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Experimental study of metal transfer of CMT welding

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Experimental study of metal transfer of CMT welding

Bachir MEZRAG

Université de Montpellier. Laboratoire de mécanique et génie civil. Montpellier, France

bachir8813@hotmail.fr

Ismail BOURI

Université Abou Bekr Belkaid. Laboratoire d’Ingénierie des Systèmes Mécaniques et Matériaux. Tlemcen, Algérie

bouri-ing@hotmail.fr

Frédéric DESCHAUX BEAUME

Université de Montpellier. Laboratoire de mécanique et génie civil. Montpellier, France

frederic.deschaux-beaume@iut-nimes.fr Mustapha BENACHOUR

Université Abou Bekr Belkaid. Laboratoire d’Ingénierie des Systèmes Mécaniques et Matériaux. Tlemcen, Algérie

bmf_12002@yahoo.fr

Abstract

The CMT welding process was investigated in this work. The transfer of Al-4043 filler metal during welding was visualized using a high-speed camera with a green laser as an illumination source. Three phases was identified during the transfer cycle. A hot phase during which a molten droplet is formed on the end of the wire electrode under a high current. A cold phase corresponds to the move of the wire toward the weld pool. The contact between the droplet and the weld pool create a short circuit during which the material transfer is initiated and the arcing current reduced. After a set time (in the middle of the short circuit phase), the wire is retracted mechanically to contribute to the droplet detachment without increasing in current welding. The effect of the current waveform on the metal transfer is investigated too. The reduction in the hot phase duration of the current waveform decreases the volume of liquid droplets at the wire tip and allows to increase the short-circuit frequency.

KeywordsCMT welding; waveform; current; transfer; phase;

droplet

I. INTRODUCTION

Understanding welding process operation mode is essential for choosing the right parameters providing good assemblies quality. For Gas Metal Arc Welding (GMAW), the transfer of molten metal across the arc from the wire electrode to the liquid weld pool (which is highly dependent on the welding current level) is the main factor determining the operation mode of a process. Three fundamental droplet transfer modes are proposed for conventional GMAW process [1]: short circuit, globular and spray transfer. The metal transfer modes have been a significant development in the last decade; due to enhanced power source control features using the welding parameters to generate unique types of droplet transfer namely

Controlled Transfer Mode. Cold Metal Transfer (CMT) is one of these developed control metal transfer processes that use a particular mechanism of transfer metal.

The CMT process is new derivate of the well-known MIG/MAG process which was invented by Fronius company [2]. The principal innovation is the integration the motions of the wire into the overall control of the process to ensure a transfer of the filler metal to the welding pool without applied voltage and current. Every time the short circuit occurs, the digital process-control both interrupts the power supply and retracts precisely the wire. The wire retraction motion assists droplet detachment during the short circuit, thus the metal can transfer into the welding pool without the aid of the electromagnetic force under minimum heat input and no or few spatters [2-5]. The filler wire then moves forward again and the cycle is repeated.

The major aim of this work is to examine the metal transfer of the CMT process and its dependence of the waveform of welding current and voltage. The second aim is to highlight the influence of the evolution of the current waveform on the transfer metal.

II. EXPERIMENTAL PROCEDURES

All welding trials were conducted using a TPS 2700 Fronius CMT power source and a monitoring system consisting of a high-speed camera (Phantom ir300) with an interference filter and a green laser of 30 W as an illumination source. Frame capture rate was set at 9000 frames/s.

Experiments consist to deposit AlSi5 selected as filler metal in 1.2 mm diameter on aluminum sheet under protection of pure argon adopted as shielding gas at a flow rate of 12 l/min. Welding speed is fixed at 60 cm/min for all tests.

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The current and voltage waveforms were captured by the corresponding sensors at frequency of 50 kHz and then written into the computer. Special control software was developed to synchronize the running of the welding system, the image capturing system and the voltage and current sampling system.

Fig. 1 illustrates the schematic of the welding experimental set-up.

Fig. 1. Schematic of the welding experimental set-up.

