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Известия ТулГУ. Технические науки. 2015. Вып. 6. Ч. 2
УДК 621.791
DETAILED MIG/MAG WELDING METAL TRANSFER CLASSIFICATION. PART 2: INTERCHANGEABLE METAL TRANSFER PHENOMENON
Vladimir Ponomarev, Americo Scotti, William Lucas
Metal transfer modes in arc welding processes have previously been classified as Natural or Controlled Metal Transfer. Modern laboratory techniques have helped to establish a new transfer classification mode in MIG/MAG welding 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 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.
Key words: MIG/MAG Welding; Interchangeable Metal Transfer.
Despite many works are carried out on metal transfer phenomenon, there is little published information on multi-mode metal transfer, hereafter referred as "Interchangeable metal transfer" observed and studied by some researchers including the present paper authors. The initial objective of this work was to carry out a detailed study of metal transfer in MIG/MAG welding with the intention of identifying all types of metal transfer, including the above mentioned "Interchangeable metal transfer". A better understanding of the metal transfer phenomenon is important for improvements in the quality and productivity of MIG/MAG welding. The experimental procedures used are presented in the first part of this work: Detailed MIG/MAG welding metal transfer classification. Part 1: General Considerations.
Interchangeable metal transfer mode characteristics. Scotti [1] and Ponomarev et al. [2], showed that there is a pattern of transfer which is not widely commented on in the current literature, most likely because the related transfers are difficult to be identified using ordinary laboratory techniques. Moreover, they are easily confused with temporary transfer instability during a setting at a transition operational envelope between two adjacent natural modes. To certain welding conditions, two or more transfer natural-like modes happen in a periodic sequence (without any interference of the operator and/or a control system), such as interchanging of modes. One important characteristic of this transfer is
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that the following mode is a consequence of the previous one (the variation of current, electrode temperature and/or plasma status due to a transfer mode gives rise to conditions for the following mode to take place). For instance: short-circuiting-Projected spray; short-circuiting-streaming spray; globular-projected spray; globular-streaming spray; globular-short-circuiting-streaming spray-globular; others.
It is also important to mention the transition zones between adjacent transfer mode 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 modes. This phenomenon can be explained by using a model proposed by Watkins et al. [3], 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 aperiodic quasi-random pattern of behaviour. Haidar & Lowke [4] 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 detected by Clark et al. [5] and Johnson et al. [6] in similar conditions (MIG/MAG, Ar-2%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 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. Madigan et al. [7] also observed electrode extension changes during metal transfer. 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 detachment. These authors considered the electrode extension to be the sum of the solid cylinder and the droplet diameter.
Despite the evidence that there might be some distinctive metal transfer modes happening in the transition zone between two adjacent transfer mode fields, most researchers describe them as transfer mode instability of a chaotic character. However, Scotti [1] and Ponomarev et al. [2], reported that, under certain welding conditions, two or more natural-like transfer 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
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metal transfer mode is not restricted to the transition zones between adjacent fields but may also occur in different combinations. For example, Short-circuiting - Projected Spray, Short-circuiting - Streaming Spray, Globular-Projected Spray, Globular-Streaming Spray, Globular-Short-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. Below, some Interchangeable metal transfer modes are described in details.
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. 1. 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. 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 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 electrode melting rate becomes less than the WFS, the wire tip returns to approaching the 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 cycle is initiated.
As can also be seen in fig. 1, switching of the natural metal transfer modes is cyclical 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 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 indirectly, the short-circuiting current level. These preconditions substantiate the reason for occurrence of the "short-circuiting - spray" interchangeable metal transfer mode.
Interchangeable "globular - spray" mode. As seen in fig. 2, 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 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
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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. 3 and 4.
Short-circuiting
A.
Streaming spray
Short-circuiting
TRANS
Fig. 1. 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 -Ar + 5%02
Due to a progressive reduction of the electrode melting rate, the electrode tip with a globular droplet attached approaches the weld pool, sometimes even causing incipient short-circuits. 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
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starts to increase again. Thus, the welding current can reach values above the transition current which is low for these low C02 Ar based gas mixtures. This results in a projected (see fig. 2) or even streaming spray transfer. The resulting high electrode melting rate coincident with this high current causes the arc length to increase and the current to reduce. The conditions are now reestablished for the globular transfer and a new cycle sets in.
Range of the electrode tip oscillation
0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
Fig. 2. An example of an interchangeable metal transfer mode of the "globular - spray" type and the correspondent Ua, /„, 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 % 02
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% C02. Then, the question is whether shielding gas mixtures with more than 12% C02 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. 4. 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 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.
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Fig: 3. Voltage trace showing the arc voltage variation as a function of the droplet growing and detachment (globular transfer): /„, = 182 A; WFS = 6.7 m/min; CTWD = 20 mm; shielding gas =Ar + 2%02
Ar+1 ... 5% CO.
Ar+10 ... 12% CO,
Ar+20 ... 100% CO.
