Laser metal cutting with tailored beam patterns

Fleming Ove Olsen

Has potential to outperform fiber and CO₂ lasers

High brightness, high power solid state laser sources such as fiber lasers show potential in the next generation of laser metal cutting. At IPU, (formerly the Institute for Product Development), the development of a new concept for laser cutting with single-mode fiber lasers has been initiated within two projects: DOEFLAC and ROBOCUT, in co-operation with a number of project partners.1

Initial experimental tests have demonstrated that this new approach has the potential to outperform state-of-the-art fiber and CO₂ laser cutting,2 producing faster, better and cheaper cutting than both of these laser cutting technologies. It is projected that after the completion of the two projects, this cutting technology will be matured to a level where it can be manufactured and marketed on normal commercial terms.

Tailored beam patterns

The projects rely on a principle where the cutting process is performed with a complex laser beam pattern as opposed to traditional laser cutting using a single round laser beam. By using dedicated beam patterns created by utilizing the unique focusing properties of high-power single-mode fiber lasers, it becomes possible to assign a part of the total laser power to create a keyhole, such as that used in laser welding or in laser cutting. The remaining power will be distributed on the melt behind the main beam to create a suitable high vapor pressure distribution over the molten material surface. This makes it possible to establish a local pressure distribution on the melt flowing out of the cut kerf, which far exceeds the pressures that can be obtained by classical coaxial gas jets in laser cutting. As a result, very narrow cut kerfs can be produced. The new process has the potential of producing burr free cutting in a wide cutting rate range, high-speed cutting capability and quality cut kerfs in narrow contour cutting (FIGURES 1 and 2).

Further, by tailoring the laser beam pattern correctly (adding a "lid" beam pattern), the melt flow can be directed out of the kerf opposite to the incoming laser beam even without applying a cutting gas. Thus a one-pass remote cutting technology can be developed that has a clear potential in removing the molten material from the cut kerf much more efficiently than state-of-the-art remote laser cutting, making remote laser cutting interesting industrial technology.

Beam shaping

The core of this laser cutting technology is the beam shaping, which can be established in different ways, for example:

Designing a system with a number of single-mode fiber lasers. Transmit these beams to the cutting head via a beam combiner structure instead of putting all the single-mode beams into one large transmission fiber, as used in high power multimode fiber laser configurations.

Applying one single-mode fiber laser source. Apply an advanced optical system with a specially designed artificial hologram, also known as a diffractive optical element (DOE), to transform the input laser beam to a radiation pattern that is optimal for a given laser cutting production (FIGURE 3). As the beam pattern (FIGURE 2) is asymmetric, the DOE must be turned according to the actual cutting direction as sketched in FIGURE 3.

The mechanism of creating vapor pressure by intense laser light has been known for several decades from laser drilling and keyhole laser welding, where the local vapor pressure is the driving mechanism of penetration in drilling and for creating and maintaining the keyhole in welding. Keyhole laser cutting has also been known for ~20 years since a group of scientists at Fraunhofer ILT3 developed sheet metal cutting at high speed with CO₂ lasers. Keyhole laser cutting with CO₂ lasers was, however, limited to very thin sheets due to the strong plasma formation in the keyhole (as is also known in high power CO₂ laser welding).

The combination of the focusability of the high brightness fiber lasers and their wavelength makes it possible to perform keyhole cutting in thicker sections because the fiber laser can cut very fast compared with the CO₂ laser.4 Keyhole cutting can be more efficient than the classical laser cutting, where the molten material is flowing down in front of the laser beam, because the melt can much more efficiently be removed from the center line in the cut front. This makes the melt film thickness smaller and thereby ensures efficient heat conduction from the melt surface, where the laser light is absorbed in the melt front, where the energy is required for melting more material.5 However, the melt flow around the laser beam in keyhole cutting causes quality problems. The molten material is running down at the cut sides, deteriorating the cut quality because the fiber lasers can cut fast, but not with high cut qualities when the material's thickness is increased.

As in classical laser cutting and state-of-the-art keyhole cutting, the coaxial gas assist is the only driving force for melt removal; the kerfs must be enlarged, compared to the optical limitations of a single-mode fiber laser, to reduce the pressure down through the cut kerf.4 Here the pressure obtainable by laser radiation is larger, and large pressures can be maintained through the entire cut kerf. Therefore, the new approach with tailored laser beams can be designed from entirely optical limitations, which have the potential of cutting with narrower kerfs than in state-of-the-art laser cutting.

FIGURE 5. Cutting potential of remote cutting with tailored laser beam pattern.

Experiments so far

The experiments conducted so far with this new approach show a clear-cut quality improvement over a wide range of cutting rates and have demonstrated that almost burr-free cuts can be obtained. The potential of the two projects is obvious (FIGURES 4 and 5). Furthermore, results from one pass remote laser melt cutting from TRUMPF6 have demonstrated that the principle of melt removal works only with laser radiation. The TRUMPF solution, however, is based upon a round laser beam, which must be defocused to realize the mechanism. This is not an optimal solution because the melt here must run out of the kerf in front of the laser beam, which their published results show clearly. Another remote cutting technique is demonstrated by Fraunhofer Institute IWS,7 where a highly focused round single-mode laser beam is scanned over the surface multiple times, performing deep engraving. This technique is only suitable for thin sheet cutting, and the advantage of the scanning is more or less lost because of the need for multiple scans.

The key work in the project is currently focusing on the computation and manufacturing of the DOEs for the test phases and establishing experimental facilities at Aalborg University, including a 3 kW single-mode fiber laser. ✺

References

1. DOEFLAC is a EUROSTARS project with 4 partners: IPU (DK); Laser Optical Engineering (UK); CyLaser (IT); and Optoskand (SE). ROBOCUT is funded by the Danish High Technology Foundation. The project has as partners: IPU (DK); IAI A/S (DK); Grundfos A/S (DK); Micronix ApS (DK); Aalborg University (DK); Volvo Car Production (SE); and KUKA Nordic (SE).
2. Olsen, F. O., Hansen, K.S. & Nielsen, J.S., "Development of multibeam fiber laser cutting," Journal of Laser Applications, Vol. 21, Issue 3, pp. 133-138.
3. Preissig, K.-U.; Petring, D. and Herziger, G., "High speed laser cutting of thin metal sheets," SPIE, Vol. 2207, ISBN 0-8194-1508-1, pp. 96-110 (1994).
4. Wandera, C.; Salminen, A.; Olsen, F.O. and Kujanpää, V., "Cutting of stainless steel with fiber and disc laser," Paper 404, Congress Proceedings, 25th International Congress on Application of Lasers and Electro-Optics: ICALEO 2006, Orlando, FL: Laser Institute of America.
5. Olsen, F.O., "An evaluation of the cutting potential of different types of high power lasers," Paper 401, Congress Proceedings, 25th International Congress on Application of Lasers and Electro-Optics: ICALEO 2006, Orlando, FL: Laser Institute of America.
6. Olschowsky P., "Vapor-pressure fusion cutting, a new remote cutting technology," Presentation at 4th Primes Workshop, Seeheim (DE), 08.09.2010.
7. http://www.iws.fraunhofer.de/projekte/remocut/remocut.pdf.

Fleming Ove Olsen ([email protected]) is a senior engineer at IPU, Lyngby, Denmark.