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Robotics Tech Papers

Fronius Cold Metal Transfer - A Welding System For the Most Demanding Applications


Fronius ushered in a new era in automated and robot-assisted GMAW (gas metal arc welding) technology with the introduction of the CMT (Cold Metal Transfer) process. Since then the innovative process, with its revolutionary reversing wire-feed unit, has been winning new applications, year after year, in the most varied sectors of industry. Nowadays, many welding and brazing tasks are simply no longer economically feasible without CMT.

The CMT process was developed with a view to solving all the unsolved problems of conventional GMAW welding. The usual systems on the market were all technologically advanced and state-of-the-art, but the engineers and developers at Fronius saw that there was room for improvement in terms of usage. Their key interests lay in achieving significantly better seam bridging, lower thermal distortion and above all, a welding result with as little spatter as possible. It soon became clear that significant improvements could only be made if people were thinking completely outside the box. Before any development work began, the team carried out intensive analysis of droplet and energy transfer and the formation of welding spatter. This research gradually led to an innovation that caused a sensation when it was presented at the Euroblech 2004 and again when it was launched onto the market in 2005 - cold metal transfer, or CMT.

"Different" is better
The significant differences between the CMT process and conventional short-arc processes include the way in which the wire movement is integrated into the process control, how the arc is precisely established and extinguished, and the way in which the droplets are detached accurately and without any spatter. What appears to be so simple is in fact an extremely complex interplay between arc control and wire electrode movement. This is where Fronius takes its own, unusual path.

The CMT system has two wire drives: one on the inverter, the other on the welding torch. The one on the inverter continuously feeds the wire electrode through the hosepack via the torch to the weld pool. The system's secret lies in the torch drive. This is not designed to simply support the other drive by pulling in the same direction. On the contrary, it counteracts this movement and pulls the wire electrode back a short distance in a defined rhythm before feeding it forwards again. This creates a movement reminiscent of the well-known saying "two steps forward, one step back". To prevent wire blockages, a special buffer is installed between the two torches. The advantages of this unique wire movement can only be seen when considered in combination with the different phases of the welding arc.

When the current-carrying welding wire comes within a certain, critical distance of the workpiece, a short circuit is normally created. However, the digital control recognises the start of the short-circuit phase and reduces the welding 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 Fronius designed this reversing movement: 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 in the targeted transfer of the droplet from the electrode into the weld pool. 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. On the first CMT systems these cycles occurred at 70 Hz (70 cycles per second). Today, they operate at up to 90 Hz and the results are impressive: this more accurate level of control also improves material transfer and gap bridgeability. This results in increased working speed and even more precise control of material and heat input.

The CMT process is characterised by three effects: virtually spatter-free welding, much lower heat input and targeted droplet detachment. The following examples demonstrate how users achieve these in practice.

Gap bridging - targeted droplet detachment for more flexible production
ELB-Form GmbH is an internationally renowned supplier to the automotive industry and specialises in the construction of lightweight hollow bodies. Its core skills include hydroforming and the welding of complex assemblies from a number of very dissimilar individual parts. The company often has to join molded parts that do fit together precisely due to manufacturing constraints. A prime example would be bent or stamped parts that have lots of imperfections at their joining interfaces. This means that products cannot always be set precisely to a defined gap. It is this large range of gap sizes that need to be bridged using welding techniques. To further complicate matters, the components all have very different wall thicknesses. The welding process employed must therefore have two features: extremely good gap bridging and an extremely low heat input.

Helmut Haspl, Works Manager at ELB-Form, believes that the CMT process is the ideal technology for automatic and robot-assisted GMAW welding. The CMT process presented the company with completely new perspectives in terms of weld quality and workflow. The excellent gap bridging; the low heat input, the virtually spatter-free welding results, the high level of process flexibility and the ability to reproduce welding results easily and consistently all contributed to the decision to switch to the CMT process. 

Similar experiences with CMT are reported daily by other suppliers to the automobile industry, as well as the car manufacturers themselves. Often parts need to be joined that have wall thicknesses that differ by a ratio of 1:5 or more. CMT takes this in its stride. Even uneven gaps of 0 to 3 mm in individual cases no longer prevent the perfect weld, as targeted droplet transfer allows the weld seam to be "modelled" without weld seam support.

Distortion under control: lower heat input, less rework
Alstom manufactures rolling stock in Salzgitter, northern Germany. Welding is one of the most important aspects of the entire bodywork production process. Testament to this in no uncertain terms are the 200 qualified welders and 700 welding systems that the company uses. The exterior of the wagons mainly consists of chrome-nickel alloys. These materials often place high demands on the welding technology.

The welders at Salzgitter got their first taste of CMT with a semi-automated process. A motorised tool holder (tractor) moved the torch of a TransPuls Synergic 4000 CMT along the 18 metre-long overlap between the wagon side panel and the roof arch. Before the CMT process was introduced, the welders used a GMAW process to make a fillet weld, which then had to be reworked. Now they save time and personnel costs thanks to CMT. The seam is flatter, more even and above all straighter. The final benefit of the CMT process is the significantly lower heat input, which causes much less distortion of the sheet. The welders in Salzgitter are full of praise for the arc, which is much more stable than that found with conventional processes.

Contract welders capitalise on the "just in time" properties of CMT
In 1992, aged just 23, Bernd Ruß founded HABS, a robotic welding contracting company, and concentrated from the outset on robot-assisted welding. Back then this was seen a very bold step, but today Bernd is rightly considered one of the pioneers of this technology. Quality and adherence to deadlines are the defining factors for business success in contract welding companies. Therefore the right technology plays an even more important role. It therefore came as no surprise when in 2005 HABS became one of the first companies to adopt CMT. Since then they have been exploring the usage potential of the process. The company generates over 80% of its turnover from robot contract welding and now has 9 CMT systems. The remainder comes from the planning and construction of welding robot workstations. The company supplies robots, accessories, welding systems, positioning equipment and process programming services. The company's core business, however, is still the robot contract welding of standard parts from the semi-finished products supplied to them.

In addition to CMT welding, Ruß is very interested in the possibility of using CMT to braze galvanised steel plate. The greatly-reduced heat input largely prevents the zinc from vaporising and as no spatter is produced, the costly process of removing spatter by grinding and other mechanical processing is no longer required. Both factors add to the value of the brazed steel plate products as the protective quality of the zinc layer (and therefore the anti-corrosion properties) remain almost completely intact.

The CMT process is far more than a mere extension of the earlier GMAW process. It offers completely new opportunities for solving technical welding and brazing problems. Workpieces that are virtually spatter-free require no costly mechanical rework. This saves any interim handling that would otherwise be required, not to mention a second quality inspection. Welding correctly in the first place enables the workflow to be optimised, thus assisting with efforts to move towards "just in time" production.

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