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Laser welding has major attractions – and also challenges

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onsdag 31. oktober 2012


Alexander Kaplan is a professor at Luleå University of Technology, Sweden. He has conducted extensive research about laser welding over more than two decades.

One of Europe’s greatest experts explains that the main attractions of laser welding for the industry are the speed and the high quality. Development of task specific procedures are however time consuming, Professor Alexander Kaplan explains to OceanWise.

By Kent Krøyer
“The precision of laser welding is highly controllable. Once you succeed with good quality, you maintain it very precisely, and this is combined with the effects of high speed,” says Professor Alexander Kaplan from Luleå University in Sweden.

Laser welding requires precise and reproducible movements of industrial robots which also reduce the safety risks for a human operator.

There are other advantages, such as very low thermal exposure of the bulk material, due to the very local melt pools formed during welding. Also the production costs can be lowered as there is less deformation, which can mean the avoidance of postprocessing or post-machining. Often there is no need for adding filler wire under zero-gap conditions. In the end, factors such as production time, inspection time, and fatigue properties all enter into the equation. And such properties, which are very different from the known electric arc welding, are bound to affect the cost of some types of industrial production.

“It is necessary to balance the high investment cost of laser welding against the advantages of high welding speed. It easily pays back for those companies that work around the clock or perhaps in two shifts, but it will not pay back for job-shops that only weld occasionally,” says Professor Kaplan.

“It is necessary to balance the high investment cost of laser welding against the advantages, such as high welding speed.” says Professor Alexander Kaplan. Photo: Luleå Univerity

Barriers to heavy industries
Those are the main pros and cons, but the overall picture for laser welding can be more complex.
However, industries working with thin steel plates in the range of 0.1 - 8 mm have already adopted laser welding. The question is what happens when heavy industries really get interested. Are there any foreseeable barriers?

“In principle there is no upper limit to the thickness of the plates. We always use electron beam welding as a reference, and this technology goes very deep. Laser welding is basically similar to electron beam welding, except that the electron beam needs a vacuum,” Alexander Kaplan says.

The actual barrier for the laser welding of thick plates is the focusing of the laser beam.

“The achievable thickness will increase, as it has done over the years. There are some optical limits for how well it can be focused, but sometimes we see that we exceed what we thought were barriers. A lot has happened – especially in the last five years - with new laser types that can be focused much better. Now it comes close to what an electron beam can do.”

Keep focus
Thick plates of 40-60 mm can be welded from two sides.

“We started doing 40 mm from each side, and this is a good option if it is possible for the actual application. It is difficult today, but what will happen in ten years’ time I don’t dare to say.”

The focal quality of the laser beam is a compromise between focal diameter and focal depth, backed by the total power of the beam and how the laser beam is composed (multi-mode or single mode).

“Today you would say that you can get a near diffraction limited beam up to 10 kW, a few years ago this limit was 1 kW. If you get more power you can weld thicker plates as long as you are able to focus the laser beam.”

The focusing optics consist of either mirrors or – more commonly today – transmissive optics, meaning quartz lenses and/or fibers.

Difficult search for parameters
A possible problem could be spatter and fumes causing damage to the optics.

“It used to be an issue. You have to have good shielding and protection, such as directed air streams to protect the optics from the spatter and the fumes.

Another problem to deal with is classical welding defects such as spattering, incomplete grooves and formation of pores and cracks.

“Sometimes the process window becomes very narrow and it becomes difficult to find the optimum parameters. It is not straightforward, which is why we have to do a lot of research. Sometimes you get the right parameters immediately and sometimes you never find them. It is not systematic enough at the moment. But once you have found the right parameters it is very reliable in production,” he says.

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