,
b
cr
PR, Brazil, [email protected]
Federal University of Uberlandia, Faculty of Mechanical Engineering, Av. Joao Naves de
Independent Consultant, Cambridge, CB25 0HF, UK, [email protected]
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c
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Avila, 2121, CEP: 38400-902, Uberlândia, MG, Brazil, [email protected]
Corresponding author: Vladimir Ponomarev; Federal University of Uberlandia, Faculty of
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Mechanical Engineering, Av. Joao Naves de Avila, 2121, CEP: 38400-902, Uberlândia, MG, Brazil; [email protected]; tel.: +55 34 3239 4483, +55 34 9161 2050, fax: +55 34
ce pt
Abstract
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3239 4482
Metal transfer modes in arc welding processes have previously been classified as Natural or Controlled Metal Transfer. Modern laboratory techniques have helped to establish
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a new transfer classification mode in GMAW of carbon steels, which has been termed Interchangeable Metal Transfer. In order to characterise the new mode, a series of specimens was welded at different combinations of welding current (wire feed speed), arc voltage and gas composition. Laser backlighting techniques and high speed filming were employed to study metal transfer. The video was synchronized with the welding current and arc voltage signals to aid the understanding of the transfer behaviour. The results showed that this new Interchangeable Metal Transfer class is distinguished from the Natural or Controlled Metal Transfer class because of its unique characteristic of periodical changes in
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the transfer mode induced by changes in welding parameters (a self-sustained behaviour). The characteristic feature of the Interchangeable Metal Transfer class was shown to comprise of two or more natural transfer modes occurring in a regular repetitive sequence. The metal transfer sequence occurs without interference from the operator or the adaptive control system of the power source. Phenomenological explanations based on arc physics
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are given to justify the main governing factors for the particular metal transfer characteristics.
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Key words: Welding; GMAW; Metal Transfer.
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1 – Introduction
Gas Metal Arc Welding (GMAW) is a widely used process in the metal fabrication
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industry. Welds are produced by using an arc to melt a wire electrode. Metal from the melting wire is transferred to the joint in the form of droplets that detach from the electrode tip. The
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performance of this process is governed by the metal transfer mode that is the way in which the metal droplets are detached from the wire electrode and transferred to the weld pool.
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Mode (a short for metal transfer mode) defines the characteristic behaviour of the droplets transferring from the wire to the weld pool. For example, the “globular” mode describes large drops being detached from the wire and transferring under gravity to the weld
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pool whilst the “spray” mode describes small droplets being projected from the wire tip to the weld pool. Group of modes refers to a number of modes that have similar characteristics. Several characteristic transfer modes have been described in current literature. The first classification, established more than 30 years ago by the International Institute of Welding (IIW), as seen in the IIW Doc. XII-636-76 (1976), is still used by several researchers. Despite its merit, this classification is applicable to natural transfer modes only and neither encompasses recent controlled transfer types nor the metal transfer modes recognizable only when using sophisticated measurement techniques. Natural modes are defined here as a mode with transfers not forced by additional electrical parameter or wire feeding control, in
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contrast to controlled transfer modes, as explained by Scotti et al. (2012) in another publication. Scotti et al. (2012), when using a laser shadograph system with synchronized electrical signals and high speed filming, observed some new modes and described their particular characteristics. These authors proposed a revised classification for metal transfer specifically for use by scientific personnel (researchers, scholars and students). However,
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there is no one best mode covering all applications as there is a preferred mode for a specific application. As a consequence, a better understanding of the metal transfer phenomenon is
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important for improvements in the quality and productivity of GMA welding.
