LaserHybrid welding: Mastering the thickest joins

Even a full-size SUV would have no trouble fitting on the LaserHybrid system’s swivelling table installed at the TRUMPF Group’s site in Pasching, Austria, which specialises in bending machines. This giant installation is over twenty metres long and has a garage-shaped safety enclosure including large lifting gates on both sides and moveable loading stations on rails. From the outside it resembles a car wash. But this is not why those in charge of the production plant, which is used to manufacture large-scale machine frames, chose the project name “car wash” during the planning phase. In fact, the generously sized, moveable gantry in the work room was responsible for the association. It allows the welding robot hanging from it to move around the frame it is working on – just like the brushes in a car wash – and reach the workpiece from any angle so that the system can then weld it completely.

THE FIRST LASERHYBRID FOR THICK SHEET METAL

The actual key to this system lies in the welding technology used. TRUMPF is the first company to use a LaserHybrid welding head, originally developed for ship building, to weld thick sheets between eight and 200 mm. This technique is used to weld the frames for the bending machine series TruBend 5000. Until the LaserHybrid system went into series operation in October 2014, TRUMPF used to outsource this part of the work process to external companies. This was quite literally the very opposite of light work: not only do these machine frames weigh anything up to 20 tonnes, but for each frame several metres of weld seam had to be prepared manually and had to meet stringent precision and quality requirements. The plant in Pasching was assembling 20 bending machines every week, so the associated transportation was both very costly and time-consuming.

But this wasn't the only reason that TRUMPF was looking at alternative production options as a matter of priority. “The lead times with the external supplier were long, and the cost structures of manual welding were far from perfect,” says Thomas Reiter, Head of Production for TRUMPF Maschinen Austria GmbH + Co. KG. “Furthermore, with manual welding it is difficult to avoid variations in quality, for example in relation to the component tolerance.” When a new generation of TruBend 5000 was launched in 2012, TRUMPF therefore decided to build its own plant for series production of machine frames at its facility in Pasching. The cornerstone for this system was a robotassisted LaserHybrid welding system from Fronius.

TWO PROCESSES IN PERFECT HARMONY

A deciding factor behind this decision was that Fronius had already been involved with TRUMPF's production operations for many years with its MAG welding processes. With its extensive expertise in welding technology, Fronius started by checking that its LaserHybrid method was suitable for thick sheet metals, and then continued by adapting the equipment technology to the specific requirements of TRUMPF's plant. Fronius had in the past already implemented similar systems in shipbuilding for welding reinforcement profiles, and in the automotive industry. The key benefit to the LaserHybrid process is that it is actually a combination of two processes that complement one another perfectly: whilst the MIG/MAG process ensures good gap-bridging ability with relatively simple seam preparation and reduces the proneness to hot cracking, the laser guarantees a reliable and concentrated heat input together with deep penetration. 

Specialists from Fronius and TRUMPF worked together on extensive practical tests to discover whether the LaserHybrid process was suitable, as they hoped, for thick sheet metals. “All those involved pulled together in such a way that it would have been almost impossible for an onlooker to tell Fronius and Trumpf employees apart,” says Thomas Reiter in praise of the team spirit. As the success of this project was not guaranteed in the initial phase due to its complexity and the scope of the system, the production chief had a back-up plan up his sleeve: if necessary the system would have worked with MAG processes only. “This would have improved the situation in terms of costs and lead times, however it would still have included an external process with all the associated disadvantages.”

WELDING WITHOUT ANNEALING OR INDUCTIVE HEATING

In order to eliminate the stresses that occur during MAG welding, the entire frame would have to be heated to 600 °C and then slowly cooled. However, that would only have been possible as an external solution. Thomas Reiter also rules out the alternative, inductive preheating, as inefficient: “The required extensions for inductive heating limit access to the workpiece too drastically.” The LaserHybrid method provides the solution: the laser preheats the seam area to 150 to 180 °C, which means that there is no need for any annealing or inductive heating. This makes the whole process considerably more efficient.

The LaserHybrid system is impressive in terms of its results too. It achieves a penetration depth of up to ten mm with perfect weld seam quality, thereby enabling fillet welds to be welded in one weld layer with no weld-seam preparation. In this way, a LaserHybridwelded seam with a throat thickness of 5 mm can achieve the same load capacity as a MAG-welded seam with a throat thickness of 8 mm. However, this would require extensive weld-seam preparation that is restricted by tolerances and multiple weld layers. By using the hybrid process, TRUMPF has succeeded in reducing the weld layers by an average of 15%. Above all for weld seams subjected to high stresses, TRUMPF prefers the LaserHybrid system. However, the machine manufacturer cannot completely do without conventional MAG welding, partly because with this process, weld seams can only be welded from a lying position, and partly because accessibility to the frames is limited. For this reason, the robot cell is fitted with a tool changer, allowing the laser to be used for preheating for the standard MAG process. This renders the annealing process unnecessary and the entire process chain can occur on site.

SYSTEM IMPRESSES BOTH CUSTOMERS AND VISITORS

A TPS 5000 power source provides the necessary energy and computing power both for the standard MAG process and the MAG element of the LaserHybrid system. The laser source, a TruDisk 8002 disc laser, and its optics are from TRUMPF. The device technology used contributes significantly to the high system availability of over 97%. In this way the machine manufacturer has significantly increased both the cost effectiveness and profitability of their production process. No wonder the LaserHybrid system impresses those in charge as well as customers and visitors. As Thomas Reiter says: “We are noticing huge interest in the technology. Users such as wind turbine manufacturers, who have to weld thick sheets with correspondingly large throat thicknesses, want to get on board too.”

“All those involved pulled together in such a way that it would have been almost impossible for an onlooker to tell Fronius and TRUMPF employees apart”

THOMAS REITER, Head of Production at TRUMPF Maschinen Austria GmbH + Co. KG ,was able to improve the profitability and process reliability of manufacturing machine frames and reduce delivery times with the aid of the LaserHybrid welding process.