Droplet resistivity > Arc Droplet resistivity ~ Arc Droplet resistivity < Arc
column resistivity column resistivity column resistivity
Fig. 4. Schematic illustration of the alteration of the ratio between the droplet
and arc column electric resistivities as a function of the C02 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
Then, if, hypothetically, the current exceeds the transition current level, the spray transfer mode could 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, resulting in a reduction in
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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 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 metal transfer mode is usually not observed when using CO2 rich argon based shielding mixtures.
General observations on interchangeable metal transfer modes. Although the interchangeable transfer modes occur under welding conditions between those for adjacent natural ones, they should not be confused with a transition 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 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 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 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 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.
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 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 from, for instance, pulsed transfer to short-circuit transfer, to satisfy special applications.
Conclusions. 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 of metal transfer for GMA welding of carbon steels;
The interchangeable metal transfer mode is distinguished from the others classes of metal transfer because of its unique characteristic of periodical changes in the transfer mode induced from short temporal changes in welding parameters (a self-sustained behaviour);
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 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 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 mixtures with more than 12% CO2.
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 (highspeed camera, laser back-light system, synchronized frames-electrical signal data loggers).
References
1. Scotti, A. Mapping the Transfer Modes for Stainless Steel GMAW, J. of Science and Technology of Welding and Joining, 2000, 5 (4), 227-234 (ISSN 1362-1718).
2. Ponomarev, V., Scotti, A., Miranda, H.C. & Costa, A.V. Influence of the power source dynamic characteristics on the metal transfer mixed mode, 2002, IIW Doc. 212-1014-02.
3. Watkins, A.D., Smartt, H.B., Johnson, J.A. A Dynamic Model of Droplet Growth and Detachment in GMAW. In: 3rd Int. Conf. on Trends in Welding Research, ASM, Gatlinburg, Tennessee, USA, 1992, pp. 993-997.
4. Haidar, J., Lowke, J.J., Predictions of metal Droplet Formation in Arc Welding, J. Phys. D: Appl. Phys., 1996, 29, 2951-2960.
5. Clark, D.E., Buhrmaster, C.L., Smartt, H.B., Drop Transfer Mechanisms in GMAW. In: 2nd Int. Conf. on Recent Trends in Welding Science and Technology, ASM, Gatlinburg, Tennessee, USA, 1989. P. 371-375.
6. Johnson, J.A., Smart, H.B., Carson, N.M., Waddoups, M., Dynamics of droplet Detachment in GMAW. In: 3rd Int. Conf. on Trends in Welding Research, ASM, Gatlinburg, Tennessee, USA, 1992. P. 987-991.
7. Madigan, R.B., Quinn, T.P., Siewert, T.A., 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, 1992. P. 999-1002.
Vladimir Ponomarev, candidate of technical science, ponomare vamecanicajifu.hr, Brazil, Uherlandia, Federal University of Uherlandia,
Américo Scotti, candidate of technical science, ascottia ufu.hr, Brazil, Uherlandia, Federal University of Uherlandia,
William Lucas, candidate of technical science, ponomare vamecanicajifu.hr, UK, Camhridge, The Welding Institute, TWI Ltd, CB21 6AL
Известия ТулГУ. Технические науки. 2015. Вып. 6. Ч. 2
ДЕТАЛИЗИРОВАННАЯ КЛАССИФИКАЦИЯ СПОСОБОВ ПЕРЕНОСА ЭЛЕКТРОДНОГО МЕТАЛЛА ПРИ СВАРКЕ В ЗАЩИТНЫХ ГАЗАХ. ЧАСТЬ 2: ЧЕРЕДУЮЩИЙСЯ ПЕРЕНОС ЭЛЕКТРОДНОГО МЕТАЛЛА
Владимир Пономарев, Америко Скотти, Уильям Лукас
Способы переноса электродного металла обычно подразделяются на естественные и контролируемые. Современное лабораторное оборудование позволило обнаружить и классифицировать еще один способ переноса электродного металла, получивший название «чередующийся перенос электродного металла». Для его изучения были выполнены многочисленные эксперименты с различными сочетаниями защитных сред, сварочных проволок и параметров сварки. Для регистрации переноса металла использовалась скоростная видеосъемка с лазерной подсветкой и синхронизацией с электрическими параметрами дуги. Полученные результаты показали, что способы чередующегося переноса электродного металла отличаются от таковых относящихся к классам естественных и контролируемых способов, так как характеризуются особой строгой периодичностью чередующихся способов как результат изменений текущих значений параметров сварки (как в саморегулируемой системе). При этом чередоваться могут два и более естественных способов переноса металла. Такое чередование способов переноса металла осуществляется без участия оператора или каких-либо систем контроля сварочного источника питания.
Ключевые слова: сварка в защитных газах, чередующийся перенос электродного металла.
Владимир Пономарев, канд. техн. наук, ponomare vamecanicajifu.hr, Бразилия, Уберландия, Федеральный Унивеситет города Уберландия,
Америко Скотти, канд. техн. наук, ascotti@,ufu.hr, Бразилия, Уберландия, Федеральный Унивеситет города Уберландия,
Уильям Лукас, канд. техн. наук, ponomare vamecanicajifu.hr, Англия, Кембридж, Институт Сварки (TWI Ltd), CB21 6AL