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Heald et al. (1994) showed that the groups of modes, and respective transfer modes, are related to welding process parameters and shielding gas types, usually represented
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through diagrams, which are often referred to as “transfer mode maps”. Scotti (2000) presented different versions for them, having similar content, yet using different approaches,
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as illustrated in Fig. 1. Arc voltage (Ua) plotted against welding current (Iw) is the most conventional way of representing a transfer map. A second version would use “arc length”
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(La), or, more precisely, the “arc gap extension”, instead of arc voltage, since arc gap is considered to describe the influence on transfer behaviour more realistically. It is important to
ce pt
point out that the arc voltage and the arc length are in some cases incorrectly used as synonyms. A direct relationship between the arc length and the arc voltage is widely known (the higher the arc length, the higher the arc voltage), but it is valid only for a given current.
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When the correlations are established as a function of the current, as the present case, the arc length can be maintained constant for different current values, because they are independent of each other. On the other hand, the voltage will increase as the current is augmented for any arc length, since the voltage is dependent of the current for a given arc length (static characteristics of arcs). As a result, the two drawings can take slight differences in shape.
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Page 3 of 32
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Fig. 1. Schematic maps of the main natural metal transfer modes occurring in GMA welding
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as a function of the welding current (Iw), represented by either the welding voltage setting, on
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the left, or the arc length, on the right (after Scotti et al., 2012).
It is also important to mention the transition zones between adjacent transfer mode
ed
fields. In the transition zones, droplet detachment becomes intermediate between, for example, larger droplets of globular transfer and the smaller droplets of the spray transfer
ce pt
modes. This phenomenon can be explained by using a model proposed by Watkins et al. (1992), based on the Shaw’s model for water droplet growth and detachment. Shaw had observed that droplets flowing from a faucet detached at periodic intervals for low flow rates. As the flow rate increased, the flow rate changed from periodic and predictable to an
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aperiodic quasi-random pattern of behaviour. Haidar & Lowke (1996) used a theoretical approach for the prediction of the droplet formation. A two-dimensional time-dependent model, accounting for the effects of surface tension, gravity, inertia and magnetic pinch forces in the droplet, was used. The wire feed speed and gas flow rate were also incorporated into the predictions. They also predicted the presence of both small and large droplets (alternately) at the transition zone between globular and spray modes, in agreement with the above-mentioned work. Similar droplet flow characteristics obtained by the above models were experimentally
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detected by Clark et al. (1989) and Johnson et al. (1992) in similar conditions (GMAW, Ar2%O2, 0.89 mm electrode wire). Johnson et al. observed an electrode extension increase during the detachment of large droplets, justified by a slower melting rate than expected. After a series of small droplet detachments, the electrode extension decreased, since these small droplets melted off faster than the average rate. According to this author, this cycle
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sometimes repeated itself several times. For example, one or two large droplets may be
followed by a series of small droplets but then followed by other one or two large droplets.
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Madigan et al. (1992) also observed electrode extension changes during metal transfer.
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Working in the droplet – spray transition zone, with a constant current power source, they observed an electrode extension increase (arc length decrease) just before droplet
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detachment. These authors considered the electrode extension to be the sum of the solid cylinder and the droplet diameter.
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Despite the evidence that there might be some distinctive metal transfer modes happening in the transition zone between two adjacent transfer mode fields, most
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researchers describe them as transfer mode instability of a chaotic character. However, Scotti (2000) reported that, under certain welding conditions, two or more natural-like transfer
ce pt
modes can happen in a periodic sequence (without any interference of the operator and/or a control system). He also showed in his results that this periodic pattern of changes in the metal transfer mode is not restricted to the transition zones between adjacent fields but may
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also occur in different combinations. For example, Short-circuiting - Projected Spray, Shortcircuiting - Streaming Spray, Globular-Projected Spray, Globular-Streaming Spray, GlobularShort-circuiting - Streaming, Spray-Globular, etc. were observed. These patterns of transfer have not been widely commented on in the literature, most likely because the related transfers are difficult to identify using ordinary laboratory techniques. Moreover, they are easily confused with temporary transfer instability which may occur for example when operating within a transition operational envelope between two adjacent natural transfer modes. Despite the above mentioned reports, there is little published information on multi-
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mode metal transfer. Thus, the objective of this work was to study more consistently the existence of the above mentioned metal transfer class of modes, hereafter referred as “Interchangeable metal transfer”.
