Ever since human beings first saw kindled sparks reflected in their companions’ eyes, a bright flickering flame has been a primordial passion. So it’s probably no surprise that this special laser welding demonstration in Planegg near Munich left a tinge of regret even among the most experienced viewers. And yet it represents a technological victory over the undesirable side effects associated with deep penetration welding with solid state lasers, namely the hard deposits on the surface of the workpiece caused by spark residue and spatter.
From behind the low-pressure chamber’s plate-glass observation window, a golden rain of flying sparks and a white-hot plume of vaporized metal rising from the weld seam could be seen. This is what happens when welding with a 6 kW disk laser at atmospheric pressure. Now, all that can be seen above the surface of the metal is a small flame no bigger than a match flame and the occasional stray spark.
This is how the low-pressure laserwelding developed by pro-beam, TRUMPF and the ifs works. (Video: pro-beam).
This is all that remains at one tenth of normal atmospheric pressure. If the pressure is reduced by a further factor of ten, all that can be seen through the filtered glass of the observation window is a faint glow emanating from the part being processed, in this instance a gear wheel, and a very occasional spark.
A joining of forces
The vacuum and electron beam technology
pro-beam is one of the world’s largest providers of electron-beam technology, with a workforce of 249 employees. Since 1974, the company has been developing systems and technologies for use in electron-beam welding processes.
The low pressure chamber in which this before-and-after fireworks display was demonstrated is operated by the research department of pro-beam, a company more usually associated with electron-beam welding rather than laser welding. For chief executive Dr. Thorsten Löwer, this does not represent conflicting interests but is more of a logical consequence: “If you look closely at our logo, you’ll see that we’ve always given equal status to the words ‘laser’ and ‘electron beam.’” This open attitude enabled a joint project to be set up, comprising two companies and a research group, with the aim of exploring entirely new perspectives for the use of solid-state lasers in deep penetration welding.
Typical workpieces: The project partners developed low-pressure welding into an industrial process that is especially suited to powertrain components requiring weld penetration depths of between three and six millimeters and a high feed rate. Photo | ifs Braunschweig
Perfectly clean weld seams: The seams of this freshly welded component are smooth and even, and there is no spatter that needs to be removed. Photo | ifs Braunschweig
A gear wheel welded at a pressure of 10 millibar. The image shows a top view of the seam (left) and the corresponding micrograph (right). The penetration depth is three millimeters and the weld was produced with a laser output of four kilowatts and a feed rate of five meters per minute. Photo: |ifs Braunschweig
The project’s roots date back to 2009. Shortly before that, TRUMPF, the other industrial partner, had brought a series of new, high-brilliance disk lasers onto the market. “We could see that the focusability of solid-state lasers was closing in on electron beam technology,” recalls Löwer. “This prompted Klaus Löffler from TRUMPF and me to start tossing ideas around at one of our meetings. We wondered what would happen if a disk laser was used for welding in a similar environment to that of electron-beam welding – namely in a vacuum or at least under reduced pressure.”
Closing the gap
The laser technology
TRUMPF is the world market and technology leader in fabricating machinery and industrial lasers and fabricating machinery for flexible sheet metal processing.
The answer to this question was revealed during the demonstration, by a glance through the observation window of the vacuum chamber on the top floor of the pro-beam building. Dr. Löwer and Hakan Kendirci, industry manager for powertrain applications at TRUMPF, then show us what this can lead to one floor below. Between the electron-beam welding machines of pro-beam’s contract manufacturing facility is a compact manufacturing cell connected to a yellow laser light cable. No more observation windows, but instead a workflow system with a turntable.
This ist not about pitching laser against electron-beam. This is about closing a gap using know-how derived from both the technologies.”
Hakan Kendirci, industry manager for powertrain applications at TRUMPF.
Once it is delivered to the customer, the station will be integrated into the daily production cycle, welding gear components for automotive transmission systems. “That might be misconstrued to mean that the laser has scored a victory over the electron beam,” comments Kendirci. “But in fact what we have created here is an entirely new solution that combines the best of both worlds and closes the gap between them.”
Getting things up to speed
The Institut für Füge- und Schweisstechnik IFS der Universität Braunschweig (institute of joining and welding) at the Technical University of Braunschweig specializes in research into component assembly methods and uses both electron-beam and laser welding.
Indeed, the technique of low-pressure laser welding permits high-volume processing with weld penetration depths exceeding three millimeters. At this kind of depth and feed rate, a solid-state laser tends to produce spatter, while on the other hand electron-beam welding requires a high vacuum and hence all the associated technical equipment.
Moreover, the applications in question lend themselves to the use of a laser network, which would substantially increase efficiency: Multiple welding cells can have access to a single laser beam source, allowing optimal use to be made of the available capacity. This type of solution would be difficult or impossible to implement using CO2 lasers or electron-beam technology.
Sounds like a limited application? According to Dr. Löwer and Hakan Kendirci, this is anything but. “We’re talking about millions of powertrain components which require a welding process that precisely fits these parameters. In other words, there’s a tremendous demand.”
It’s not often we see a system operating in a real-life, series production environment only two years after an applied research project of this type was launched. This is one of those exceptional cases.”
Christian Börner, project manager at the ifs at the Braunschweig university
The third partner in the project, Christian Börner, confirms the accuracy of pro-beam and TRUMPF’s statement. Börner is project manager at the Institute of Joining and Welding (ifs) at the Technical University Braunschweig. “It’s not often we see a system operating in a real-life, series production environment only two years after an applied research project of this type was launched. This is one of those exceptional cases.”