III. RESULTS

A. Metal transfer cycle of the CMT process

Fig. 2 illustrates a typical CMT welding electrical signal cycle with its corresponding metal transfer process. A cycle can be defined as the period required to deposit one droplet of molten electrode into the weld pool. From the figure and according to the level of the electrical current, we can divide the cycle into three phases:

a) Hot phase:

It is corresponding to a peak current and high constant arc voltage during which a molten droplet is formed on the wire tip (image 5). These conditions facilitate also the ignition and stability of the arc.

b) Cold phase:

This phase represents the lower current and progressive decrease of the arc voltage corresponding to the feeding of the wire electrode forward until to make contact with the weld pool/base material creating a short circuit (image 6-8). This lower energy inhibits the detachment of the molten metal during this phase.

c) Short circuit phase:

During this phase the wire contacts with the weld pool (image 8-3) and the arc voltage drops into zero. In contrast to conventional MIG welding, in the CMT process the shorting- circuiting current is reduced to a low level. After a defined period the electrode is mechanically retracted (image 3), this rearward motion aiding in pinching the molten droplet into the weld pool. The arc is then reignited and the cycle repeats.

Note that a part of the drop remains attached to the electrode wire after breaking the short circuit, and is not transferred until the next short-circuit (Fig. 3).

Fig. 2. The current and voltage waveforms of the CMT process synchronized with high speed film of its metal transfer.

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Fig. 3. (a) Droplet formed before the short circuit, (b) metal remained attached to the end of the wire after the detachment of the droplet.

B. Evolution of the current waveform

After realizing seventeen tests by increasing the speed of wire feeding, it was found three types of waveform:

 The first type is characterized by a very low pulsation intensity less than the short circuit current and an important pulse duration (Fig. 4a ).

 The second type corresponds to a higher pulse intensities than short-circuit current, with increasingly shorter pulse durations gradually as the intensity increases (Fig. 4b).

 Finally, the third type corresponds to a waveform having very short pulses of constant intensity of 150 A (Fig. 4c).

We can easily observe that more the pulse intensity increases over its duration decreases, which favors increasing the frequency of a short circuit.

Note that whatever the type of waveform, the maximum voltage never exceeds 20 V, which corresponds to the upper value of the voltage domain allowing a short circuit metal transfer.

The geometry of the droplets formed at the end of wire also varies the waveform (Fig.4). For the first type of waveform corresponding to low intensities, the droplet formed is very elongated and its diameter is same as the wire. This geometry suggests a low thermal gradient with a low temperature droplet, which then has a high viscosity, does not allow it to reach a spherical geometry which minimizes the surface tensions.

When the intensity of the pulsations increases and their duration is rather high, the temperature droplet seems to be higher, since it adopts the spherical shape with a greater diameter than that of wire because of the large volume of droplets formed. The third type of waveform with very short pulses of high intensity produces small droplets of almost hemispherical shape.

Fig. 4. Types of waveforms output by the CMT process, and geometry of the droplets formed at the end of the wire.

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IV. CONCLUSION

The results of this study show the efficiency monitoring system chosen for online monitoring of arc welding process which brightness of the arc is completely filtered.

The CMT process is unique in that not only deposition controlled by the forward and rearward motion of the electrode, the electrical characteristics are also controlled with the result that molten metal transfer takes place at both low current and low voltage.

Increasing the electrode wire feed speed causes the evolution of electrical parameters that have a direct effect on the droplet shape formed thereafter.

A supplementary work is needed to study the evolution of parameters of each phase separately and the relationship between these parameters and the transfer of heat and mass resulting.

Acknowledgement

The authors would like to thank AVERROES Program for financing of this research.

References

[1] Scotti, V. Ponomarev, W. Lucas, “A scientific application oriented classification for metal transfer modes in GMA welding”, Journal of Materials Processing Technology, vol. 212, pp. 1406-1413, June 2012.

[2] Information on http: http://www.fronius.com

[3] C.G. Pickina, S.W. Williams, M. Lunt, “Characterisation of the cold metal transfer (CMT) process and its application for low dilution cladding”, Journal of Materials Processing Technology, vol. 211, pp.

496–502, March 2011.

[4] J. Feng, H. Zhang, P. He, “The CMT short-circuiting metal transfer process and its use in thin aluminium sheets welding”, Materials and Design, vol. 30, pp. 1850–1852, May 2009.

[5] H.T. Zhang, J.C. Feng, P. He, B.B. Zhang, J.M. Chen, L. Wang, “The arc characteristics and metal transfer behaviour of cold metal transfer and its use in joining aluminium to zinc-coated steel”, Materials Science and Engineering, vol. A 499, pp. 111–113, January 2009.

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