2 -Experimental Procedure
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A series of experiments was carried out with the aim of reproducing welding
conditions that would lead to differing modes of interchangeable metal transfer in GMA
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welding. Bead-on-plate welds were carried out on carbon steel plates using a 1.0-mm-
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diameter wire of the AWS ER70S-6 class with DCEP. The approach was to select a different shielding gas (Ar + 2% or 5%O2) and then set a combination of inductance, welding current
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and arc voltage to produce the desired droplet transfer mode. An electronic constant voltage output characteristic power source was used in these experiments.
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The main methodological approach applied was based on a system for metal transfer visualization as used by Lin et al. (2001) and Bálsamo et al. (2000), among others. The
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experimental rig was set up as shown in Fig. 2. A shadow of the non-transparent components (contact tube, electrode, metal drops, weld pool and plate) of the arc region was
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projected onto the lens of a camera, a technique known as backlighting. A high-speed digital camera working at 2,000 fps and a 632.2 nm He-Ne laser were used. To enable the arc to be seen also, optical filters of different intensity were employed. The electric signals were
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synchronised with the film frames to correlate the variations in arc voltage and welding current with the formation and detachment of the droplets. Synchronization was carried out using a dedicated program built in a LabView® platform.
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8 7 6 5
5
9 Synchronization signal
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4
Ua
3 A
400
Iw
300 200 100
Ua
0 V
20
11
0
Iw 10
0
10
20
30
40
t, ms
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1
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2
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Fig. 2. Details of the optical laser system used for metal transfer visualization. 1, light source (laser); 2, neutral filters; 3, divergent lens; 4, convergent lens; 5, protection glass; 6, band-
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pass and neutral filters; 7, high-speed video camera; 8, monitor; 9, image recording unit; 10,
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3 - Analysis of Results
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computer; 11, current hall probe (after Scotti et al., 2012).
Different types of interchangeable metal transfer modes (two or more transfer naturallike modes happening in a periodic sequence) were generated. The characteristics and the
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reason for the occurrence of each interchangeable metal transfer mode are described and discussed as follows.
3.1 - Interchangeable “short-circuiting – spray” mode The two natural transfer modes during this type of the interchangeable metal transfer are the short-circuiting mode and the streaming/projected spray one, as illustrated in Fig. 3. The welding conditions (arc voltage and welding current instant values) initially favour the natural short-circuiting transfer which includes the droplet growth and short-circuit stages.
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However, during the post short-circuit period, a higher mean current level leads to a high post short-circuit current, which remains temporarily above the transition current level. Due to this augmented current, the electrode melting rate becomes momentarily higher than the wire feed speed (WFS) and the arc length progressively increases. This has the effect of preventing short-circuiting transfer as the process has sufficient time to enable more than
ip t
one tiny droplet to detach sequentially. With a constant voltage power source, as a longer arc makes the welding current decrease, the electrode melting rate also falls gradually. As
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electrode melting rate becomes less than the WFS, the wire tip returns to approaching the
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weld pool. The combination of a low current and a short arc reinstates the conditions required for short-circuiting to occur. Normally only one drop is transferred before a new
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cycle is initiated.
Streaming spray
Short-circuiting
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Transition current
Ua
10
Iw
20
30
40
Short-circuiting
50
t (ms)
Projected Spray
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(A) 400 300 200 100 0 20 (V) 0 0
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Short-circuiting
(A) 400
Iw
300 200 100
Ua
0 20
(V) 0
0
10
20
30 t (ms)
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Fig. 3. Examples of an Interchangeable Metal Transfer mode of the type “short-circuiting – spray” (above “streaming” and below “projected” spray) and the correspondent arc voltage (Ua) and welding current (Iw) traces: mean Ua = 23.5 V; mean Iw = 170 A; set WFS = 7 m/min; travel speed = 36 cm/min; contact-tube to work distance (CTWD) = 18 mm; shielding gas =
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Ar + 5%O2.