The beauty of spatter-free welds
The ifs joined the project around two years after the original “brainstorming” session. The institute’s director, Professor Klaus Dilger, conducts research into both laser and electron-beam welding processes. In 2011, when he replaced the existing pro-beam electron-beam machine with a new one, a vacuum chamber became vacant. Dr. Löwer and Klaus Löffler saw this as an opportunity and came to an arrangement with Professor Dilger. pro-beam agreed to let the institute continue using the vacuum chamber and TRUMPF provided a six-kilowatt TruDisk 6002 with the necessary optics and laser light cables.
Improved weld quality of a steel gear component produced using a solid-state laser at reduced ambient pressure. (Video: ifs – Institute of Joining and Welding Technology at the Technical University of Braunschweig).
The very first experimental tests already produced clean weld seams with hardly any spattering. Cross-section images showed seams with a very narrow profile and parallel edges. The typical nail-head shape disappeared. “The welds we looked at were beautiful and extremely clean,” says Börner. “That’s what sparked the decisive idea: Instead of aiming to achieve the maximum weld depth, we decided to concentrate our efforts on developing a new welding process for powertrain components that wouldn’t require any post-processing.”
And that’s the reason why
In the course of the two-year, publicly funded research project entitled “Laser beam welding at reduced ambient pressure” (IGF 17.560 N), Börner and his team succeeded in explaining most of the mechanisms involved in the process. They identified the flow behavior of the expelled metal vapor as the cause of spatter and plume formation.
As the metal vapor swirls and intermingles with the ambient air, it forms a plume surrounded by areas of turbulence. This turbulence causes the plume to dance around, repeatedly hitting the molten layer at the capillary rear wall. Fragments of the molten material are stripped off by the plume, rise up into the air, and shower the workpiece with a stream of red-hot particles.
The images captured by the institute’s high-speed cameras show that the faster the beam penetrates the metal, the more the capillary wall tends to bend away from the welding direction. And the more oblique the angle of the outflowing gases, the greater the force with which the plume hits the melt pool.
Mark the difference
These two images show the amount of sparks generated at a feed rate of 6 meters per minute and a weld penetration depth of three millimeters: At atmospheric pressure (left) and at 10 millibar (right). Photos | pro-beam
At atmospheric pressure, a white-hot plume of metal vapor rises up above the keyhole (left). The plume sways around as a result of the turbulence caused when the expelled metal vapor intermingles with the ambient air, generating spatter. At a pressure of 10 millibar, this phenomenon is virtually eliminated and there is hardly any more spatter (right). Photo | ifs Braunschweig
Comparison of the two weld seams, both produced at a feed rate of 6 meters per minute: As above, at atmospheric pressure (left) and at 10 millibar, (center). Photo | ifs Braunschweig
Below an ambient pressure of around 50 millibar, an increase in the feed rate has no further impact on spark formation- Photos | ifs Braunschweig
The graph shows how the weld quality increases as the pressure is reduced. Photo | ifs Braunschweig
“By reducing the ambient pressure, we can increase the mean free path of the metal atoms in the plume, that is, the average distance they can travel without colliding with another atom or molecule,” Börner explains. “At atmospheric pressure, the mean free path is around 68 nanometers. If we reduce the pressure to one millibar, the mean free path is multiplied by 1,500 to 100 micrometers.”
On a macroscopic scale, this means that the flow of vaporized metal is less restricted and more unidirectional, and the vortex above the keyhole is dissipated. This causes the turbulence to disappear, along with the collisions and resulting spatter. “The full effect already takes place at a pressure between ten and one millibar,” adds Börner. “We therefore refer to the process as low-pressure welding or welding at reduced pressure, so as to differentiate it from the high-vacuum process of electron-beam welding.”
Automotive industry joinging in
At the welding cell in Planegg, Hakan Kendirci places a freshly welded gear component on the airlock of the workstation. “This is the result we were looking for: Clean components with zero spatter. And we can make better use of the allocated cycle times by using the same beam source to weld components alternately in different chambers,” he says. “We only require a slight drop in pressure, which can be achieved in a matter of seconds for components of this size. This considerably reduces cycle times compared with fine-vacuum processes.”
No surprise, then, that two major automakers had representatives on the research project’s steering committee, and both are currently preparing to introduce the new process. And it’s also no surprise that laser manufacturer TRUMPF has been operating a low-pressure chamber in its own applications center for some time now, which is being used to investigate the practical aspects of the process.
“As the pressure diminishes, the metal vapor plume disappears. But there are also other promising changes,” explains Haan Kendirci. “That’s why we are studying parameters such as gap bridging capability and the welding behavior of like materials or different combinations of unlike materials.”
I can well imagine very efficient systems that combine laser welding at atmospheric pressure with low-pressure welding.”
Dr. Thorsten Löwer, chief executive of pro-beam
Leading the way
Hakan Kendirci and Dr. Löwer see the future in much the same way. While many people still regard laser welding and electron-beam welding as rival techniques, all that really counts when designing processes is their efficiency, operating cost, and the capital outlay.
“Why does it always have to be either/or?” Dr. Löwer wonders. “There are so many welded parts that need to be assembled in successive stages, each involving different levels of difficulty. With our expertise in low-pressure and airlock technology, I can well imagine very efficient systems that combine laser welding at atmospheric pressure with low-pressure welding.” He points once again to the pro-beam logo, where the laser and the electron beam have co-existed for so long.
TRUMPF Industry Manager Automotive Powertrain
phone: +49 7156 303-36890