Robotic welding the cool way

voestalpine Automotive Components Schmölln GmbH in Germany manufactures various structural components for the automotive industry from galvanised and high-strength sheets as well as aluminium sheets. In doing so, the company is reliant on robot assistance: to ensure it can supply its customers around the world, voestalpine has a total of twelve automated welding cells at its plant in the German state of Thuringia. Half of these use Fronius technology: since 2011, voestalpine has been converting more and more of its systems from the standard MAG process to the innovative CMT (Cold Metal Transfer) welding process. voestalpine's experience with CMT was altogether positive, and so they also decided to adopt it as their standard process for arc welding of sheet steels up to 3 mm thick in the requirements specification for new welding systems. Those in charge at voestalpine accordingly reacted with curiosity when the second generation of CMT technology was launched in early 2016. Since then this technology has also become available in combination with the latest power source platform TPS/i. Fronius suggested to voestalpine that they trial using it in series production. “We selected an A-pillar made from high-strength galvanised steel for the initial welding trials. This complex component is ideally suited to putting the new process through its paces, not only because of the demanding nature of the material, but also due to the limited accessibility and the large gap widths of the manufacturing process,” explains Michael Nowasz.

DYNAMIC CONTROL ENABLES SPATTER-FREE RESULTS

As in the previous generation, the new CMT process excels with its low heat input and high gap-bridging ability. This is achieved through dynamic arc control. “In the Fronius system, droplet detachment and droplet transfer are not only controlled electronically by the power source, but are also aided mechanically,” says Nowasz. To achieve this, the wire electrode is always precisely pulled back slightly with the aid of a push-pull welding torch when the current has reached its maximum and is switched off. This creates an especially stable arc that is spatter‑free and allows high welding speeds. The first welding trials were conducted at the Fronius Institute of Technology at its headquarters in Wels in the summer of 2016. “I visited the laboratory with our head of development and experienced the new CMT process in action,” says Nowasz. “What we saw there was simply fantastic. It was immediately clear to us that Fronius had made another huge leap forward.” The foundation stone for this technological leap is formed by the latest power source TPS/i. Fronius has redeveloped all of its components, from the controller, communication bus and wirefeeder to the user interface, cooling system and hosepack, completely from scratch. This has allowed the company to use the latest technology and thus prime the whole system for maximum precision and performance.

NEW ALGORITHM – NEW ROBOT WELDING TORCH

To enable the CMT process to gain the maximum possible advantage from the TPS/i, Fronius redeveloped the algorithm. Even the push-pull Robacta Drive welding torch was equipped as a CMT variant with a digitally controlled, gearless AC servo motor for even higher frequencies of the oscillating wire electrode movement. “An additional advantage of the new robot torch is that it is more compact than the previous model, guaranteeing the improved access that we require ever more frequently for components of greater complexity,” adds Nowasz. The results of the welding trials on a real component were so impressive that voestalpine decided to use the new CMT generation in their series production. In autumn 2016, it was time. In just one day the first of the six robot cells was converted by Fronius and then used to produce brackets from 2- to 3-mm-thick steel plates with CO₂ shielding gas. This involves welding roughly a metre of weld seams comprising 48 individual seams with a maximum length of 35 mm. “The next morning we worked together to program the robot, then welded the first parts on the same day,” recalls Michael Nowasz. “The right parameters can be found much more simply and quickly with the new system.” This meant that only slight adjustments had to be made on the third day in order to achieve the optimal operating values. This also allowed the head of department to convince himself of the functionality of the new magnetic CrashBox that Fronius had included in the design of its robot welding system: “We allowed the welding torch to come into contact with a sheet – the protective device reacted so quickly that no damage to the welding torch, robot, device or workpiece could occur.” After a collision, the magnetic lock simply needs to be re-engaged, meaning the reference point is retained and welding can continue immediately. Tedious exchanging of the CrashBox and the reference travel therefore become obsolete.

HIGH-SPEED WELDING WITH IMPROVED AVAILABILITY

“An additional bonus is the excellent availability of the CMT system,” states Nowasz. “If the arc is extinguished for any reason, the process will reignite it automatically. Ignition errors, the wire electrode welding to the contact tip or other similar annoyances are now a thing of the past for us.” Last but not least, the joining specialist is impressed by the variety of options for influencing the arc geometry. This also includes the option of combining CMT with a pulse process to better control the heat input, allowing faster welding. voestalpine managed to resume series production with the new CMT configuration on the same day as the tests were concluded. “I was absolutely delighted that the conversion went so smoothly. In the whole start-up phase we haven't had any production downtimes and haven't had to request additional support from Fronius,” says an elated Michael Nowasz. “The second generation of the CMT process represents another quantum leap in comparison to the previous version. We have noticeably improved our quality thanks to the numerous innovations and further developments of the TPS/i in combination with the tailored CMT process and also significantly extended our room for manoeuvre with regards to joining processes.”

“CMT has already allowed us to significantly increase our output. What's more, there's now hardly ever any spatter to be removed, which means that our operatives can focus completely on what is important to our customers: the quality of the weld seam.”

MICHAEL NOWA SZ, Head of the Joining Department, at the voestapline site in Schmölln.