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As can also be seen in Fig. 3, switching of the natural metal transfer modes is cyclical
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which mainly depends on the inductance of the power source (dynamic response of the current, i.e., current rising and falling rates), arc length and the combination of the electrode
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and the shielding gas which influences the surface tension. The latter determines the transition current level and the others act together to determine the short-circuit duration and
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indirectly, the short-circuiting current level. These preconditions substantiate the reason for
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occurrence of the "short-circuiting - spray" interchangeable metal transfer mode.
3.2 - Interchangeable “globular - spray” mode
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As seen in Fig.4, globular and spray natural transfer modes are interchanging giving rise to an interchangeable transfer mode. It is considered that the reason for this mode is that when using shielding gas mixtures with less than 12% CO2 and a carbon steel wire, the
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electrical resistivity of the droplet becomes higher than that of the arc column. During a globular transfer under such conditions in which the droplet resistivity is greater than the arc column resistivity, the growth of the droplet overcomes the effect of the shortening of the arc regarding the resistance variation. The increase in the summation of the electric resistances consequently reduces progressively the welding current and resulting in a reduction of the wire melting rate. Thus, even though the arc length became shorter, the voltage measured between the contact tip and the work piece increases, as illustrated in Figs. 5 and 6. Due to a progressive reduction of the electrode melting rate, the electrode tip with a globular droplet attached approaches the weld pool, sometimes causing incipient short-
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circuits, as shown in Fig. 7. Together with an increase in the electrode extension, the total electric resistance starts to reduce so that the lower resistivity of the wire dominates the resistivity of the arc column and the welding current starts to increase again. Thus, the welding current can reach values above the transition current which is low for these low CO2 Ar based gas mixtures. This results in a projected (see Fig. 4) or even streaming spray
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transfer (see Fig. 7). The resulting high electrode melting rate coincident with this high
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re-established for the globular transfer and a new cycle sets in.
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current causes the arc length to increase and the current to reduce. The conditions are now
Range of the electrode tip oscillation
Projected spray
Globular
620 700
726
751
752
29 27 25 0.45
Ua 0.5
(Ohm)
0.55
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0.22 0.20 0.18 0.16 0.14 0.12 0.45
794 804 805 812
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(V)
781 783
843 852 950 1035 1058 1059 1082 1090
(A)
ed
Transition current
760
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an
Globular
200 180 160
Iw
140 120
0.6
0.65
0.7
0.75
0.8
0.85
0.9
t (s)
0.6
0.65
0.7
0.75
0.8
0.85
0.9
t (s)
Ra
0.5
0.55
Fig. 4. An example of an interchangeable metal transfer mode of the “globular – spray” type and the correspondent Ua, Iw and instantaneous arc resistance (Ra) traces: mean Ua = 27.9 V; mean Iw = 166 A; WFS = 6.3 m/min; travel speed = 30 cm/min; CTWD = 18 mm; shielding gas = Ar + 5%O2.
