Under steam
There’s no way to use the laser without ripples, spatters and vapors. But maybe there is? New research is attempting to tame the beast.
The simulation of a metal block. And suddenly its interior is ablaze, in all the rainbow’s colors. A swirling funnel eats its way into the material. The maelstrom pulses at its edges. Droplets and clouds of vapor spew from the hole in every direction. In a thousandth of a second it is all over- at least in the simulation by Prof. Andreas Otto, physicist and lecturer at the Vienna University of Technology.
Otto is one of 30 laser researchers and users who meet annually at the snowy ski resort of Hirschegg in Kleinwalsertal, Austria. The explosive topic of this workshop: What actually happens in the metal when the laser beam’s brute energy welds, cuts or marks it?
Dr. Rudolf Weber, Institute for Laser Tools (IFSW), University of Stuttgart
Up to now the honest answer to this question was: “We haven’t a clue.” The dynamics of the molten material in the “keyhole” – that’s what experts call the capillaries the laser burns into the metal – are largely unknown. “Our goal is to better understand what’s happening in the sauce,” explained Rudolf Weber from the Institut für Strahlwerkzeuge IFSW (Institute for Laser Tools) at the University of Stuttgart, who organized the workshop. Weber knows that “half the participants go to Hirschegg because they are looking for answers to unanswered questions in laser processing.”
And the other half work at the IFSW to find answers to these questions. Laser users urgently need these answers. Because until now they have been adjusting output, defocusing, feed speed and other parameters according to the principle of trial and error.
Simulation instead of experiment
This functions just fine as long as the laser beam welds or cuts steel. Yet when it comes to modern lightweight construction materials like aluminum or special types of steel like cast steel with a high percentage of carbon, the cost and effort for tests quickly become extremely high.
Prof. Andreas Otto, Vienna University of Technology
For simulations that could shorten the test series, the findings concerning the processes and interactions inside the keyhole are not sufficient in most cases. That is why promising application ideas often do not get beyond the starting phase. This is unfortunate because joining cast material and steel opens up new geometric opportunities and saves weight.
Aluminum and copper bonds, for example, are urgently needed for current-carrying components in electric and hybrid autos. That also means that parameters like spot size or laser intensity have to be reconsidered in order to be able to weld such materials.
Stubborn copper
In the central lab at the Ulm-based Wieland-Werke AG, Dirk Herrmann is also working on the difficult task of welding copper. The company delivers semi-finished products like coils, tubes and profiles made of copper and copper alloys to customers who turn them into radiators, plug-in connectors or solar absorbers, for example.
These customers, too, would like composite materials “for customized products that combine numerous functions,” says Herrmann. Plugs, for example: It would be ideal if they were wear-resistant and hard in the front connector and yet bendable and ductile in the rear cable connector.
Due to its high electrical and thermal conductivity, copper is indispensable in many applications. Yet it is precisely these properties that make the welding of copper materials so difficult. Getting the energy to the point quickly is the name of the game, according to Herrmann, in order to outmaneuver the thermal conductivity of the reddish metal.
TRUMPF is currently participating in a research project on a green laser which should manage this and apply up to 60 percent of the energy to the metal instead of only 2 percent. The benefits of the green laser for welding copper materials are currently being intensely explored in a joint project (CuBriLas) sponsored by Germanys Federal Ministry of Education and Research. In the future, we will be able to weld copper as smoothly as steel,” Herrmann asserts.
Until then, however, there are still a few difficulties to overcome. The new measuring equipment from IFSW should contribute to these efforts.
5,000 new images per second
Volker Rominger, an IFSW graduate student working at Trumpf, has produced high-speed videos of the capillary that show the laser beam drilling into the metal during welding. The videos demonstrate that with a slower feed speed and lower laser output, fewer ripples and splatters are produced.
But slowing the laser down cannot be the solution. Rominger is looking for settings in which ripples and splatters do not occur at all. And if they do, they are specifically produced. After all, what is a disaster for one user, can be a blessing for another. In other words, those who cut sheet metal are pleased when ripples form in the molten material, because ripples act as a kind of perforation that help the laser do its job. Dr. Rudolf Weber is pragmatic: “If you cannot solve a problem, you should capitalize on it.”
Nevertheless, the problem still needs to be understood. The x-ray machine the IFSW is building should help with understanding. It delivers live images from inside the metal during welding. The institute intends to take up to 5,000 images per second – a world record.
The x-ray should illuminate what is happening in the molten bath instead of seeing just what is on the surface – “everything else was speculation,” admits Felix Abt, who is responsible for building the machine at IFSW. The images should tell us, among other things, how much the welding depth is dependent on the laser settings.
It is presumed that air bubbles will not show up in the x-ray images because they are hard to image. The machine is also only somewhat suited for materials other than steel, like copper, which is very dense for the x-ray light. The good news is there are still enough research questions that future workshops will be held in Kleinwalsertal.
Contact:
Vienna University of Technology
Prof. Andreas Otto
andreas.otto@tuwien.ac.at
University of Stuttgart
Dr. Rudolf Weber
weber@ifsw.uni-stuttgart.de
This article was first published in fall 2010.
