COLD METAL TRANSFER – THE CMT PROCESS

Somoskői Gábor Ügyvezető

Froweld Hegesztéstechnikai és Kereskedelmi kft.

Doctoral supervisor: Dr. Török Imre ME, Institute of Mechanical Technology

ABSTRACT

The article is about the industrial demand drive to developing the CMT process, the theory of the welding. The requirement for safe joining processes, mainly in the light-gauge area, requires welding processes with maximum process stability and adjustable, reduced heat input. The spatter and the heat impact of the ground material should be decreased; the economical figures of the whole process have to be increased. The CMT, this new welding approach which incorporates the wire feeding into process control satisfies these different demands. KEYWORDS

CMT welding, thin sheet, low heat input, low distortion, material transfer control, aluminum-steel joint, dip-arc, spray arc THE ORIGIN OF THE CMT-PROCESS Genesis

There are some serious problems using traditional GMAW welding process. In the one hand, because the economical ground always thinner and thinner sheets are used to construct structures. On the other hand new emerging technologies born, new material is necessary, high Ni base alloys, special aluminum alloy, just name a few.

To decrease the cost of pre-producing we use wider tolerance, but doing so we have problems with the gap bridging. The welding gap bridability and filling up of larger welded joints occur in practice quite often, but they remain almost untreated in literature. [1]

The next important aspect is the low thermal distortion, to produce more ac-curate product or to decrease the building of residual stress. Residual stress exists in the structure without external load, and is a result of localized heating and cooling of the sheet by welding.

Spatter is also important aspect of the welding seam. However in author’s opinion the spatter could be dramatically decreased by setting up the right voltage and torch angle.

In the past couple of decades engineers and researchers had been trying to solve the different aspects. Ideas were coming from many sides, for instant devel-

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oping a lot of numbers shielding gases, trying to use smaller wire diameters and flux wires, modifying the electrical behaviors of the arc and the welding source, deploying twin or also three wire systems [2], etc.

The reason of the CMT-Process development, together with other processes was to solve the above mentioned problems using traditional GMAW welding. The first step was the demand to joint aluminum and steel in 1991. The micro-welding was interesting industrial application to produce common light bulbs, and the so called spatter free ignition made possibly to begin the serial producing in 2004.

1.2. The CMT arc process There are other developments to solve the above mentioned questions: con-

trol the current in the droplet phases. This process’s name is the STT (Surface Tension Transfer), dedicated mainly to weld the root passes of energetic pipe-line welding seams. STT welding power source provide stabile main welding parame-ters during welding process which enable welding by “short circuit arc”. The ma-terial transfer in electric arc is founded on surface tension force between weld pool and melted bead in electric arc. STT unit frequently and precisely controls welding current during welding. It sets an optimal welding parameters (which are stabile) by significant changing of arc length and “stick out”. [3] Other processes try to modify the inductivity and the electrical behavior of the welding circuit, with more or less outcome.

The most significant difference between traditional short, or spray arc and the CMT material transfer is the involving the wire motion in transfer process. The system has two fire feeding machine: the first is in the welding source and his main task is the continuously feed the wire with stable speed. The second is in the torch integrated, and this one is able to push and also to pull back the wire, couple dozens times per seconds. This creates a movement reminiscent of the well-known saying "two steps forward, one step back". To supply the feeding, a special buffer is installed between the two wire feeding machines.

When the welding wire touches work piece, a short circuit is created. The digital control recognizes the start of the short-circuit phase and reduces the weld-ing current accordingly. The control interrupts the arc for fractions of a second, thus preventing the usual spattering associated with short-circuit arcs. Another advantage is the reduced heat input into the weld pool, as heat input only occurs during droplet detachment in the non-arc phase. This explains just why this re-versing movement has been designed: the torch drive pulls the wire electrode out of the weld pool for a fraction of a second during the non-arc phase. This slight reverse movement assists the transfer of the droplet from the electrode into the weld pool with the help of the surface tension [4]. Immediately after the droplet detaches, the torch drive releases the wire so that it can move again towards the weld pool. In parallel, the control increases the current again and recreates the arc - a new cycle begins. See Figure 1.

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3

Figure 1. Droplet functions in the CMT process

To make a long story short following the main characteristic of the new process: Incorporated wire motions No spatter Extremely low thermal input Extremely stable arc

TYPICAL APPLICATIONS CMT on steel

Advantages are the increased speed, deeper penetration using CO2 shield gas and the practically no spatter. Figure 2. shows a typical seam, where we where able to increase the welding speed from 70 cm/min to 150 cm/min. The application is made at a Hungarian automotive supplier.

Figure 2.

Dip-transfer seam and CMT seam on steel CMT on aluminum

Dip-transfer arc

CMT

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At the aluminum joint with the CMT process we make possible impossible task, extreme thin sheets welding and also joint aluminum and steel. The newer is very important in the car industry because the aluminum chasses are more often. Figure 3 shows the former, Figure 4 the newer application.

Figure 3.

Ultra-light-gauge joints, higher welding speeds

Material aluminum 0,3 mm Pulsed arc not possible CMT speed = 6,4 m / min

Figure 4. Steel – aluminum joint (car crash-box)

Cladding

A very interesting task is solved in Hungary in energetic cladding industry. The extreme low heat input made possible to reduce the number of the welding seams. At the high nickel alloy cladding the ferrite-number was decreased. The producing such kind of cladded tubes became more economical. See Figure 5.

CMT

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5

Figure 5. Fe content has been decreasing by CMT welding

The other advantages were the extreme low dilution (the distortion was de-

creased by 75%) and speeding from 40 cm/min (TIG-Hot wire) up to 80 cm/min. SUMMARY

The CMT process is more than an extension of traditional GMAW welding process. It opens up new windows to solve welding applications. Work pieces that are virtually spatter-free require no costly mechanical rework. This saves af-ter handling that would otherwise be required, not to mention a second quality in-spection. The new process shows extremely stable arc and gives not seen oppor-tunities before. Welding correctly in the first place enables the workflow to be optimized, thus assisting with efforts to move towards "just in time" production. ACKNOWLEDGEMENT

The described work was carried out as part of the TÁMOP-4.2.1.B-10/2/KONV-2010-0001 project in the framework of the New Hungarian Devel-opment Plan. The realization of this project is supported by the European Union, co-financed by the European Social Fund. LITERATURA [1] J. Tušek,: Faculty of Mechanical Engineering, Ljubljana, Slovenia, METALURGIJA 42 (2003) 1, 21-25 [2] J. Tušek: Functions of Electrodes in the Formation of Weld in Triple- Electrode Submerged Arc Welding. Doc. 212-696-88. International Institute of Welding, Wien, 1988 [3] Dr.sc. Marko Dunđer - Prof.dr.sc. Ivan Samardžić: MONITORING OF MAIN WELDING PARAMETERS AT STT WELDING PROCESS 9th Interna-tional Research/Expert Conference ”Trends in the Development of Machinery

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and Associated Technology” TMT 2005, Antalya, Turkey, 26-30 September, 2005 [4] Somoskői Gábor: A CMT eljárás elméleti alapjai és gyakorlati alkalmazási lehetőségei 25. Jubileumi Hegesztési Konferencia Budapest, 2010. május 19-21 page 271-273

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