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55
59
60
61
76
80
95
103
109
115
120
123
124
125
U (V)
ip t
31 30
cr
29
27 0.3
0.35
0.4
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28 0.45
0.5
0.55
t (s)
an
Fig. 5. Voltage trace showing the arc voltage variation as a function of the droplet growing and detachment (globular transfer): Iw = 182 A; WFS = 6.7 m/min; CTWD = 20 mm; shielding
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gas = Ar + 2%O2
Ar+10 … 12% CO2
U
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Ar+1 … 5% CO2
U
U
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t
t
Iw
t
Iw
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Iw
Ar+20 … 100% CO2
Droplet resistivity > Arc column resistivity
Droplet resistivity ≈ Arc column resistivity
Droplet resistivity < Arc column resistivity
Fig. 6. Schematic illustration of the alteration of the ratio between the droplet and arc column electric resistivities as a function of the CO2 content in an argon based gas mixture. The droplet and arc column electric resistivities are illustrated by lines, where the thicker line means lower resistivity
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Range of the electrode tip oscillation
750
800
Short-circuiting
850
900
950
958
959
Projected Spray
962
964 970
Streaming spray
980 985 990
Iw
Transition current (A)
45
35
Ua
1350 1400
1450
1.05
1480 1485
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1200 1250 1300
0.95
1.15
ed
0.85
M
15
5 0.75
175 150 125 100
an
25
200
us
Iw
1487
1490
75 50 25 0 -25
1.25
1.35
t (s)
1550 1600 1650 1700 1750 1800 1850 1900
Ra
0.85
0.95
1.05
1.15
1.25
1.35
t (s)
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0.24 0.2 0.16 0.12 0.08 0.04 0 0.75
995 1000 1050 1100 1150
cr
Ua (V)
(Ohm)
ip t
Globular
Fig. 7. An example of an interchangeable metal transfer mode of the “Globular - Streaming Spray” type: mean Ua = 28.4 V; mean Iw = 177 A; WFS = 6.5 m/min; travel speed = 36 cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
Thus, the reason for a "globular - spray" interchangeable metal transfer mode is a lower specific resistance of the arc column compared to that of the metal droplet, which is conditioned by the use of shielding gas mixtures with less than 12% CO2.
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Page 12 of 32
Then, the question is whether shielding gas mixtures with more than 12% CO2 could promote or not an interchangeable transfer “globular – spray” as the specific resistance of the arc column is now higher than that of the droplet, as illustrated by Fig. 6. During the globular transfer stage, as long as the droplet is growing and, consequently the arc length is reducing, the arc voltage reduces as well. The reason is that the reduction of the total arc
ip t
column voltage drop is more significant than the voltage drop in the growing droplet which when using constant voltage power sources, causing a current increase. Then, if,
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hypothetically, the current exceeds the transition current level, the spray transfer mode could
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be established followed by an increase of the electrode melting rate and the arc lengthening of the arc. This behaviour could be accompanied by a rise in the arc column resistance,
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resulting in a reduction in the current. Finally, there might be a re-establishment of the globular transfer mode. A new cycle would be re-established. However, the higher the CO2
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content in a gas mixture, the higher the transition current value becomes, and, thus, it becomes more difficult to be exceeded. This is the main reason why the interchangeable
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metal transfer mode is usually not observed when using CO2 rich argon based shielding
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mixtures.
3.3 - Interchangeable “globular – short-circuiting - streaming spray” mode Fig. 8 illustrates the interchangeable mode "globular – short-circuiting – streaming
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spray". It is considered that, in this case, the conditions described in both sections 3.1 (high post short-circuiting current) and 3.2 (a lower specific resistance of the arc column and of the wire electrode compared to that of the metal droplet) are acting in combination. There was a gradual reduction of the current between frames 792 and 900, caused by an increase in the growing droplet resistance. Due to the consequent reduction of the electrode melting rate, the electrode tip with a droplet starts to move towards the weld pool, from frames 900 to 1028. Together with an increase in the electrode extension, the total electric resistance starts to reduce and the lower resistivity of the wire now starts to dominate over that of the arc column so that the current increasing again. Although the increment in the current was not
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sufficient to exceed the globular-spray transition, as there was a short-circuit, the metal transfer mode transition became possible due to a high post short-circuiting current. The reduction of the arc voltage as compared with the case shown in Fig. 7 was one of the favourable conditions for this mode to happen.
Short-circuiting
Streaming spray
Globular
780
792
850
900
950
980
1028
1029
1040
1045
1050
1065
1090
1110 1140
(A)
an
750
us
cr
Globular
ip t
Range of the electrode tip oscillation
300
Iw
Transition current (V) 30 20
0 0.74
0.78
0.82
(Ohm) 0.2
Ra
0.12 0.08 0.04
0.78
150
0.82
100 50 0
0.86
0.9
0.94
0.98
t (s)
0.86
0.9
0.94
0.98
t (s)
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0 0.74
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0.16
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Ua
10
200
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40
250
Fig. 8. An example of an interchangeable metal transfer mode of the “globular – shortcircuiting – streaming spray” type: mean Ua = 27.5 V; mean Iw = 170 A; WFS = 6.5 m/min; travel speed = 36 cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
3.4 - Interchangeable “projected spray - streaming spray” mode There are certain energy-related conditions which can make the current vary periodically generating the "projected spray - streaming spray" interchangeable metal transfer mode, Fig. 9. The intensive generation of metallic vapour in an arc under streaming
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spray transfer creates the potential to alter the plasma properties in such a way as to force the current to reduce due to a lower ionization potential and/or heat transfer losses. The transfer would, then, turn into projected spray with less metallic vapour generation, which would make the current increase again, and so on. An alternative reason for the current starting to reduce at the end of the streaming
ip t
spray stage is that, when using a constant voltage power supply, a progressively increasing arc length occurs. Vice-versa, during the projected spray transfer, a low current causes the
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arc to shorten, inducing a current rise and moving the transfer mode transition towards the
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streaming spray mode. In either case, there is a clear manifestation of the interchangeable metal transfer mode fundamental principles, when variations of conditions due to a previous
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transfer mode give rise to conditions for the following mode to take place.
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Range of the electrode tip oscillation
(V) 30
Ua
895
899
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29
855
903
904
906
950
1100
Spray projected
1206 1255
1258 1300
1350 1402 1480
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820
Streaming spray
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Spray projected
(A)
Iw
215 210 205 200 195
28
27 1.75
1.8
1.85
1.9
1.95
2
2.05
2.1
2.15
1.9
1.95
2
2.05
2.1
2.15
t (s)
(Ohm) 0.15
Ra 0.14 0.13 1.75
1.8
1.85
t (s)
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Fig. 9. An example of an interchangeable metal transfer mode of the “spray projected – streaming spray” type: mean Ua = 28.7 V; mean Iw = 207 A; WFS = 8.7 m/min; travel speed = 36 cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
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4 – Classification of Metal Transfer Modes
Although the interchangeable transfer modes occur under welding conditions
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between those for adjacent natural ones, they should not be confused with a transition
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transfer mode, because they are characterized by sequential periodic repeatability. It is not a phenomenon of occasional natural instability between two modes. The most important
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characteristic of Interchangeable transfer is that the following mode is a consequence of the previous one. In particular, the variation of current, electrode temperature and/or plasma
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status due to a transfer mode gives rise to conditions for the following mode to take place. An interchangeable metal transfer mode takes place only if all the necessary conditions are
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present, i.e., a combination of welding current, arc length, material and diameter of the wire, shielding gas, contact tube to work distance and favourable dynamics (inductance) of the
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power source. It is important to note that further research work is still required to establish the ranges for the conditions promoting each of the interchangeable transfer modes. The Interchangeable Transfer Mode cannot be attributed to either Natural Transfer
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Modes because its characteristic sequential periodic changing between two or even more natural transfer modes or to Controlled Transfer Modes because there is no in-line or off-line control. These types of transfer mode possess all characteristics of an individual class of modes which has been called Class of Interchangeable Transfer Modes, as summarized in Fig. 10.
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ip t cr us
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Fig.10. GMAW Metal Transfer Classification based on hierarchical order: classes, groups
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and modes
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Interchangeable metal transfer modes are not identifiable by welders and operators, even though characterized by a low frequency of metal transfer interchanging (3 to 5 Hz). If
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the transfer is interchanging from globular to spray, it is unstable as occurs with a globular transfer. But if it is interchanging from short-circuiting and spray, the welder may not feel the any difference in performance from the normal short-circuiting operation. In fact, modern power source manufactures are trying to develop controlled metal transfers interchanging
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from, for instance, pulsed transfer to short-circuit transfer, to satisfy special applications. Including the new “Interchangeable Metal Transfer” class in the overall classification
of GMA Metal Transfer Modes for arc welding completes the classification. It is now possible to describe all modes of metal transfer from the simple “Natural Metal Transfer” class to the often quite complex multi-mode type of transfer categorised in the “Interchangeable Metal Transfer” class which has been identified in this study.
5 – Conclusions
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Modern laboratory techniques, especially high speed video filming synchronized with welding parameters acquisition, brought out evidences that: - There is a new metal transfer class “Interchangeable Metal Transfer”, which with the well known Natural and Controlled Metal Transfer classes completes the classification
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of metal transfer for GMA welding of carbon steels;
- The interchangeable metal transfer mode is distinguished from the others classes of
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metal transfer because of its unique characteristic of periodical changes in the transfer
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mode induced from short temporal changes in welding parameters (a self-sustained behaviour);
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- The interchangeable metal transfer mode may comprise two or more natural transfer modes happening in a periodic repetitive sequence, one following the other, as a
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consequence of the previous one. There is no operator or adaptive control system interference;
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- The interchangeable metal transfer mode can only take place if all the necessary conditions are present, i.e., a combination of welding current, arc length, material and
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diameter of the wire, shielding gas, contact-tube to work distance and favourable dynamics (inductance) of the power source; - The interchangeable metal transfer mode does not occur when using shielding gas
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mixtures with more than 12% CO2.
6 – Acknowledgements
The authors would like to thank the Brazilian agencies for research and development (CNPq and Fapemig), which have provided the financial backing for the specialized equipment used in this work (high-speed camera, laser back-light system, synchronized frames-electrical signal data loggers).
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7 – References
Bálsamo, P.S.S., Vilarinho, L.O., Vilela, M., Scotti, A., 2000. Development of an experimental technique for studying metal transfer in welding: synchronised shadowgraphy. Int. J. Join. Mater. 12(1), 1-12, ISSN 0905-6866.
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Clark, D.E., Buhrmaster, C.L., Smartt, H.B., 1989. Drop Transfer Mechanisms in GMAW. In: 2nd Int. Conf. on Recent Trends in Welding Science and Technology, ASM, Gatlinburg,
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Tennessee, USA, pp. 371-375.
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Haidar, J., Lowke, J.J., 1996. Predictions of metal Droplet Formation in Arc Welding, J. Phys. D: Appl. Phys., 29, 2951-2960.
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Heald, P. R., Madigan, R. B., Siewert, T. A., Liu, S., 1994. Mapping the Droplet Transfer Modes for an ER100S-1 GMAW Electrode". Weld. J. 73 (2), 38s-44s.
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IIW (International Institute of Welding), 1976. Classification of Metal Transfer, IIW Doc. XII636-76.
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Johnson, J.A., Smart, H.B., Carson, N.M., Waddoups, M., 1992. Dynamics of droplet Detachment in GMAW. In: 3rd Int. Conf. on Trends in Welding Research, ASM,
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Gatlinburg, Tennessee, USA, pp. 987-991.
Lin, Q., Li, X., Simpson, S.W., 2001. Metal transfer measurements in gas metal arc welding. J. Phys. D: Appl. Phys., 34 (3), 347–353, doi: 10.1088/0022-3727/34/3/3172000.
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Madigan, R.B., Quinn, T.P., Siewert, T.A., 1992. Sensing Droplet Detachment and Electrode Extension for Control of Gas Metal Arc Welding. In: 3rd Int. Conf. on Trends in Welding Research, ASM, Gatlinburg, Tennessee, USA, pp. 999-1002. Scotti, A., 2000. Mapping the Transfer Modes for Stainless Steel GMAW. J. Sci. Technol. Weld. Join. 5 (4), 227-234, ISSN 1362-1718. Scotti, A., Ponomarev, V., 2008. MIG/MAG Welding: better understanding – better performance. Artliber Editora Ltda., São Paulo, Brazil, pp. 268 - 277 (in Portuguese). Scotti, A., Ponomarev, V., Lucas, W., 2012. A Scientific Application Oriented Classification for Metal Transfer Modes in GMA Welding. Journal of Materials Processing Technology,
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212, 1406-1413, doi:10.1016/j.jmatprotec.2012.01.021. Scotti, A., Ponomarev, V., Resende, A., 2006. The influence of the electrode materials and shielding gas mixture on the specific electric resistances of the drop/column of the arc in GMA welding, IIW Doc. XII-1909-06. Watkins, A.D., Smartt, H.B., Johnson, J.A., 1992. A Dynamic Model of Droplet Growth and
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Detachment in GMAW. In: 3rd Int. Conf. on Trends in Welding Research, ASM,
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Gatlinburg, Tennessee, USA, pp. 993-997.
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Figure Captions
Fig. 1. Schematic maps of the main natural metal transfer modes occurring in GMA welding as a function of the welding current (Iw), represented by either the welding voltage setting, on
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the left, or the arc length, on the right (after Scotti et al., 2012).
Fig. 2. Details of the optical laser system used for metal transfer visualization. 1, light source
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(laser); 2, neutral filters; 3, divergent lens; 4, convergent lens; 5, protection glass; 6, band-
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computer; 11, current hall probe (after Scotti et al., 2012).
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pass and neutral filters; 7, high-speed video camera; 8, monitor; 9, image recording unit; 10,
Fig. 3. Examples of an Interchangeable Metal Transfer mode of the type “short-circuiting –
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spray” (above “streaming” and below “projected” spray) and the correspondent arc voltage (Ua) and welding current (Iw) traces: mean Ua = 23.5 V; mean Iw = 170 A; set WFS = 7 m/min;
ed
travel speed = 36 cm/min; contact-tube to work distance (CTWD) = 18 mm; shielding gas =
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Ar + 5%O2.
Fig. 4. An example of an interchangeable metal transfer mode of the “globular – spray” type and the correspondent Ua, Iw and instantaneous arc resistance (Ra) traces: mean Ua = 27.9
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V; mean Iw = 166 A; WFS = 6.3 m/min; travel speed = 30 cm/min; CTWD = 18 mm; shielding gas = Ar + 5%O2.
Fig. 5. Voltage trace showing the arc voltage variation as a function of the droplet growing and detachment (globular transfer): Iw = 182 A; WFS = 6.7 m/min; CTWD = 20 mm; shielding gas = Ar + 2%O2 (after Scotti et al., 2006).
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Fig. 6. Schematic illustration of the alteration of the ratio between the droplet and arc column electric resistivities as a function of the CO2 content in an argon based gas mixture. The droplet and arc column electric resistivities are illustrated by lines, where the thicker line
ip t
means lower resistivity (after Scotti and Ponomarev, 2008).
Fig. 7. An example of an interchangeable metal transfer mode of the “Globular - Streaming
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Spray” type: mean Ua = 28.4 V; mean Iw = 177 A; WFS = 6.5 m/min; travel speed = 36
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cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
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Fig. 8. An example of an interchangeable metal transfer mode of the “globular – shortcircuiting – streaming spray” type: mean Ua = 27.5 V; mean Iw = 170 A; WFS = 6.5 m/min;
ed
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travel speed = 36 cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
Fig. 9. An example of an interchangeable metal transfer mode of the “spray projected –
ce pt
streaming spray” type: mean Ua = 28.7 V; mean Iw = 207 A; WFS = 8.7 m/min; travel speed =
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36 cm/min; CTWD = 18 mm; shielding gas = Ar + 2%O2.
Fig.10. GMAW Metal Transfer Classification based on hierarchical order: classes, groups and modes (described in more details by Scotti et al., 2012).
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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Figure 9
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Figure 10
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